assignment of transfer of heat

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Heat Transfer – Conduction, Convection, Radiation

Heat transfer occurs when thermal energy moves from one place to another. Atoms and molecules inherently have kinetic and thermal energy, so all matter participates in heat transfer. There are three main types of heat transfer, plus other processes that move energy from high temperature to low temperature.

What Is Heat Transfer?

Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics . The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal). The unit for the rate of heat transfer is the kilowatt (KW).

The Three Types of Heat Transfer With Examples

The three types of heat transfer differ according to the nature of the medium that transmits heat:

• Conduction requires contact.
• Convection requires fluid flow.
• Radiation does not require any medium.
• Conduction is heat transfer directly between neighboring atoms or molecules. Usually, it is heat transfer through a solid. For example, the metal handle of a pan on a stove becomes hot due to convection. Touching the hot pan conducts heat to your hand.
• Convection is heat transfer via the movement of a fluid, such as air or water. Heating water on a stove is a good example. The water at the top of the pot becomes hot because water near the heat source rises. Another example is the movement of air around a campfire. Hot air rises, transferring heat upward. Meanwhile, the partial vacuum left by this movement draws in cool outside air that feeds the fire with fresh oxygen.
• Radiation is the emission of electromagnetic radiation. While it occurs through a medium, it does not require one. For example, it’s warm outside on a sunny day because solar radiation crosses space and heats the atmosphere. The burner element of a stove also emits radiation. However, some heat from a burner comes from conduction between the hot element and a metal pan. Most real-life processes involve multiple forms of heat transfer.

Conduction requires that molecules touch each other, making it a slower process than convection or radiation. Atoms and molecules with a lot of energy have more kinetic energy and engage in more collisions with other matter. They are “hot.” When hot matter interacts with cold matter, some energy gets transferred during the collision. This drives conduction. Forms of matter that readily conduct heat are called thermal conductors .

Examples of Conduction

Conduction is a common process in everyday life. For example:

• Holding an ice cube immediately makes your hands feel cold. Meanwhile, the heat transferred from your skin to the ice melts it into liquid water.
• Walking barefoot on a hot road or sunny beach burns your feet because the solid material transmits heat into your foot.
• Iron clothes transfers heat from the iron to the fabric.
• The handle of a coffee cup filled with hot coffee becomes warm or even hot via conduction through the mug material.

Conduction Equation

One equation for conduction calculates heat transfer per unit of time from thermal conductivity, area, thickness of the material, and the temperature difference between two regions:

Q = [K ∙ A ∙ (T hot – T cold )] / d

• Q is heat transfer per unit time
• K is the coefficient of thermal conductivity of the substance
• A is the area of heat transfer
• T hot  is the temperature of the hot region
• T cold  is the temperature of the cold region
• d is the thickness of the body

Convection is the movement of fluid molecules from higher temperature to lower temperature regions. Changing the temperature of a fluid affects its density, producing convection currents. If the volume of a fluid increases, than its density decreases and it becomes buoyant.

Examples of Convection

Convection is a familiar process on Earth, primarily involving air or water. However, it applies to other fluids, such as refrigeration gases and magma. Examples of convection include:

• Boiling water undergoes convection as less dense hot molecules rise through higher density cooler molecules.
• Hot air rises and cooler air sinks and replaces it.
• Convection drives global circulation in the oceans between the equators and poles.
• A convection oven circulates hot air and cooks more evenly than one that only uses heating elements or a gas flame.

Convection Equation

The equation for the rate of convection relates area and the difference between the fluid temperature and surface temperature:

Q = h c  ∙ A ∙ (T s  – T f )

• Q is the heat transfer per unit time
• h c  is the coefficient of convective heat transfer
• T s  is the surface temperature
• T f  is the fluid temperature

Radiation is the release of electromagnetic energy. Another name for thermal radiation is radiant heat. Unlike conduction or convection, radiation requires no medium for heat transfer. So, radiation occurs both within a medium (solid, liquid, gas) or through a vacuum.

Examples of Radiation

There are many examples of radiation:

• A microwave oven emits microwave radiation, which increases the thermal energy in food
• The Sun emits light (including ultraviolet radiation) and heat
• Uranium-238 emits alpha radiation as it decays into thorium-234

Radiation Equation

The Stephan-Boltzmann law describes relationship between the power and temperature of thermal radiation:

P = e ∙ σ ∙ A· (Tr – Tc) 4

• P is the net power of radiation
• A is the area of radiation
• Tr is the radiator temperature
• Tc is the surrounding temperature
• e is emissivity
• σ is Stefan’s constant (σ = 5.67 × 10 -8 Wm -2 K -4 )

More Heat Transfer – Chemical Bonds and Phase Transitions

While conduction, convection, and radiation are the three modes of heat transfer, other processes absorb and release heat. For example, atoms release energy when chemical bonds break and absorb energy in order to form bonds. Releasing energy is an exergonic process, while absorbing energy is an endergonic process. Sometimes the energy is light or sound, but most of the time it’s heat, making these processes exothermic and endothermic .

Phase transitions between the states of matter also involve the absorption or release of energy. A great example of this is evaporative cooling, where the phase transition from a liquid into a vapor absorbs thermal energy from the environment.

• Faghri, Amir; Zhang, Yuwen; Howell, John (2010). Advanced Heat and Mass Transfer . Columbia, MO: Global Digital Press. ISBN 978-0-9842760-0-4.
• Geankoplis, Christie John (2003). Transport Processes and Separation Principles (4th ed.). Prentice Hall. ISBN 0-13-101367-X.
• Peng, Z.; Doroodchi, E.; Moghtaderi, B. (2020). “Heat transfer modelling in Discrete Element Method (DEM)-based simulations of thermal processes: Theory and model development”. Progress in Energy and Combustion Science . 79: 100847. doi: 10.1016/j.pecs.2020.100847
• Welty, James R.; Wicks, Charles E.; Wilson, Robert Elliott (1976). Fundamentals of Momentum, Heat, and Mass Transfer (2nd ed.). New York: Wiley. ISBN 978-0-471-93354-0.

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Heat Transfer

Heat transfer refers to the phenomenon in which heat or thermal energy is transferred from one region to another. The movement of atoms, ions, and molecules is responsible for carrying the energy. Heat is transferred from a high-temperature region to a low-temperature region. It can also occur between two thermodynamic systems at different temperatures, resulting in changes in internal energies.

Types of Heat Transfer

Three types of heat transfer occur in nature – conduction , convection , and thermal radiation .

1. Conduction

Heat is transferred between two atoms or molecules in direct contact. The transfer occurs when agitated molecules at high temperatures strike slower molecules at low temperatures, resulting in collisions. Heat transfer occurs through vibrations if the atoms are fixed in a lattice. Conduction takes place in solid, liquid, and gas. For example, when we heat one end of a metal rod, the energy quickly transmits to the other.

According to Fourier’s Law for heat conduction, the heat transfer rate is proportional to the negative temperature gradient and the area at right angles to the gradient through which heat flows. Mathematically, the equation is given by

Q: Heat transfer per unit time

K: Thermal conductivity

A: Cross-sectional area of the conductor

\( \frac{\Delta T}{\Delta x} \): Temperature gradient

2. Convection

Energy is transferred through a medium like liquid or gas. Molecules carry the energy due to the motion of the fluid. For example, when we boil water in a pan, the molecules at the bottom get heated first and carry the energy to the top. In nature, we experience convection currents due to hot air close to the ground that expands, rises, and cools down, resulting in a circular movement.

3. Thermal Radiation

Energy is transferred without any contact between the atoms or molecules. Energy in the form of electromagnetic radiation is emitted by a heated body and absorbed by another. A medium is not required for energy to travel. Electromagnetic waves carry thermal energy. For example, Sun emits heat through radiation, and we feel its warmth.

