Science 7

Lesson 19: Transferring Energy

It’s a chilly Alberta night to be outside riding snowmobiles. It has been fun, but now you are going back to the roaring campfire! You are sitting in front of it—warm on one side and cold on the other. Why aren’t you warm all the way around your body?

Well, you have already dealt with the transfer of thermal energy! You know that if you were to touch one of the glowing embers, the sudden transfer of a large amount of heat would burn you. You know that when roasting marshmallows, holding them above the embers browns them much faster than holding them off to the side. You know that to warm your back, you are going to have to turn around. And, the closer you get to the fire, the faster you will warm up. But still, a question remains: why is it warm on one side and cold on the other?

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In this lesson you will look at thermal energy transfer. The particle model of matter will be used for this. You will also use a new model—a model that does not even have particles.

Three Methods of Energy Transfer

Heat has been talked about a lot. It is finally time to define the term heat. Heat is, simply, energy that moves from a warmer substance to a cooler substance. Heat can transfer in three different ways:

(1) conduction (by touch)
(2) convection (carried upward by particles)
(3) radiation (as an energy wave)

You know from experience that thermal energy always moves from a hotter substance to a cooler substance. Perhaps the following examples will be somewhat familiar to you.

Example 1

You decide to run a hot bath for yourself. You need to relax after a particularly long day. You set the tap at the right position and add some bubble bath. A few minutes later, you return to ease yourself into the tub. The temperature of the bath is 43°C. Your body temperature is 37°C. Since the water is warmer than your body, thermal energy transfers from the bath water to you. Now you can start to relax.

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Example 2

You decide to run a hot bath for yourself. You need to relax after a particularly long day. You set the tap at the right position and add some bubble bath. A few minutes later, you return to ease yourself into the tub. Unfortunately, your little brother just had a shower and two full loads of laundry were just done. Needless to say, the hot water tank was drained. The temperature of the bath is 28°C. Your body temperature is 37°C. Since your body is warmer than the water, thermal energy transfers from your body to the bath water. You feel frozen. A moment later you jump out of the tub, shivering and looking for a towel.

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Write definitions for the terms heat, conduction, convection, and radiation.

Predict the primary (main) form of heat transfer that takes place in the following.
1. roasting marshmallows over a fire
2. sitting a few metres from the fire and only the front of your body is warmed
3. climbing into a warm (or cold) tub of bath water

Radiation

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What do light bulbs, toasters, and electric stoves have in common? Actually, several things! They are all appliances; they all convert (change) electrical energy into thermal energy; and they all produce light energy. In each case, the light energy is released as radiant energy or electromagnetic radiation (EMR).

Turn to page 226 of the textbook. Read the first paragraph of “Topic 6: Transferring Energy.” Then read “Radiation Transfers Energy.” These readings introduce this method of energy transfer.

Define radiant energy (electromagnetic radiation).

List three characteristics of all forms of radiant energy.

Does radiant energy transfer require particles to hold/carry the energy?

In the sunshine, surfaces can become very hot. Have you ever touched the surface of a black car after it has been sitting in the sunshine for a few hours? Have you ever wondered why some windshield sunblocks—along with some tents and outdoor equipment—are silver in colour?

How do different surfaces affect radiant energy transfer? You will find out in the next activity.

Find Out

Absorb That Energy

Read through the entire activity on page 227 of the textbook.

You may find it easier to create paper and aluminum foil envelopes. Just slip the thermometer into them one at a time. Shine the bulb down on each envelope for the needed time. Remember to control your variables as closely as possible. Try to keep the texture, thickness, and area of paper the same. You should only be changing the colour or shininess.

You can change the procedure a bit so that it will work for you. You should create your data table as your first step. You can model it after the one that follows. Note: The bulb gives off both radiant light and heat. Light energy absorbed by your surfaces will be converted (changed) into heat energy. The thermometer will indicate the result of absorbing both types of energy.

Record your measurements in your data table. Draw a line graph to illustrate your results.

Answer questions 1, 3, and 4 of “What Did You Find Out?” Modify them as necessary to suit your situation.

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Investigation 3-G: Comparing Surfaces

Read the entire investigation on page 228 of the textbook. Then do the activity.

