2.6 Heat Transfers in Three WaysPS3-3 The students will be able to apply scientific principles to design, construct, and test a device that either minimizes or maximizes thermal energy transfer.

Why do the cookies on the cookie sheets get hot in an oven?

What Is Conduction?Conduction is the transfer of thermal energy between particles of matter that are touching. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Conduction is one of three ways that thermal energy can be transferred (the other ways are convection and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature. In the opening photo above, conduction occurs between particles of metal in the cookie sheet and anything cooler that comes into contact with it—hopefully, not someone’s bare hands!

Pass It OnTo understand how conduction works, you need to think about the tiny particles that make up matter. The particles of all matter are in constant random motion, but the particles of warmer matter have more energy and move more quickly than the particles of cooler matter. When particles of warmer matter collide with particles of cooler matter, they transfer some of their thermal energy to the cooler particles. From particle to particle, like dominoes falling, thermal energy moves through matter. Click on the animation “Conduction”:

http://www.hk-phy.org/contextual/heat/hea/condu/conduction_e.html

Examples of ConductionThe cookie sheet in the opening image transfers thermal energy to the cookies and helps them bake. There are many other common examples of conduction. The figure below shows a few situations in which thermal energy is transferred in this way.

Hot Iron: A hot iron removes the wrinkles in a shirt. Hot Cocoa: Holding a cup of hot cocoa feels good when you have cold hands. Camp Stove: This camp stove can be used to cook food in a small pot. Snow: Ouch! Can you imagine how cold this snow must feel on bare feet?

Q: How is thermal energy transferred in each of the situations pictured in the figure above ?

A: Thermal energy is transferred by conduction from the hot iron to the shirt, from the hot cup to the hand holding it, from the flame of the camp stove to the bottom of the pot as well as from the bottom of the pot to the food inside, and from the feet to the snow. The shirt, hand, pot, food, and snow become warmer because of the transferred energy. Because the feet lose thermal energy, they feel colder.

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Watch the video about conduction at the following URL. Then write a paragraph explaining how conduction is related to state of matter.

http://www.youtube.com/watch?v=xAcZ9-zNYlY

Conduction Assessment1) What is conduction? (DOK1)2) How does conduction occur? (DOK3)3) Give three examples of conduction. (DOK1)4) A cold metal spoon is placed in a hot bowl of soup. What will happen to the

temperature of the soup and the spoon? Why? (DOK3)5) Which would bake faster - brownies baked in an aluminum pan or brownies

baked in a glass pan of the same size and mass. Explain your answer. (DOK3)

Convection

How does all the water in the pan get hot?

Heat Can Transfer by ConvectionConvection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid). Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Convection is one of three ways that thermal energy can be transferred (the other ways are conduction and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature.

How Does Convection Occur?The figure below shows how convection occurs, using hot water in a pot as an example. When particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal energy, they move more quickly, have more collisions, and spread farther apart. This decreases the density of the particles, so they rise up through the fluid. As they rise, they transfer their thermal energy to other particles of the fluid and cool off in the process. With less energy, the particles move more slowly, have fewer collisions, and move closer together. This increases their density, so they sink back down through the fluid. When they reach the bottom of the fluid, the cycle repeats. The result is a loop of moving particles called a convection current . You can learn more about convection currents by watching the cartoon video called “Convection” at this URL: http://www.sciencehelpdesk.com/unit/science2/3

Examples of ConvectionConvection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earth’s surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds. You can see one way this happens in the figure below . The land heats up and cools off faster than the water because it has lower specific heat. Therefore, the land gets warmer during the day and cooler at night than the water does. During the day, warm air rises above the land and cool air from the water moves in to take its place. During the night, the opposite happens. Warm air rises above the water and cool air from the land moves out to take its place.

 

Q: During the day, in which direction is thermal energy of the air transferred? In which direction is it transferred during the night?

A: During the day, thermal energy is transferred from the air over the land to the air over the water. During the night, thermal energy is transferred in the opposite direction.

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Watch the video at the following URL, and then answer the questions below.http://video.google.com/videoplay?docid=1902141755519014330#docid=73796470044669448571) Describe what you observed in the video.2) Explain your observations.3) Predict what you would observe if the procedure in the video continued.

Convection Assessment

1) What is convection? (DOK1)2) Describe how convection occurs and why convection currents form. (DOK2)

3) Add arrows representing convection currents to the room in the figure shown at the right to show how thermal energy moves from the radiator to the rest of the room. Label areas of the room that are warm and cool. (DOK3)

Radiation

How is warmth from the fire transferred to his hands?

Thermal Radiation is Another Way to Transfer Heat

The bonfire from the opening image has a lot of thermal energy. Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Thermal energy from the bonfire is transferred to the hands by thermal radiation. Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space, as shown in the figure below. When the waves of thermal energy reach objects, they transfer the energy to the objects, causing them to warm up. This is how the fire warms the hands of someone sitting near the bonfire. This is also how the sun’s energy reaches Earth and heats its surface. Without the energy radiated from the sun, Earth would be too cold to support life as we know it.