The intensity of radiation depends on the temperature and the surface characteristics. The radiant heat energy emitted per second per unit area by a surface is proportional to the fourth power of its absolute temperature. It is the basis of Stefan-Boltzmann law , which is represented as follows:

P: Radiant heat energy emitted per second

A: Surface area of the object emitting heat

σ: Stephan-Boltzmann constant

T: Temperature of the object

• Heat Transfer – Hyperphysics.phy-astr.gsu.edu
• Principles of Heating and Cooling – Energy.gov
• The Transfer of Heat Energy – Weather.gov
• Heat Transfer – Teachengineering.org

Article was last reviewed on Monday, January 2, 2023

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1.4 Heat Transfer, Specific Heat, and Calorimetry

Learning objectives.

By the end of this section, you will be able to:

• Explain phenomena involving heat as a form of energy transfer
• Solve problems involving heat transfer

We have seen in previous chapters that energy is one of the fundamental concepts of physics. Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. As we learned earlier in this chapter, heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature. Heat transfer is fundamental to such everyday activities as home heating and cooking, as well as many industrial processes. It also forms a basis for the topics in the remainder of this chapter.

We also introduce the concept of internal energy, which can be increased or decreased by heat transfer. We discuss another way to change the internal energy of a system, namely doing work on it. Thus, we are beginning the study of the relationship of heat and work, which is the basis of engines and refrigerators and the central topic (and origin of the name) of thermodynamics.

Internal Energy and Heat

A thermal system has internal energy , which is the sum of the microscopic energies of the system. This includes thermal energy, which is associated with the mechanical energies of its molecules and which is proportional to the system’s temperature. As we saw earlier in this chapter, if two objects at different temperatures are brought into contact with each other, energy is transferred from the hotter to the colder object until the bodies reach thermal equilibrium (that is, they are at the same temperature). No work is done by either object because no force acts through a distance (as we discussed in Work and Kinetic Energy ). These observations reveal that heat is energy transferred spontaneously due to a temperature difference. Figure 1.9 shows an example of heat transfer.

The meaning of “heat” in physics is different from its ordinary meaning. For example, in conversation, we may say “the heat was unbearable,” but in physics, we would say that the temperature was high. Heat is a form of energy flow, whereas temperature is not. Incidentally, humans are sensitive to heat flow rather than to temperature.

Since heat is a form of energy, its SI unit is the joule (J). Another common unit of energy often used for heat is the calorie (cal), defined as the energy needed to change the temperature of 1.00 g of water by 1.00 ° C 1.00 ° C —specifically, between 14.5 ° C 14.5 ° C and 15.5 ° C 15.5 ° C , since there is a slight temperature dependence. Also commonly used is the kilocalorie (kcal), which is the energy needed to change the temperature of 1.00 kg of water by 1.00 ° C 1.00 ° C . Since mass is most often specified in kilograms, the kilocalorie is convenient. Confusingly, food calories (sometimes called “big calories,” abbreviated Cal) are actually kilocalories, a fact not easily determined from package labeling.

Mechanical Equivalent of Heat

It is also possible to change the temperature of a substance by doing work, which transfers energy into or out of a system. This realization helped establish that heat is a form of energy. James Prescott Joule (1818–1889) performed many experiments to establish the mechanical equivalent of heat — the work needed to produce the same effects as heat transfer . In the units used for these two quantities, the value for this equivalence is

We consider this equation to represent the conversion between two units of energy. (Other numbers that you may see refer to calories defined for temperature ranges other than 14.5 ° C 14.5 ° C to 15.5 ° C 15.5 ° C .)

Figure 1.10 shows one of Joule’s most famous experimental setups for demonstrating that work and heat can produce the same effects and measuring the mechanical equivalent of heat. It helped establish the principle of conservation of energy. Gravitational potential energy ( U ) was converted into kinetic energy ( K ), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. Joule’s contributions to thermodynamics were so significant that the SI unit of energy was named after him.

Increasing internal energy by heat transfer gives the same result as increasing it by doing work. Therefore, although a system has a well-defined internal energy, we cannot say that it has a certain “heat content” or “work content.” A well-defined quantity that depends only on the current state of the system, rather than on the history of that system, is known as a state variable . Temperature and internal energy are state variables. To sum up this paragraph, heat and work are not state variables .

Incidentally, increasing the internal energy of a system does not necessarily increase its temperature. As we’ll see in the next section, the temperature does not change when a substance changes from one phase to another. An example is the melting of ice, which can be accomplished by adding heat or by doing frictional work, as when an ice cube is rubbed against a rough surface.

Temperature Change and Heat Capacity

We have noted that heat transfer often causes temperature change. Experiments show that with no phase change and no work done on or by the system, the transferred heat is typically directly proportional to the change in temperature and to the mass of the system, to a good approximation. (Below we show how to handle situations where the approximation is not valid.) The constant of proportionality depends on the substance and its phase, which may be gas, liquid, or solid. We omit discussion of the fourth phase, plasma, because although it is the most common phase in the universe, it is rare and short-lived on Earth.

We can understand the experimental facts by noting that the transferred heat is the change in the internal energy, which is the total energy of the molecules. Under typical conditions, the total kinetic energy of the molecules K total K total is a constant fraction of the internal energy (for reasons and with exceptions that we’ll see in the next chapter). The average kinetic energy of a molecule K ave K ave is proportional to the absolute temperature. Therefore, the change in internal energy of a system is typically proportional to the change in temperature and to the number of molecules, N . Mathematically, Δ U ∝ Δ K total = N K ave ∝ N Δ T Δ U ∝ Δ K total = N K ave ∝ N Δ T The dependence on the substance results in large part from the different masses of atoms and molecules. We are considering its heat capacity in terms of its mass, but as we will see in the next chapter, in some cases, heat capacities per molecule are similar for different substances. The dependence on substance and phase also results from differences in the potential energy associated with interactions between atoms and molecules.

Heat Transfer and Temperature Change

A practical approximation for the relationship between heat transfer and temperature change is:

where Q is the symbol for heat transfer (“quantity of heat”), m is the mass of the substance, and Δ T Δ T is the change in temperature. The symbol c stands for the specific heat (also called “ specific heat capacity ”) and depends on the material and phase. In the SI system, the specific heat is numerically equal to the amount of heat necessary to change the temperature of 1.00 1.00 kg of mass by 1.00 ° C 1.00 ° C . The SI unit for specific heat is J/ ( kg × K ) J/ ( kg × K ) or J/ ( kg × °C ) J/ ( kg × °C ) . (Recall that the temperature change Δ T Δ T is the same in units of kelvin and degrees Celsius.)

Values of specific heat must generally be measured, because there is no simple way to calculate them precisely. Table 1.3 lists representative values of specific heat for various substances. We see from this table that the specific heat of water is five times that of glass and 10 times that of iron, which means that it takes five times as much heat to raise the temperature of water a given amount as for glass, and 10 times as much as for iron. In fact, water has one of the largest specific heats of any material, which is important for sustaining life on Earth.

The specific heats of gases depend on what is maintained constant during the heating—typically either the volume or the pressure. In the table, the first specific heat value for each gas is measured at constant volume, and the second (in parentheses) is measured at constant pressure. We will return to this topic in the chapter on the kinetic theory of gases.

In general, specific heat also depends on temperature. Thus, a precise definition of c for a substance must be given in terms of an infinitesimal change in temperature. To do this, we note that c = 1 m Δ Q Δ T c = 1 m Δ Q Δ T and replace Δ Δ with d :

Except for gases, the temperature and volume dependence of the specific heat of most substances is weak at normal temperatures. Therefore, we will generally take specific heats to be constant at the values given in the table.

Example 1.5

Calculating the required heat.

• Calculate the temperature difference: Δ T = T f − T i = 60.0 ° C . Δ T = T f − T i = 60.0 ° C .
• Calculate the mass of water. Because the density of water is 1000 kg/m 3 1000 kg/m 3 , 1 L of water has a mass of 1 kg, and the mass of 0.250 L of water is m w = 0.250 kg m w = 0.250 kg .
• Calculate the heat transferred to the water. Use the specific heat of water in Table 1.3 : Q w = m w c w Δ T = ( 0.250 kg ) ( 4186 J/kg ° C ) ( 60.0 ° C ) = 62.8 kJ . Q w = m w c w Δ T = ( 0.250 kg ) ( 4186 J/kg ° C ) ( 60.0 ° C ) = 62.8 kJ .
• Calculate the heat transferred to the aluminum. Use the specific heat for aluminum in Table 1.3 : Q Al = m A1 c A1 Δ T = ( 0.500 kg ) ( 900 J/kg ° C ) ( 60.0 ° C ) = 27.0 kJ . Q Al = m A1 c A1 Δ T = ( 0.500 kg ) ( 900 J/kg ° C ) ( 60.0 ° C ) = 27.0 kJ .
• Find the total transferred heat: Q Total = Q W + Q Al = 89.8 kJ . Q Total = Q W + Q Al = 89.8 kJ .