Complete the investigation by answering the following.
1. questions 1 and 2 of “Procedure”
2. question 3 of “Analyze”

Conduction

Mr. Casey: Appliances again! What do these three appliances have in common? Austin: They convert electrical energy into thermal energy, too. Mr. Casey: Yes. These appliances transfer heat by conduction to their target—a dress shirt, coffee pot, and popcorn kernels. Conduction is a method of heat transfer that requires contact. The particles of the substances in contact do bump into each other to transfer energy.

Turn to page 229 of the textbook. Read “Conducting Energy Through Solids.” Study the figures carefully to get a visual image of the process of conduction.

Define the term heat insulator.

Explain why thermal conduction is like a chain reaction.

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Find Out

Activity: The Super Stirrer

Now it’s time to test your knowledge of conductors and insulators. Read the activity on page 229 of the textbook.

Make sure you work under the direct supervision of your home instructor. The hot water needs to be close to boiling for this activity to be successful. Be careful not to burn yourself!

Perform the entire activity. Then answer questions 1 and 2 of “What Did You Find Out?”

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Convection

Heat from a stove moves through a room by convection currents. Convective heat transfer can only occur in fluids. (Liquids and gases are both fluids.) The particles in fluids are free to move individually, carrying heat energy with them.

Turn to page 230 of the textbook. Read “Convection, Energy on the Move.” Carefully study Figure 3.25. It shows the parts and sequence of a convection current. You should note that warmer air does not rise by itself. It is actually pushed up by the colder, denser air surrounding it.

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Write definitions for the following terms:
• convection current
• density
• fluid

Do solids transfer heat by convection? Why?

Going Further

Read “Find Out Activity: Displaced Drops” on page 231 of the textbook.

Perform the entire activity. A clear, colourless glass or jar can be used instead of a beaker. Handle the hot water with care under the direct supervision of your home instructor.

Once you have completed the activity, answer questions 1 to 4 of “What Did You Find Out?”

For more information on convection, view the segment “How Convection Works” on the Science 7 Multimedia CD.

Find Out

Activity: Displaced Thermal Energy

Get some personal experience with heat transfer. Under the direct supervision of your teacher or home instructor, light a candle; then do the following:

Situation 1: Hold a wire or a piece of metal cutlery in the flame. Keep it there until you feel the thermal energy from the flame being transferred to your hand. Do not hold the object long enough to burn yourself.

Situation 2: Slowly lower your hand above the flame. Do not get low enough or hold your hand over the flame long enough to burn yourself.

Situation 3: Hold your hands on either side of the flame. Bring them close enough to experience heat transfer. Again, do not burn yourself.

Identify the primary method of heat transfer in each of the three situations.

How did thermal energy reach your hand in the first situation?

What method(s) of heat transfer did you experience when you held your hand above the flame?

Which method is the primary method of heat transfer (transferred the most heat) in a fluid? (Remember, gases and liquids are both fluids.)

Controlling Heat Transfer

Fire! Beautiful or terrifying? Useful or destructive? Controlled or out of control? How do you view fire? Conduction, convection, and radiation are the three ways heat transfers from one substance or location to another. You and everyone else rely on these three methods to transfer energy from the Sun to the Earth, to circulate heat around the atmosphere and your home, and to cook your food. Heat transfer is very much a part of your everyday life. However, thermal energy can be very dangerous. The transfer of heat energy within buildings and appliances must be carefully controlled. You must be able to turn the heat source on and off and up and down. You have to keep thermal energy from leaking out from where you want it—say, from under your blankets. Effective and efficient energy transfer systems are vital in the modern world.

Turn to pages 231 to 233 of the textbook. Learn more about these vital systems by reading “Analyzing Energy Transfer Systems” and “Features of Energy Transfer Systems.” Be sure to study the figures carefully.

Is heat a substance or something else?

There are five features that are common to energy transfer systems:

(1) Energy Source: This is the part of the system that supplies the energy.

(2) Direction of Energy Transfer: Heat energy always moves from the area or substance with the higher temperature to areas or substances with lower temperatures.

(3) Energy Transformations (Conversions): Energy is often changed from one form to another as it moves through the chain of events involved in an energy transfer system.