Thermal radiation is one of three ways that thermal energy can be transferred. The other two ways are conduction and convection, both of which need matter to transfer energy. Radiation is the only way of transferring thermal energy that doesn’t require matter. To learn more about thermal radiation, watch “Radiation” at the URL: http://www.sciencehelpdesk.com/unit/science2/3

Sources of Thermal RadiationYou might be surprised to learn that everything radiates thermal energy, not just really hot things such as the sun or a fire. For example, when it’s cold outside, a heated home radiates some of its thermal energy into the outdoor environment. A home that is poorly insulated radiates more energy than a home that is well insulated. Special cameras can be used to detect radiated heat. In the figure below, you can see an image created by one of these cameras. The areas that are yellow are the areas where the greatest amount of thermal energy is radiating from the home. Even people radiate thermal energy. In fact, when a room is full of people, it may feel noticeably warmer because of all the thermal energy the people radiate!

Q: Where is thermal radiation radiating from the home in the picture?

A: The greatest amount of thermal energy is radiating from the window on the upper left. A lot of thermal energy is also radiating from the edges of the windows and door.

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Watch the video “Thermal Radiation” at the following URL. Then answer the questions below. http://www.teachertube.com/video/thermal-radiation-1863881) What does a night vision camera detect?2) The kind of light an object gives off depends on its __________.3) Thermal radiation occurs when an object gives off __________ light.4) Explain why some objects appear brighter than others when viewed through a

night vision camera.5) In the video, the scientist places her hands inside a black plastic trash bag.

You can’t see her hands with your eyes but you can see them with a night vision camera. Explain why.

Radiation Assessment1) What is thermal radiation? (DOK1)2) If you sit close to a campfire, the fire warms you. Describe how thermal energy

is transferred from the fire to you. (DOK2)3) Explain how radiation differs from conduction and convection. (DOK3)

Conductors and Insulators

Why do the wires turn red inside of a toaster, but the outside of it does not?

Thermal ConductorsConduction is the transfer of thermal energy between particles of matter that are touching. Thermal conduction occurs when particles of warmer matter bump into particles of cooler matter and transfer some of their thermal energy to the cooler particles. Conduction is usually faster in certain solids and liquids than in gases. Materials that are good conductors of thermal energy are called thermal conductors. Metals are especially good thermal conductors because they have freely moving electrons that can transfer thermal energy quickly and easily.

Besides the heating element inside a toaster, another example of a thermal conductor is a metal radiator, like the one in the figure below. When hot water flows through the coils of the radiator, the metal quickly heats up by conduction

and then radiates thermal energy into the surrounding air.

Q: Thermal conductors have many uses, but sometimes it’s important to prevent the transfer of thermal energy. Can you think of an example?

A: One example is staying warm on a cold day. You will stay warmer if you can prevent the transfer of

your own thermal energy to the outside air.

Thermal InsulatorsOne way to retain your own thermal energy on a cold day is to wear clothes that trap air. That’s because air, like other gases, is a poor conductor of thermal energy. The particles of gases are relatively far apart, so they don’t bump into each other or into other things as often as the more closely spaced particles of liquids or solids. Therefore, particles of gases have fewer opportunities to transfer thermal energy. Materials that are poor thermal conductors are called thermal insulators . Down-filled snowsuits, like those in the figure below, are good thermal insulators because their feather filling traps a lot of air.

Another example of a thermal insulator is pictured in the figure below . The picture shows fluffy pink insulation inside the attic of a home. Like the down filling in a snowsuit, the insulation traps a lot of air. The insulation helps to prevent the transfer of thermal energy into the house on hot days and out of the house on cold days. Other materials that are thermal insulators include plastic and wood. That’s why pot handles and cooking utensils are often made of these materials. Notice that the outside of the toaster pictured in the opening image is made of plastic. The plastic casing helps prevent the transfer of thermal energy from the heating element inside to the outer surface of the toaster where it could cause burns. You can learn more about thermal insulators at this URL: http://www.school-for-champions.com/science/thermal_insulation.htm .

Home insulation looks a little like cotton candy. Like cotton candy, it is light and fluffy and full of tiny holes that trap air.

Q: Thermal insulators have many practical uses besides the uses mentioned above. Can you think of others?

A: Thermal insulators are often used to keep food or drinks hot or cold. For example, Styrofoam® coolers and thermos containers are used for these purposes.

With the interactive activity at the following URL, you can test different materials to see how well they prevent the transfer of thermal energy. http://www.bbc.co.uk/schools/scienceclips/ages/8_9/keeping_warm_fs.shtml

Explore MoreComplete the worksheet at the following URL: http://www.bp.com/liveassets/bp_internet/bpes_new/bpes_new_uk/STAGING/protected_assets/flash/ysi_full/downloads/KeepingCosy_Worksheet1.pdf

Conduction and Insulators Assessment

1) What is a thermal conductor? Give an example. (DOK1)2) Why do metals often feel cool to the touch? (DOK2)3) Define thermal insulator. Describe a scenario in which thermal insulators are

needed. (DOK3)

S.T.E.M. Challenge

BUILDING A BETTER COFFEE CUP CHALLENGEYou are tasked with the challenge of engineering a more effective insulated coffee cup. You will work in teams to design a cup that will hold heat longer than the cup I use. Your group will then develop a plan to market your new device to the teacher as the best cup to keep the coffee warm. Your group will present its sales pitch to the teacher (think of the show “Shark Tank”). Only proof of insulating ability and a creative commercial will win the “Shark” over. The best insulating data and best sales pitch combine to make the winner! Who will win the competition?