Significance

Example 1.6 illustrates a temperature rise caused by doing work. (The result is the same as if the same amount of energy had been added with a blowtorch instead of mechanically.)

Example 1.6

Calculating the temperature increase from the work done on a substance.

Calculate the temperature increase of 10 kg of brake material with an average specific heat of 800 J/kg · °C 800 J/kg · °C if the material retains 10% of the energy from a 10,000-kg truck descending 75.0 m (in vertical displacement) at a constant speed.

Because the kinetic energy of the truck does not change, conservation of energy tells us the lost potential energy is dissipated, and we assume that 10% of it is transferred to internal energy of the brakes, so take Q = M g h / 10 Q = M g h / 10 . Then we calculate the temperature change from the heat transferred, using

where m is the mass of the brake material. Insert the given values to find

In a common kind of problem, objects at different temperatures are placed in contact with each other but isolated from everything else, and they are allowed to come into equilibrium. A container that prevents heat transfer in or out is called a calorimeter , and the use of a calorimeter to make measurements (typically of heat or specific heat capacity) is called calorimetry .

We will use the term “calorimetry problem” to refer to any problem in which the objects concerned are thermally isolated from their surroundings. An important idea in solving calorimetry problems is that during a heat transfer between objects isolated from their surroundings, the heat gained by the colder object must equal the heat lost by the hotter object, due to conservation of energy:

We express this idea by writing that the sum of the heats equals zero because the heat gained is usually considered positive; the heat lost, negative.

Example 1.7

Calculating the final temperature in calorimetry.

• Use the equation for heat transfer Q = m c Δ T Q = m c Δ T to express the heat transferred from the pan in terms of the mass of the pan, the specific heat of aluminum, the initial temperature of the pan, and the final temperature: Q hot = m A1 c A1 ( T f − 150 ° C ) . Q hot = m A1 c A1 ( T f − 150 ° C ) .
• Express the heat gained by the water in terms of the mass of the water, the specific heat of water, the initial temperature of the water, and the final temperature: Q cold = m w c w ( T f − 20.0 ° C ) . Q cold = m w c w ( T f − 20.0 ° C ) .
• Note that Q hot < 0 Q hot < 0 and Q cold > 0 Q cold > 0 and that as stated above, they must sum to zero: Q cold + Q hot = 0 Q cold = − Q hot m w c w ( T f − 20.0 ° C ) = − m A1 c A1 ( T f − 150 ° C ) . Q cold + Q hot = 0 Q cold = − Q hot m w c w ( T f − 20.0 ° C ) = − m A1 c A1 ( T f − 150 ° C ) .
• Bring all terms involving T f T f on the left hand side and all other terms on the right hand side. Solving for T f , T f , T f = m A1 c A1 ( 150 ° C ) + m w c w ( 20.0 ° C ) m A1 c A1 + m w c w , T f = m A1 c A1 ( 150 ° C ) + m w c w ( 20.0 ° C ) m A1 c A1 + m w c w , and insert the numerical values: T f = ( 0.500 kg ) ( 900 J/kg ° C ) ( 150 ° C ) + ( 0.250 kg ) ( 4186 J/kg ° C ) ( 20.0 ° C ) ( 0.500 kg ) ( 900 J/kg ° C ) + ( 0.250 kg ) ( 4186 J/kg ° C ) = 59.1 ° C . T f = ( 0.500 kg ) ( 900 J/kg ° C ) ( 150 ° C ) + ( 0.250 kg ) ( 4186 J/kg ° C ) ( 20.0 ° C ) ( 0.500 kg ) ( 900 J/kg ° C ) + ( 0.250 kg ) ( 4186 J/kg ° C ) = 59.1 ° C .

Check Your Understanding 1.3

If 25 kJ is necessary to raise the temperature of a rock from 25 °C to 30 ° C, 25 °C to 30 ° C, how much heat is necessary to heat the rock from 45 °C to 50 ° C 45 °C to 50 ° C ?

Example 1.8

Temperature-dependent heat capacity.

We solve this equation for Q by integrating both sides: Q = m ∫ T 1 T 2 c d T . Q = m ∫ T 1 T 2 c d T .

Then we substitute the given values in and evaluate the integral:

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FREE K-12 standards-aligned STEM

curriculum for educators everywhere!

Find more at TeachEngineering.org .

• TeachEngineering
• What Is Heat?

Lesson What Is Heat?

Grade Level: 6 (5-7)

(three 60-minute class periods)

Lesson Dependency: None

Subject Areas: Physical Science

NGSS Performance Expectations:

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• Keep It Hot!

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Engineering connection, learning objectives, worksheets and attachments, more curriculum like this, pre-req knowledge, introduction/motivation, associated activities, lesson closure, vocabulary/definitions, additional multimedia support, user comments & tips.

Understanding heat transfer is essential knowledge for the engineering of mechanical, chemical and biological systems. Design of internal combustion engines, air conditioning and heating systems, chemical and biological reactors and even clothing technology requires an understanding of heat transfer. Design of insulating materials for homes, buildings and even beverage containers also requires an understanding of heat transfer.

After this lesson, students should be able to:

• Explain that heat is the flow of energy from hot materials to cold materials.
• Describe that molecules in a material begin to vibrate (or move) more quickly when the material is heated.
• Identify conduction as heat transfer within and between solids.
• Identify convection as heat transfer involving gases or liquids.
• Identify radiation as heat transfer carried by little packets of energy that can travel through almost any material—even empty space.
• List examples of each type of heat transfer.

Educational Standards Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards. All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN) , a project of D2L (www.achievementstandards.org). In the ASN, standards are hierarchically structured: first by source; e.g. , by state; within source by type; e.g. , science or mathematics; within type by subtype, then by grade, etc .

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State Standards

California - science.

A familiarity with basic concepts about energy and its different forms, as well as a basic understanding of temperature.

Raise your hand if you ever put on a jacket? Or turned on a heater? Or melted an ice cube in your hand? (Expect every student to raise their hand.)

You probably appreciate heat on a cold day. But today, and over the next couple of days, we are going to talk about how scientists and engineers think about heat.

Lesson Background and Concepts for Teachers

Demonstration Materials: A few simple and powerful demonstrations are suggested for this lesson. A thermal energy demonstration requires two transparent containers that are capable of holding hot water, plus hot water, ice water and a few drops of food coloring. The conduction demonstration requires one candle, matches three small nails/thumb tacks, an oven mitt, and a hacksaw blade or metal rod (not stainless steel). An additional quick conduction demonstration requires five to 10 inflated balloons. Demo preparation and presentation instructions are provided on the slides and notes of slides 4 and 14.

The Additional Background Material section (below) provides a very detailed discussion about heat. While this material is generally above the sixth-grade level, it presents key background information for the teacher so they are able to answer advanced student questions.

Use the 21-slide What Is Heat? Presentation , a Microsoft PowerPoint® file, to directly deliver the lesson content, using the guidance provided below; alternatively, use the presentation to inform other teaching methods. Note that each slide includes background and discussion information in the notes sections that is not provided below and is unavailable in the PDF version. In addition, the slides are animated, so clicking brings up the next text or component on the slide.

( Slide 1 ) What is heat? Do the images on this slide give you any hints? Heat is energy that has something to do with temperature and is an important concept used by engineers to design many of the products we use every day.

( Slide 2 ) Open a discussion about what will happen to the temperature of the beverage in each case (hot chocolate, iced tea) when left unattended for 30 minutes. Why do some things get warmer while other things get colder when they are left out? Given time, both eventually become room temperature. The hot drink releases energy; the cold drink absorbs energy.