(4) Waste Heat: Any thermal energy that does not reach the target is transferred into the surroundings.

(5) Control Systems: These are devices or means to control the original heat source.

The following photograph is an example of a very simple heat transfer. Identify the following components: energy source, direction of energy transfer, an energy transformation, wasted heat, and a control system.

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Going Further

Apply and expand your knowledge of energy transfer by building your own heat-transfer system. Get together with your teacher or home instructor and try “Investigation 3-H: Making a Transfer” on page 234 of the textbook.

Preventing Heat Transfer

Living in Canada, you are probably very familiar with insulation. Things like wool mitts, a warm jacket, and the itchy, pink stuff they put in the walls of houses all prevent heat transfer. For the most part, people are concerned about preventing heat from transferring out of a system. For example, the heat that is lost from your body or house on a cold day. However, heat transfer into a system can also be unwanted. For example, the less heat that transfers into your home on a hot day, the more comfortable you will be.

Insulation can be used to prevent all three types of heat transfer in one or both directions. Read “Unit 3: Ask an Expert” on pages 258 and 259 of the textbook. Then turn to page 235 of the textbook. Read “Find Out Activity: Keeping in the Warmth” up to the Procedure.

How is the heat energy in your home lost to the outside air?

What does the RSI value indicate?

Which of the listed materials is the best insulator? the best conductor?

What is the RSI value of 10 cm of fibreglass?

Investigation 3-H: The Big Meltdown

Challenge—Protecting an Ice Cube

Ice cubes melt quite quickly on hot, summer days. Can you make one last five times longer than usual? Design and build an insulated structure that will protect an ice cube from melting.

Materials

• two or more ice cubes as close to the same size as possible
• a variety of structural and insulating materials

Specifications

1. Your goal is to design and build a structure that will greatly increase the time it takes to melt an ice cube.
2. You can only use everyday materials you can find around the house.
3. You can build and test as many insulating structures as you like.
4. If possible, have your friends, family, teacher, or home instructor take part in the activity. Brainstorm ideas about how to build the structure, like
   • materials that make good insulators
   • the thickness of the insulating material
   • heat transfer methods—radiation, convection, and conduction
5. Sketch a labelled design proposal for each structure.
6. Plan a table to store your data in.
7. If your structure doesn’t give good enough results, suggest changes to the design. Modify it accordingly, and test again.

Evaluate

There is a lot of information in this module that can help you. Create a point-form summary of the ideas you used to design your structures.

How could you or did you improve your structure to repeat this experiment?

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Turn to page 236 of the textbook. Answer questions 1 to 4 of “Topic 6 Review.”

Looking Back

After completing this lesson, you will know that it is mainly radiant energy that keeps you warm when you sit beside a campfire on a cool evening. Radiant energy travels as an energy wave. It travels in a straight line and is absorbed at the surface of your body. It does not reach the far surface of your body—usually your back. That’s why you feel warm on one side and cold on the other.

You applied the particle model to the concept of heat transfer. You learned that there are three methods of heat transfer—conduction, convection, and radiation. You also learned that every heat transfer system shares five common features. In the end, you applied your knowledge. You designed and built structures meant to prevent energy transfer.

Suggested Answers

Lesson 19

Heat is thermal energy transferred from a warmer substance to a cooler substance.

Conduction is a method of heat transfer from particle to particle by touch.

Convection is a method of heat transfer in which lighter, warmer, less-dense fluids (gas, liquid) are pushed upward by heavier, colder, denser fluids.

Radiation is a method of heat transfer from particle to particle using electromagnetic waves.

1. convection
2. radiation
3. conduction

Radiant energy (electromagnetic radiation) is energy that transfers in the form of a wave.

All radiant energy behaves like waves, can be absorbed or reflected, and travels at very high speeds through space (a vacuum or the spaces between particles). It is interesting to note that radiant light energy from the Sun takes about eight minutes to reach Earth! A fourth characteristic—radiant energy travels in straight lines—explains why your back is cold when you sit facing a raging fire. The infrared radiation cannot curve around to warm your back.

Radiation is the only method of energy transfer that does not require the assistance of particles.

Check your responses with your teacher or home instructor.