( Slide 3 ) Remind students about energy and some of its different forms. Expect them to recall that moving objects have kinetic energy. Show the animation to help visualize the relationship between temperature and kinetic energy: https://commons.wikimedia.org/wiki/File:Translational_motion.gif .

( Slide 4 ) Conduct a class demonstration to show temperature and kinetic energy using food coloring : Prepare separate transparent cups of hot and cold water (ice water is best; remove the ice for the demo). Into each cup, place a drop of food coloring and direct students to observe what happens. Expect them to notice that the food coloring in the hot water spreads out more quickly than that in the cold water. It is helpful to repeat this experiment after explaining the mechanism. Alternative: If conducting this demo is not possible, show a 2:52-minute video, "Moving Water Molecules"  (link also provided in the Additional Multimedia Support section).

( Slide 5 ) Talk about what students observed in the demo. The faster jiggling hot water dispersed the dye more quickly. Then show the animation of Brownian Motion at https://commons.wikimedia.org/wiki/File:Brownian_motion_large.gif . We can think of the small dots as water molecules, and the yellow dot as a much larger dye molecule being bounced around by the water molecules' thermal jiggling. This was discovered by Scottish botanist Robert Brown, who used a microscope to look at pollen samples in water. He could not see the water molecules, but noticed that pollen in hotter water jiggled around more than in colder water. The phenomenon was named in his honor: Brownian Motion.

( Slide 6 ) Make the point that thermal energy is in everything—even if it is something we consider cold.

( Slide 7 ) Explain the definition of heat as flowing thermal energy and clarify the direction of heat flow—from the hotter object to the cooler object. Energy transfers always occur from higher to lower states of energy.

( Slides 8-13 ) Use the provided images of a hot cup of coffee, an ice cream cone and a tea kettle on a burner as examples to talk about the direction of heat flow. Have students draw arrows to show the direction of heat flow; circulate around the room to verify their understanding. Make sure students realize that 1) heat is a form of energy that is transferred by a difference in temperature; a difference in temperature is needed for heat to flow, 2) heat always flows from hot to cold, or more precisely, heat flows from higher temperature to lower temperature, and 3) the units of heat are Joules, just like kinetic energy. The three different types of heat transfer (the movement of thermal energy) are conduction, convection and radiation. The "thought experiments" on slide 13 using the examples of hot soup and snowballs give students practice in using correct terminology and full sentences to explain how heat flows. Make sure students are able to realize that no heat transfer occurs between objects of the same temperature.

( Slide 14 ) Introduce the first type of heat transfer, conduction, which is heat transfer within or between solid objects. With our hands, we experience heat transfer by conduction any time we touch something that is hotter or colder than our skin.

At this point, present a conduction demonstration that you have prepared in advance . Before the activity, use drops of candle wax to "glue" two or three small nails or thumb tacks to a hacksaw blade or metal rod. Space the nails about 1 inch apart, with the first one located one to two inches from the end of the blade/rod. Hold the other end of the blade/rod with an oven mitt or nail it to a block of wood. Heat the end of the rod with a candle flame. As heat conducts down, the wax holding the nails melts and drops the nails, one by one, in sequence. This shows students the heat traveling down the rod.

Then conduct another class demonstration on heat conduction . Give each of five to 10 student volunteers an inflated balloon and have them hold them together, touching, in a line. Start to jiggle one end of the line and observe how this jiggling travels down the line of balloons.

( Slides 15-19 ) Introduce and go over the other two ways heat can move from one object to another: convection and radiation. Each slide starts with a discussion and examples and then gives a definition that can be used for building students' vocabulary.

( Slide 20 ) Introduce the concept of insulation, which is important in heat transfer and necessary background to understand the associated activity Keep It Hot! . Besides the oven mitt and pop can cozy, other examples of insulation include the walls and roof of houses, multi-pane windows, beverage thermos, insulation around car engines to keep passengers cool, inside a jet engine, material on the outside of the space shuttle, plastic casing on wires, a sweater or jacket, and refrigerator and oven walls.

( Slide 21 ) Wrap up with a brief review of key terms: heat, conduction, convection, radiation, insulation, and that heat flows from hot (or higher temperature) to cold (or lower temperature).

Additional Background Material

Heat in Engineering: Heat is the flow of thermal energy that arises from temperature differences. Whenever two things of different temperatures are near one another, thermal energy flows. This flowing energy is called heat. The fans heard whirring in computers are designed to remove heat generated by the electronics. Without these fans, computers would melt or create fires. On a winter morning, we put on coats to stay warm. Heat and how it flows within and between objects is something we experience every day and a fundamental engineering concern.

Thermal Energy and Heat: Every object in the universe has thermal energy stored within it. Thermal energy is the energy embodied in the vibrations, rotations and translations of atoms and molecules. This motion is extremely fast, significantly faster than indicated in the animations typically shown, and significantly faster than bulk translation (such as the flow of water molecules in a river). Expect the presence of energy in a system of jiggling, bouncing, molecules to be very obvious to students who already understand the concept of kinetic energy; indeed, the underlying physical mechanism is similar.

The energy contained in thermal "jiggling" is a function of many factors such as the mass of the particles and the speed of their motion. However, for a given material, faster molecular movement means more thermal energy is present.

Thermal energy is almost impossible to confine to a location. Rather, it can be causally observed every day. A cup of tea left on the counter cools off. Touching a hot pot lid burns one's hand. Objects that are in thermal contact tend towards thermal equilibrium, that is, they exchange thermal energy until both objects have the same temperature. When thermal energy moves around, the flowing thermal energy is called heat. This is somewhat confused by the engineering terminology of "heat transfer" (the study of just how that heat is moved around), which is somewhat redundant since the word "heat" already conveys the motion of thermal energy. In this document, "heat," "heat flow" and "heat transfer" all mean the flow of thermal energy.

One common example of thermal equilibrium is a cup of hot tea. Thermal energy in hot tea will flow (as heat) into the air because the tea temperature is higher than the air temperature. Heat leaving the tea causes the tea's temperature to decrease. Heat going into the air causes the air's temperature to increase. This process continues until the temperature of the tea and air is exactly the same, that is, until thermal equilibrium has been reached and no more impetus exists for thermal energy to move as heat. This is discussed further in the presentation using the analogy of a skier on a hill.

The mechanism of heat flow can be understood by remembering thermal "jiggling." Imagine placing a room temperature pot on a hot stove. Initially, the pot is 25 °C while the cooking element might be 600 °C. We know that heat is flowing from the element to the pot, because the pot's temperature increases. If we had a sufficiently powerful microscope, we could observe the atoms in the element and the pot. The lower temperature pot atoms would be jiggling around much more slowly than the atoms in the element. Since the two are touching, eventually a vigorously jiggling element atom collides with a slower jiggling pot atom. Just as a fast-moving cue ball collides with an eight ball and transfers some of its kinetic energy, the element transfers its thermal energy to the pot through countless such collisions.

The following is a very subtle point. The slowly jiggling pot atoms in the previous example might collide with the swiftly jiggling element atoms and transfer some kinetic energy FROM THE POT TO THE ELEMENT. This is quite the opposite from the established direction of heat transfer, that is, from high temperature to low temperature (or "hot to cold" in the easier-to-repeat shorthand phrase). Although this "opposite" mechanism may occur in isolated interactions, averaging the flow of heat over billions and billions of collisions always results in the "hot to cold" direction with which we are all familiar. Thermal equilibrium is reached when these collisions (again on average) involve the same amount of energy flowing into and out of the pot. At this point, both items are at the same temperature, and heat ceases to flow. Along these lines, "cold" is not a substance that flows. What happens when holding an icy soda can is NOT "cold flowing into my hand." The person holding the can experiences the sensation of a cold hand because the thermal energy in the hand has flowed, as heat, into the lower temperature soda can and given enough time, the two reach thermal equilibrium.

Types of Heat Transfer: Heat flows from objects of higher temperature to objects of lower temperature, and occurs in three forms, referred to by engineers as heat transfer: conduction, convection and radiation.

Conduction is heat flow in or between solid objects. If one touched the top edge of the pot in a previously described example, they would be burned. It is well known that heat flows from the bottom of a pot and into the upper edge, lid and handle. The mechanism of this heat flow is just as described in the pot and element example. Atoms in the bottom of the pot are jiggled by the hotter element atoms. The "front line" pot atoms then collide with their neighbors and then the next neighbors, eventually transferring thermal energy all through the pot.

A cast iron pan, left on the stove long enough, requires an oven mitt to handle. Heat flows from the element, into the pan, up the edge and along the handle. A pan with a wooden or plastic handle does not suffer from this problem because those materials have much lower thermal conductivity (the materials property that describes how well something conducts thermal energy) than the iron pot handle. Insulators such as wool, wood and Styrofoam have low thermal conductivity and are useful for slowing the flow of heat. Materials with high thermal conductivity such as copper, aluminum and glass are used to help heat move more quickly. As evidenced in the choice of materials used for electrical conductors and insulators, most materials with high electrical conductivity also have high thermal conductivity.

Convection is the flow of heat in gases or liquids; both are called "fluids" by engineers. A hair drier provides an excellent example of convection. Just as in the stove element, a piece of metal inside a hair drier is heated with electricity. Imagine if no fans were included inside hair driers. The air molecules near the hot elements atoms would be collided with, and heat would flow into them. In the case of the solid pot, the pot atoms are prevented from large movements because the pot is a solid. The pot atoms might jiggle and vibrate, but cannot go flying off across the room (unless heated to a very high temperature indeed). In the hair drier, the gaseous air molecules are much freer to move. They do this naturally in a process called free convection, which can be described by the familiar mechanism of "hot air rises." The rising hot air allows fresh cold air molecules to come into contact with the hot element atoms. Forced convection is what occurs in the hair drier—a fan blows high-speed air molecules over the hot element. In both cases of convection, the jiggling air molecules continue their jiggling when pushed away from the element. Depending on how fast the new air molecules are pushed past the element, convection can move heat over much larger distances, and much more quickly than conduction. The best remedy for a burned finger is to put it under flowing tap water. The subtleties of forced vs. free convection are beyond the scope of a sixth-grade class. The presentation simply refers to all heat transfer in liquids and gases as convection, with examples of the simpler fan-driven forced convection provided.

Radiation is the flow of heat carried by little packets of energy called photons. Radiation can transfer heat between two objects even in empty space, which is how the energy from the Sun gets to Earth. Although radiation does not need air to travel, it can travel through gases, liquids and even some solids. The cause of radiation is fairly complex. When a charged particle is accelerated, it emits a bit of radiation called a photon. Everything in the universe emits radiation because thermal energy causes electrons to accelerate and emit radiation (everything in the universe has some thermal energy). The amount of radiation an object emits is proportional to its temperature to the fourth power, so radiation is the dominant form of heat transfer only at fairly high temperatures. Just as before, the mechanism of heat flow through radiation can be imagined with the billiard ball collision example (although this is not as accurate an explanation of the underlying physics with radiation, it suffices). A photon from a high-temperature object strikes an atom in a lower-temperature object, causing it to jiggle more, raising the cooler object's temperature. Just as with the aside in the original pot/element discussion, some subtlety exists. Since all objects (even -400 °F comets) emit some radiation, an ice cube next to a red hot piece of iron is transferring energy from itself to the iron through radiation. But, for every one photon from the ice cube that strikes an iron atom, many thousands of photons transfer heat from the iron to the ice. So, on average, heat flows from hot to cold.

All three forms of heat flow occur at the same time, though some typically dominate, which permits engineers to ignore the others. Blowing a large fan over a 100 °C piece of metal involves almost entirely convection, but a little conduction (into the ground say) and a little radiation (heating the walls of the room) does occur.

Watch this activity on YouTube

(After the associated activity) We have discussed that heat is simply the flow of thermal energy that always goes from ________ to ________. (Expect everyone to chant out loud "from hot to cold.") We also know the three ways that heat can be transferred, which are _____________. (Answer: Conduction, convection and radiation.) Now, putting it all together and using what we understand about insulators, write and explain one way you can stay cool in the summertime and one way you can keep warm in the wintertime.

conduction: Heat transfer within or between solid objects.

convection: Heat transfer into or out of fluids.

heat: Thermal energy that flows due to a difference in temperature. Heat flows from hot to cold.

heat transfer: A method by which heat flows (conduction, convection, radiation).

insulation: A material that slows down heat transfer.

radiation: Heat transfer due to packets of energy called photons that can travel through many substances, even empty space.

temperature: the measure of the average speed of all particles.

thermal energy: the total energy of all particles in an object.

Pre-Lesson Assessment

Class Discussion & Assignment: To get students thinking about heat, lead a discussion and present a few everyday examples of heat, such as hot beverages, grabbing hot pans or touching ice cubes. Ask students to write a few sentences about how temperature and energy might be related. Also have each student draw an example of an everyday hot object. Provide a list of some examples: hot cocoa, a coal from a fire and a pan right out of the oven. Then ask students to draw a cold object near the hot one. This might be an ice cube, a can of soda from the refrigerator or cold air. Then ask students to draw arrows in their pictures that show what direction the energy flows (from the hot to the cold object, regardless of orientation).

Post-Introduction Assessment

Drawing Arrows: Use slide 8 of the What Is Heat? Presentation as an example and then have each student work individually during slides 9-11 to identify the direction of heat transfer by drawing arrows and writing a sentence. Circulate the room to verify and/or correct their understanding of the concepts.

Lesson Summary Assessment

Post-Quiz: After the lesson, and before starting the associated activity, administer the 10-question What Is Heat? Post-Quiz . Review students' answers to assess their comprehension of the thermal energy concepts.

Written Examples: As part of the Lesson Closure after completing the associated activity, assign students to write and explain one way they can stay cool in the summertime and one way they can keep warm in the wintertime. Require that they use scientific terminology as part of their explanations.

As an alternative to the thermal energy class demo, show this 2:52-minute video, "Moving Water Molecules" as a good illustration of the same demonstration: https://www.youtube.com/watch?v=CXY02tcgiBY .

With the help of simple, teacher-led demonstration activities, students learn the basic physics of heat transfer by means of conduction, convection and radiation. They also learn about examples of heating and cooling devices, from stove tops to car radiators, that they encounter in their homes, scho...

Students learn about the nature of thermal energy, temperature and how materials store thermal energy. They discuss the difference between conduction, convection and radiation of thermal energy, and complete activities in which they investigate the difference between temperature, thermal energy and ...

Students learn the scientific concepts of temperature, heat and the transfer of heat through conduction, convection and radiation, which are illustrated by comparison to magical spells found in the Harry Potter books.

Students explore heat transfer and energy efficiency using the context of energy efficient houses. They gain a solid understanding of the three types of heat transfer: radiation, convection and conduction, which are explained in detail and related to the real world.

Other Related Information

Browse the NGSS Engineering-aligned Physics Curriculum hub for additional Physics and Physical Science curriculum featuring Engineering.

Contributors

Supporting program, acknowledgements.

The contents of this digital library curriculum were developed by the Renewable Energy Systems Opportunity for Unified Research Collaboration and Education (RESOURCE) project in the College of Engineering under National Science Foundation GK-12 grant no. DGE 0948021. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: October 31, 2021

• Thermodynamics
• Heat Transfer

Modes of Heat Transfer (Conduction Examples)

In our everyday life, it has been observed that when a pan full of water is boiled on a flame, its temperature increases, but when the flame is turned off, it slowly cools down.

Table of Contents

What are the different modes of heat transfer, recommended videos, factors affecting heat transfer.

• Frequently Asked Questions – FAQs

This is because of the phenomenon of heat transfer taking place between the pan full of water and the flame. It has been established that heat transfer takes place from hotter objects to colder objects.

When there are objects which are at different temperatures or there is an object at a different temperature from the surroundings, then the transfer of heat takes place so that the object and the surrounding, both reach an equilibrium temperature.

There are three modes of heat transfer.

1. Conduction of Heat

Heat conduction is a process in which heat is transferred from the hotter part to the colder part in a body without involving any actual movement of the molecules of the body. Heat transfer takes place from one molecule to another molecule as a result of the vibratory motion of the molecules. Heat transfer through the process of conduction occurs in substances which are in direct contact with each other. It generally takes place in solids.