1. 
2. 1. The best reflector is one that is light-coloured with a shiny reflector.
   2. The worst reflector is one that is dark-coloured with a dull surface.
   3. The best absorber is one that is dark-coloured with a dull surface.
   4. The worst radiator is one that is light-coloured with a shiny surface.

3. You could choose surfaces to absorb or reflect radiant energy to modify the temperature of your house or heat water.

A heat insulator is a material that conducts heat poorly.

Thermal conduction is like a chain reaction in that one particle touches and passes energy to another particle, which touches and passes energy to another particle, and so on.

Textbook questions 1 and 2 of “What Did You Find Out?,” p. 229
1. 1. wood or plastic
      These substances are poor conductors. Heat will not be transferred to your fingers.
   2. copper or other metal
      The bottom should be a good conductor. This way the heat of the stove is transferred to the food in the can.
   3. wood or plastic
      These are poor conductors. Heat will not be transferred to your hands.
   4. wood (In real life, cardboard is used because it is lighter, cheaper, and still insulates pizza.)
      The insulating qualities of a container help keep the pizza hot.
2. The particles in an insulator could be spaced further apart than the particles in a conductor. They have higher specific heat capacities (must absorb more heat to increase 1°C in temperature). So, they take longer to warm up and then transfer energy. The greater the difference in temperature, the faster heat energy transfers.

A convection current is a roughly circular pattern that results from the uneven heating or cooling of a fluid.

Density refers to how closely packed or thick a substance is. It is a measure of the amount of matter (number of particles) packed into a particular volume.

A fluid is a liquid or a gas.

Solids cannot transfer heat by convection. The particles are not free to move individually. They are fixed in place.

In Situation 1, conduction was the primary method of heat transfer. In Situation 2, the primary method was convection. In Situation 3, it was radiation.

Particles in the metal absorbed energy from the flame. They moved faster, and in the tightly packed solid, they collided with the next particle(s). During collisions, the faster moving particles transferred energy to the cooler (lower kinetic energy) particle(s). The newly energized particles then collided with their neighbouring particles and transferred energy to them. This continued in a series of chain reactions until it spread throughout the metal from the heat source to your hand.

Actually, you experienced all three. Radiant energy transfers equally in all directions, so some of the heat energy moved directly from the flame to your hand as radiation. Most of the heat from the flame is carried upward from the flame by particles (convective heat transfer). Your skin absorbs the thermal energy from these particles by conduction when the hot particles touch your skin.

Convection is the primary method of heat transfer in a fluid. You quickly notice the heat above the flame. Heat is reaching your hand by both convection and radiation. Radiant heat transfer is equal in all directions. Convective heat transfer is only upward. At the side of the candle, you only experience radiation. Did you notice how much closer you can get to the side of the flame and how much longer you can hold your hands there? Much more heat is transferred by convection than by radiation.

Heat is not a substance. It is a form of energy.

The energy source is the fire.

The direction of energy transfer is from the fire to the surroundings.

Energy transformations include the following:

• chemical energy of the burning wood to heat and light energy
• heat and light energy to kinetic energy of air particles, the particles on the surface of the boy’s skin, and the food

Heat is wasted to the surrounding air particles if you and your food are the target.

The control system is as follows:

• amount of wood on the fire
• water to lower the flame or douse the fire
• amount of oxygen the fire receives (Have you ever lifted a log in the fire to allow more oxygen at the coals?)
• how close you and your food get to the fire

Heat is lost through the chimney, walls, around doors and windows, as well as through the window glass.

RSI is an indicator of a material’s resistance to heat conduction. It is based on using a 1-cm thick piece.

Blue plastic foam panels are the best insulators (highest RSI/resistance). Glass is the best conductor (lowest RSI/resistance).

10 cm × 0.24 RSI/cm = 2.4

The RSI is 2.4.

Answers will vary. The following ideas may be included:
• Use materials that are better insulators. For example, use a down-filled comforter as an additional layer of insulation.
• Design a structure to provide for less heat transfer by radiation. For example, place aluminum foil just under the final layer of insulation. The foil should prevent radiant heat from reaching the ice cube.

Check your responses with your teacher or home instructor.

Lesson Glossary