Conduction example: When frying vegetables in a pan. Heat transfer takes place from flame to the pan and then to the vegetables.

Based on the conductivity of heat , substances can be classified as conductors and insulators. Substances that conduct heat easily are known as conductors and those that do not conduct heat are known as insulators.

2. Convection of Heat

In this process, heat is transferred in the liquid and gases from a region of higher temperature to a region of lower temperature. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer.

For example. Heating of milk in a pan.

3. Radiation of Heat

It is the process in which heat is transferred from one body to another body without involving the molecules of the medium. Radiation heat transfer does not depend on the medium.

For example: In a microwave, the substances are heated directly without any heating medium.

Now we will discuss the rate of heat transfer or the factors on which it depends. The rate of heat transfer depends on the following:

ΔQΔt ∝ A(T1–T2)x

So the heat transfer equation comes out to be, ΔQΔt = K A(T1–T2)x where, K is the heat transfer coefficient. Here if heat flow is positive then we can infer T1 > T2. So heat flows from higher temperature to lower temperature. We can see that an analogy with electricity can be drawn, here temperature plays the role of potential difference and rate of heat transfer is like current while the rest of expression is like Electric Resistance . Now that we have drawn an analogy, so there must be series and parallel connections here also,

1. Heat Transfer in Series

Let the temperature of the junction be T. Therefore for the first rod,

⇒ ΔQΔt = K1 A1(T1–T)L1 —- (1)

Also for the second rod,

⇒ ΔQΔt = K2 A2(T–T2)L2 —- (2)

Since the temperature of conjunction remains constant, so the rate of heat transfer in (1) and (2) must be the same. Using the equation we can find the Value of temperature ‘T′.

2. Heat Transfer in Parallel

ΔQΔt = K1A1(T1–T2)L —- (3)

ΔQΔt = K2 A2(T1–T2)L —- (4)

So, net heat flow is the summation of (3) and (4). Suppose that the outside temperature is T and the depth of the lake is h. How much time will it take to freeze the entire lake? The latent heat of ice is L and thermal conductivity is K.

At this point, the rate of heat transfer is, ⇒ dQdt = KATx

⇒ dQ = KATx dt —- (5)

This heat is taken out and dx layer of ice is formed.

dm = ρA.dx —- (6)

Putting values from (5) and (6) we get,

KATx dt = ρA.dx.L

⇒ ∫t0 dt = ρLKT∫h0x.dx

Integrating with limits we get,

t = ρLh22KT

Frequently Asked Questions – FAQs

What are the different modes of heat transfer.

There are primarily three modes of heat transfer: Conduction, Convection and Radiation.

What is heat conduction?

Heat conduction is a process in which heat is transferred from the hotter part to the colder part of a body without involving any actual movement of the body’s molecules. Example: When frying vegetables in a pan. Heat transfer occurs from the flame to the pan and the vegetables.

What is heat convection?

Heat convection is a process in which heat is transferred in the liquid and gases from a higher temperature region to a lower temperature region. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer. Example. Heating of milk in a pan.

What is heat radiation?

Heat radiation is a process in which heat is transferred from one body to another without involving the medium’s molecules. Radiation heat transfer does not depend on the medium. Example: In a microwave, the substances are heated directly without any heating medium.

What is the cause of heat transfer?

The difference in temperature is the primary cause of heat transfer.

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CBSE Class Notes Online – Classnotes123

CBSE Class Notes, Worksheets, Question Answers, Diagrams , Definitions , Diffrence between , Maths Concepts, Science Facts Online – Classnotes123

Class 7 science -Chapter 4 – Heat -Detailed Notes

Table of Contents

Chapter 4 – Heat -Notes – Explained in details

Heat is a fundamental concept in science that affects everything around us, from the weather to how we cook our food. This chapter introduces the idea of heat, its measurement, and how it’s transferred between objects. You’ll learn about the difference between hot and cold objects, how we measure temperature with thermometers, and the scales used for measurement. We explore different ways heat moves—through conduction in solids, convection in liquids and gases, and radiation without any medium. The chapter also touches on practical applications of heat, like in designing energy-efficient buildings and choosing appropriate clothing for different temperatures. By the end of this chapter, you’ll understand the basic principles of heat and how it impacts our daily lives.

Hot and Cold Objects

Our ability to feel temperature through touch is a basic yet crucial part of understanding the world around us. When we touch an object, we can often tell if it’s hot or cold. This section explains how we differentiate between hot and cold objects using our sense of touch and discusses the reliability and limitations of this method.

Using Touch to Determine Temperature

• Sense of Touch- Our skin is sensitive to temperature changes, allowing us to perceive whether an object is hot or cold.
• Immediate Perception- As soon as we touch something, we can usually tell its temperature. This helps us in everyday activities, like testing water temperature or feeling if food is cooked.

Reliability of Touch

• Subjective Nature- The sense of touch is not always reliable. What feels hot to one person might feel warm to another.
• Surrounding Environment- Our perception of temperature can be influenced by the environment. For instance, a metal object feels colder than a wooden one in the same room because metal conducts heat away from our hand faster.

Safety Precautions

• Risk of Burns- Hot objects can cause burns. It’s important to be cautious, especially with very hot items like cooking utensils or irons.
• Using Protection- When handling hot objects, use protective gear like oven mitts or tongs.
• Children’s Safety- Teach children to be cautious around hot objects and supervise them to prevent accidents.

Also Check – Class 7 science -Chapter 4 – Heat – Definition and Explanation of Important Keywords

Temperature and Thermometers

The concept of temperature is fundamental in understanding heat. This section elaborates on what temperature signifies, introduces thermometers as measurement tools, and explains the Celsius and Fahrenheit scales.

What is Temperature?

• Basic Definition- Temperature is a measure indicating how hot or cold an object is, directly related to the kinetic energy of the particles in the substance.
• Heat Energy Indicator- It reflects the amount of heat energy in an object. The higher the temperature, the more energetic the particles within the substance.

Thermometers- Precision in Measurement

• Role of Thermometers- They are devices specifically designed to measure temperature accurately and consistently.
• Types and Uses- There are various kinds of thermometers, such as digital, mercury, and alcohol, each suitable for different situations like medical assessments, scientific experiments, or everyday use.

Understanding Celsius and Fahrenheit Scales

• Celsius Scale – This scale, where water freezes at 0°C and boils at 100°C, is commonly used globally. It’s based on the properties of water and is crucial for scientific research, cooking, and weather reporting.
• Fahrenheit Scale- Primarily used in the United States, it sets the freezing point of water at 32°F and boiling point at 212°F. This scale is often used in daily weather forecasts and household appliances like ovens.
• Scale Conversion- Understanding how to convert temperatures from one scale to another .F = (9/5)C + 32″ for Fahrenheit and “C = (5/9)(F – 32)

Engaging in Temperature Activities

• Experiential Learning- Activities like measuring the temperature of various objects or environments help understand the abstract concept of temperature.
• Practical Demonstrations- Experiments that involve observing the effects of temperature changes on substances (e.g., melting ice, boiling water) provide tangible experiences with temperature concepts.

Clinical and Laboratory Thermometers

This section covers two specialised types of thermometers- clinical and laboratory thermometers. Each serves a different purpose, and understanding their usage and handling is crucial.

Clinical Thermometers

• Purpose- Clinical thermometers are specifically designed for measuring human body temperature. They are commonly used in medical settings and at home.
• Design- These thermometers are usually slender and have a constriction in the tube to hold the mercury or other temperature-sensitive liquid in place once removed from the body.
• Usage- To measure body temperature, the thermometer is placed under the tongue or in the armpit. After a few minutes, the temperature can be read from the scale.
• Reading- Careful reading is essential to get an accurate measurement. The temperature is usually measured in degrees Celsius or Fahrenheit.

Laboratory Thermometers

• Broader Range- Laboratory thermometers are designed to measure a wider range of temperatures compared to clinical thermometers.
• Applications- They are used for various scientific experiments in schools, colleges, and professional labs.
• Design and Usage- Unlike clinical thermometers, they don’t have a constriction in the tube and can measure both high and low temperatures. They are used by immersing or holding them in the substance whose temperature is to be measured.

Safe Handling and Reading

• Handling Precautions- Both types of thermometers should be handled with care to avoid breakage, as they can contain substances like mercury which are harmful.
• Reading Accuracy- Ensure the thermometer is clean and read at eye level to avoid parallax errors.
• After Use- It’s important to clean and disinfect clinical thermometers after use, especially when used by multiple people.

Also Check -Chapter 3 Heat Activities: Simple Experiments for Class 7

Heat Transfer

In this section, we delve into the concept of heat transfer, which is the process of heat moving from one object to another. Understanding how heat transfer works is crucial in both natural phenomena and everyday applications.

How Heat Moves

• Basic Principle- Heat always flows from a hotter object to a cooler one until both reach the same temperature.
• Transfer Methods- Heat can move in three primary ways- conduction, convection, and radiation. Each method has its unique characteristics and occurs in different circumstances.

The Concept of Heat Flow

• Energy Transfer- Heat flow is essentially the transfer of thermal energy between objects or within an object.
• Equilibrium- The ultimate goal of heat transfer is to reach thermal equilibrium, where no net heat flow occurs between objects in contact; they are at the same temperature.

Also Check – Heat facts for kids

Practical Implications

• Daily Life- We experience heat transfer daily, like when a spoon in a hot cup of tea becomes warm (conduction), or when we feel a warm breeze (convection).
• Technology and Industry- Heat transfer principles are critical in designing systems like heaters, refrigerators, and even buildings.
• Natural Processes- Understanding heat transfer helps explain natural phenomena like sea breezes, land breezes, and global weather patterns.

Also Check – NCERT Solutions For Class 7 Science Chapter 4 – Heat

Conduction is a key method of heat transfer, particularly in solids. This section explores how conduction works, the role of different materials in conducting heat, and how to experiment with this concept.

Heat Transfer in Solids

Heat Transfer in Solids through Conduction

• Direct Contact Mechanism- In solids, conduction occurs when heat is transferred through direct contact between particles. The particles themselves do not move significantly, but they vibrate and pass their kinetic energy to adjacent particles.
• Vibration of Particles- When a part of a solid object is heated, the particles in that area gain energy and start vibrating more vigorously. These vibrations are then transferred to neighbouring particles.
• Efficiency Due to Solid Structure- Solids are particularly good at conducting heat because their particles are closely packed. This close packing allows energy to be transferred more efficiently from particle to particle compared to liquids or gases.

Conductivity of Materials

• Varied Conductivity- Not all materials conduct heat with the same efficiency. This variation is primarily due to the differences in their molecular structure and bonding.
• Metals as Good Conductors- Metals like copper and aluminium are excellent conductors of heat. This high conductivity is due to the presence of free electrons. In metals, some electrons are not bound to any particular atom and can move freely throughout the material. When one part of a metal is heated, these free electrons gain energy and move rapidly, transferring the heat across the metal.
• Poor Conductors – Wood and Plastic- Materials like wood and plastic are poor conductors of heat. They lack free-moving electrons and their molecular structure does not allow for efficient transfer of vibrational energy.

Conductors and Insulators

• Conductors- Conductors are materials that allow heat to pass through them easily. Copper and aluminium are common examples used in cookware, electrical wiring, and heat exchangers because of their ability to transfer heat quickly.
• Insulators- Insulators are materials that resist the flow of heat. They are used to prevent the loss or gain of heat. Common insulators include wool, fibreglass, and even air. These materials are used in building insulation, protective clothing, and thermoses to maintain temperature stability.

Activities and Experiments

• Hands- On Learning- Simple experiments, such as comparing how quickly different materials heat up, can demonstrate conduction.
• Observing Heat Transfer- Touching different materials left in the sun (like metal, stone, and wood) shows how some conduct heat more effectively than others.

Also Check – NCERT Exemplar Solutions- Class 7 Science – Chapter 3 – Heat

Convection is another form of heat transfer, primarily occurring in liquids and gases. This section will explore the mechanics of convection, provide practical examples, and discuss its role in natural phenomena like sea and land breezes.

Heat Transfer in Liquids and Gases

• Fluid Movement- Unlike conduction, convection involves the movement of the fluid itself. When a part of a liquid or gas is heated, it becomes less dense and rises. Cooler, denser fluid then takes its place, creating a circular motion.
• Efficiency in Fluids- Convection is an efficient way to transfer heat in fluids because it involves the actual movement of mass, carrying heat energy with it.

Explanation of Convection

• Everyday Examples- A common example of convection is the heating of water in a pot. The water at the bottom heats up, becomes less dense, and rises. The cooler water then descends to take its place, heating up in turn.
• Heating Systems- In homes, convection is used to circulate warm air. Heaters warm the air, which then rises and spreads throughout the room. As it cools, it descends and is heated again, creating a cycle.

Role in Natural Phenomena

Sea breezes and land breezes.

• Convection on a Large Scale- Sea and land breezes are natural phenomena that occur due to convection, which involves the movement of air caused by temperature differences.

Daytime – Sea Breeze-

• Process- During the day, land surfaces heat up faster than water bodies. This heating causes the air above the land to warm up, become less dense, and rise.
• Air Movement- As the warm air over the land rises, a pressure difference is created. Cooler, denser air from over the sea moves in to replace the rising warm air.
• Result- This movement of air from the sea to the land creates a sea breeze, a cooling wind that can be felt near coastlines.

Nighttime – Land Breeze-

• Reverse Process- At night, the reverse occurs. Land surfaces lose heat more quickly than water. As a result, the air over the land becomes cooler and denser than the air over the sea.
• Air Movement- The cooler air over the land moves towards the sea, replacing the warmer air above the water that rises due to its lower density.
• Result- This creates a land breeze, a flow of air from the land towards the sea.

Global Weather Patterns

• Convection’s Impact- Convection is not just limited to local phenomena like sea and land breezes but also plays a crucial role in shaping global weather patterns.
• Air Movement in Atmosphere- The sun heats different parts of the Earth unevenly. This uneven heating causes variations in air temperature and density, leading to the movement of air masses.
• Formation of Wind- As warm air rises in one region, cooler air from surrounding areas moves in to replace it, creating wind patterns.
• Weather Changes- These movements of air masses, driven by convection, are fundamental in forming clouds, precipitations, and various weather conditions globally.

Also Check – Sea Breezes and Land Breezes – Class 7 Science explained in details

Also Check – Difference Between Sea Breeze And Land Breeze

Radiation stands out as a unique method of heat transfer, characterised by its ability to occur without any intervening medium. This section delves deeper into the mechanics of radiation and its various implications in everyday life.

Heat Transfer Without a Medium

• Fundamental Nature- Unlike conduction and convection, radiation does not require a material medium to transfer heat. This means radiation can occur in a vacuum, where there are no particles to facilitate the transfer of energy.
• Mechanism of Energy Transfer- Radiation involves the emission of electromagnetic waves, which carry energy from the emitting to the receiving body.

Also Check – Conduction, Convection, and Radiation- Class 7 Science Explained

Sun as a Prime Example

• Solar Radiation- The Sun is a powerful source of radiant energy. It emits a broad spectrum of electromagnetic waves, including visible light, ultraviolet light, and infrared radiation.
• Transmission through Space- These waves traverse the vacuum of space and reach the Earth, delivering energy that warms the planet’s surface and atmosphere.

Understanding How Radiation Works

• Emission of Electromagnetic Waves- Any object with temperature above absolute zero emits radiation. The amount and type of radiation depend on the object’s temperature. For instance, a very hot object may emit visible light (like the filament in a light bulb), while cooler objects emit mostly infrared radiation.
• Spectrum of Radiation- The electromagnetic spectrum encompasses a wide range of wavelengths, from short gamma rays to long radio waves. The type of radiation an object emits falls somewhere within this spectrum.

Absorption and Emission

• Interaction with Surfaces- When these electromagnetic waves encounter a surface, they can be absorbed, reflected, or transmitted.
• Impact of Surface Properties- Dark and rough surfaces tend to absorb more radiation and thus heat up more than light or smooth surfaces. This is why wearing a black shirt on a sunny day feels hotter than wearing a white one.
• Balancing Act- Objects not only absorb radiation but can also emit it. The balance between absorbed and emitted radiation determines the object’s temperature change.

Everyday Implications

• Feeling the Sun’s Heat- We feel the heat from the Sun due to radiation. Even on a cold day, standing in direct sunlight can feel warm because of the radiated heat.
• Home Heating and Cooking- Radiation is used in household appliances like toasters and infrared heaters. These devices emit infrared radiation to cook food or heat a room.
• Technology and Industry- Radiation principles are applied in various technologies, such as solar panels, which convert solar radiation into electricity.

Energy-Efficient Building Design

This section explores how energy-efficient building design, particularly through the use of insulation and specific building materials like hollow bricks, can significantly enhance a building’s energy efficiency.

Role of Trapped Layers of Air in Insulation

• Insulation Basics- Insulation in buildings is crucial for maintaining a comfortable indoor temperature and reducing energy consumption.
• Trapped Air as an Insulator- One of the key principles of insulation is the use of trapped air layers. Air, when not allowed to circulate freely, is an excellent insulator. This is because air molecules, when stationary, minimise the transfer of heat by conduction.
• Application in Buildings- Insulation materials often contain numerous small pockets of trapped air. These pockets help prevent the flow of heat into or out of a building, keeping it warmer in winter and cooler in summer.

Using Hollow Bricks for Energy Efficiency

• Hollow Bricks Design- Hollow bricks, with their internal cavities, naturally trap air within these spaces.
• Enhanced Insulation Properties- These air pockets in hollow bricks reduce the rate of heat transfer through the walls. As a result, they help maintain a stable temperature inside the building regardless of external weather conditions.
• Additional Benefits- Besides thermal insulation, hollow bricks also provide sound insulation and reduce the overall weight of the structure, contributing to its stability and efficiency.

Clothing and Temperature

In this section, we delve into the vital role clothing plays in temperature regulation and how the colour of clothing can impact heat absorption.

Choosing Appropriate Clothing for Different Temperatures

• Temperature Regulation- Clothing acts as a personal insulation system, helping to maintain our body’s temperature balance. The type of clothing we choose depends on the environmental temperature.
• Layering in Cold Weather- In colder temperatures, wearing multiple layers helps trap body heat. Materials like wool and fleece, known for their insulation properties, are commonly used.
• Light Clothing in Warm Weather- Conversely, in warmer temperatures, light and breathable fabrics like cotton are preferred. These materials allow for air circulation and help in dissipating body heat.

Experiments on Color and Heat Absorption

• Colour Impact- The colour of clothing can significantly influence how much heat it absorbs or reflects. Dark colours tend to absorb more heat, while light colours reflect it.
• Simple Experiment- A practical experiment to understand this concept involves placing pieces of cloth of different colours under the sun. After some time, by touching them, one can feel that darker colours are warmer than lighter ones.
• Applying the Concept- This knowledge is applied in choosing clothing colours based on the weather – darker colours for colder days to absorb more heat and lighter colours for hot days to keep cool.

Keeping Warm with Wool

In this section, we’ll explore the unique insulating properties of wool and how the concept of trapped air in fabrics contributes to its effectiveness in keeping us warm.

Insulating Properties of Wool

• Natural Insulator- Wool is a natural fibre known for its excellent insulation. This is due to its unique structure and composition.
• Fibre Structure- The fibres in wool are crimped, meaning they have a natural wave or bend. This structure creates tiny pockets of air throughout the woollen fabric.
• Heat Retention- These air pockets trap body heat, preventing it from escaping into the colder external environment. This makes wool exceptionally good at keeping us warm.

Trapped Air in Fabrics

• Air as an Insulator- Air, when trapped and still, is a poor conductor of heat. It’s this property that makes trapped air an effective insulator.
• Wool’s Advantage- The crimped nature of wool fibres maximises the amount of air trapped within the fabric. More trapped air means better insulation.
• Versatility in Weather- Wool can regulate temperature due to its ability to trap air. It keeps us warm in cold conditions by retaining heat and can also be comfortable in warmer conditions due to its breathability and moisture-wicking properties.

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CD match, raise, or 9% APY! Promos heat up before Fed rate cut. Hurry to get the best rate

The fight to keep your money is heating up with a couple of credit unions now offering to match or beat the return you’re getting at your current financial institution, according to findings from rate watcher CD Valet .

For a limited time, RiverLand Federal Credit Union in New Orleans, will top any 6- to 36-month certificate of deposit (CD) rate by 0.05% annual percentage yield (APY) when the new CD is opened. Consumers Federal Credit Union in Brooklyn, New York will match the 5-year CD rate of any U.S. financial institution.

The offerings can be a win-win for both credit unions and savers with the Federal Reserve expected to begin cutting rates in September .

“Consumers are particularly attuned to interest rates and how any changes will impact the returns they can earn on their savings,” and credit unions are “eager to hold on to their existing deposits and bring in new deposits with these eye-catching promotional offerings,” said Mary Grace Roske, CD Valet’s senior vice president of marketing communications.

What are details of these CD offerings?

RiverLand Federal Credit Union offers a Match & Raise Certificate .  If members move funds from another financial institution, Riverland will top the best certificate of deposit (CD) rate they can find by 0.05% annual percentage yield (APY) when they open a new CD with RiverLand.

CD terms are limited to a minimum of six and a maximum of 36 months. Proof of the advertised rate, including terms and offer date, is required as well as a minimum $1,000 investment.

Consumers Federal Credit Union in Brooklyn will match the 5-year CD rate of any U.S. financial institution if you open a new one at Consumers. Only members can receive the offer and the minimum investment is $500 with no maximum, said Stephen Jacoby, the credit union's chief executive.

"If they don’t have rate proof, we will make every best effort to look it up for them and make it easy for them, too," he said.

Now, that's long-term investing: A 100-year CD puts a new spin on long-term investing. Is it a good idea?

What are the highest CD rates available?

One-year CDs are the most popular and are offering high rates, according to CD Valet. It found 600 rates nationwide of 5.00% APY and above with 1-year terms and from April through June, 65% of online searches were for 1-year CDs.

Shoppers can earn even more with rates of 6.00% APY and higher if they qualify for membership at select credit unions, it said. CD Valet has seen twice as many rates at 6.00% and higher APY in April through June than it did in the first three months of the year.

Express Credit Union in Washington state tops the list with its 90 th Anniversary Special . Through December 9 th , it will pay 9% APY on a 1-year CD if you open the CD on the 9 th of the month, on balances up to $5,000. You must be a member to receive this special, and it’s one CD per member, it said.

“That means you could earn $450 in 12 months just by opening the CD at 9%,” it said on its website.

Are buying CDs now a good investment?

CDs: Best credit union cd rates

“Now is the perfect time,” said Ronnie Thompson, investment adviser representative and owner of True North Advisors, a financial advising firm in Northville, Michigan.

With Federal Reserve Chairman Jerome Powell suggesting Wednesday the central bank could begin cutting its benchmark short-term federal funds rate in September , Thompson said financial institutions will immediately start slashing the interest rates they pay on deposits.

“Banks will be waiting with bated breath to cut rates because rates have been high for such a long time,” Thompson said. “So, it’s reasonable for people to look into locking in these higher rates.”

Lower rates would encourage businesses and consumers to borrow , which means more lending by financial institutions.

Many people may also have CDs they’ve bought over the last year or two that are coming due and need somewhere to park their money, Thompson said.

What CD terms are best to buy?

When you buy a CD, remember that money will be locked in for the duration of the CD unless you pay a penalty to withdraw your money early.

So, if you have enough money, Thompson suggests creating a CD ladder , which is splitting your money and buying a variety of terms.

“Maybe buy a 1, 3 and 5-year, which allows some money to become (available) during that period,” he said. “They all have different rates so you take the aggregate of those rates and collectively, that’s a bigger rate over the 5-year period.”

Medora Lee is a money, markets, and personal finance reporter at USA TODAY. You can reach her at [email protected] and  subscribe to our free Daily Money newsletter  for personal finance tips and business news every Monday through Friday.