Heat moves from one place to another by means of conduction, convection, or radiation.  The following experiments will help us understand how each of these work.

Heat moves through solids mainly by conduction.  When a solid is heated, heat moves away from the source of the heat through the solid.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: An old metal butter knife; candle or alcohol lamp with safety pan.

Procedure: Light your heat source, and hold one end of the knife in the flame.  Hold the knife just until it begins to feel warm. Remove it from the flame as soon as it does so that you don’t get burned.
 
What Happened: The heat moved from the end of the knife in the flame to the end in your hand by means of conduction. 

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Observing Conduction in Another Solid

 

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: A metal rod such as a piece of coat hanger wire; sandpaper; candle with safety holder; pliers or tongs.

Procedure: Have an adult to cut a piece of straight wire from the bottom of a coat hangar.  Sand all the paint off the wire.  (You’re going to heat this wire, and this will prevent fumes from burning paint!) Light your candle.  Allow it to drip a single drop of wax about of every 3 cm (1 in) or so along the metal rod as shown.  Let the wax harden.

Place your candle in the safety pan and hold one end of the rod with pliers or tongs.  Place the other end of the rod in the flame.  Observe the drops of wax along the length of the rod.

What Happened: As the heat moved along the rod, you could see the drops of wax begin to melt.  The melting began closest to the flame and moved outward.  This shows how heat moves through the rod away from the heat source by conduction.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: A piece of solid copper wire (See Procedure); candle with safety pan.

Procedure:   Find a piece of solid copper wire the same length as the steel rod from the last experiment, and as nearly the same thickness as possible.  Use the candle to drip wax along the copper wire just as you did on the steel rod in the last experiment.

Place the candle in the safety pan and hold one end of the wire in the flame.  Notice how long it takes for the heat to melt the drops of wax along the length of the wire.

What Happened: The wax melted faster in the copper than in the steel rod.  Heat moves through different substances at different rates. Copper is a better conductor of heat than steel, and the better a substance conducts heat, the faster heat moves through it. 

In the butter knife, the clothes hangar rod, and the copper wire, heat moved away from the heat source because the heat caused the molecules near the heat source to vibrate faster.  As the molecules vibrated faster, they began to bump into the molecules next to them.  Like a line of falling dominoes, the molecules passed the heat energy along the entire length of the rod. This is how conduction works. You’ll learn more about how heat is related to the moving molecules a little later. 

We have now seen how heat travels by conduction through a solid.  However, there are some substances that don’t conduct heat very well, and these substances are called “insulators”.  Insulators are very useful because they allow us to keep things warm or cool longer by preventing heat from moving.

Materials Needed: Four waterproof thermometers; measuring cup; food tin; glass or porcelain coffee cup; Styrofoam ® cup; Thermos ® bottle; hot tap water.

Procedure: Allow the tap water to run until it is very hot.  Fill each container with 250 ml (1/2 cup) hot tap water.  Place the thermometers in each container and measure the temperature.  (If you only have one thermometer, you can do each container one at a time.)  Write down your results.

Measure and record the temperature every 10 minutes for an hour.  Use a chart similar to the one below to record your results.

What Happened: The temperature inside the food tin and the glass container dropped much more quickly than the Styrofoam ® or the Thermos ® bottle.  Glass and metal are better conductors of heat, so they are poor insulators.  The material used to make the Styrofoam ® cup is a very poor conductor of heat.  The Thermos ® bottle has a double layered inner shell which has had most of the air removed from between the layers.  This makes it a very poor conductor of heat as well.  A poor conductor of heat is a good insulator and will not allow heat to move from one place to another very quickly.

Going Further: Repeat this experiment using very cold water.  Heat moves from warmer areas to cooler ones, so is cold being trapped by the insulators or is heat being kept out?

Have you ever felt a draft in a cold room on a winter day? How about a cool spot in a swimming pool?  Both are caused by convection currents. Convection currents are currents of moving matter, usually gases or liquids, caused by uneven heating or cooling. These next experiments will allow you to observe and study convection currents.

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Convection Currents in Gases

Materials Needed: An ice cube; tongs or pliers.

Procedure: Hold the ice cube with the tongs or pliers.  Hold your hand just underneath the ice cube.  Slowly move your hand downward about a meter underneath the ice cube.  What do you feel?

Next, hold your hand just above the ice cube.  Slowly raise your hand about a meter above the ice cube.  What do you feel now? 

You may see mist around the ice cube.  If so, what is happening to the mist?

What Happened: When you placed your hand under the ice cube, you felt cool air.  As you moved your hand downward, you probably felt cool air for some distance.  However, when you placed your hand over the ice cube, you felt cool air near the ice cube only. 

If you were able to see mist coming off the ice cube, you probably noticed that the mist fell downward.  Hot air generally rises above cooler air.  Likewise, cooler air drops below warmer air.  It is this movement of air caused by temperature differences that creates convection currents. 

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Convection Currents in Gases - Air and Heat

Materials Needed: Compass; construction paper; scissors; ruler, pencil with an eraser; small needle; lamp.

Procedure:   Using a compass, draw a 12 cm (4 in) diameter circle on the paper.  Copy the spiral diagram below onto the circle and cut along the line on the pattern to make a spiral “snake.”  

If you prefer, you can download a larger version of the above diagram by clicking here.  You can then print it and cut it out.  

Push the needle into the top of the eraser. Place the center of the paper spiral on top of the needle and press down just enough to male a small dimple in the paper to hold the paper in place. When you hold the pencil straight, the snake should be able to turn on the needle freely. Hold your paper “snake” directly over the light bulb of a lamp that has been turned on.  Hold the snake as steady as possible for a few moments.  What do you see?

What Happened: You should see the “snake” begin to turn, and continue to turn for as long as it is over the bulb.  The air over the light bulb is heated.  When this air is heated, it begins to rise, and creates a current of moving air as cooler air moves in to take its place.  This type of current is called a convection current.  As this convection current moves up and away from the bulb, it moves through the paper snake, causing the snake to spin.

You have already seen that convection currents are created in air when part of that air is heated and rises.  These next two experiments will demonstrate that convection currents exist in liquids as well.

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Observing Convection Currents in Liquids

Materials Needed: Four clear soft drink bottles of the same size; warm tap water; cold tap water; food coloring or other coloring agent; two small pieces of cardboard.

Procedure: Place a drop or two of food coloring (or a few grains of instant coffee or powdered drink mix) into the bottom of two of the bottles.  Fill these two bottles completely with hot tap water.  Fill the other two bottles with cold tap water.

Place a piece of cardboard over the mouth of one of the hot bottles, and place this bottle upside down over one of the cold ones.  Carefully remove the cardboard so that the water in the two bottles can mix. What happens?

Next, place a piece of cardboard over the mouth of the remaining cold bottle, and place this bottle upside down over the remaining hot bottle.  Again, carefully remove the cardboard so that the water in the two bottles can mix. What happens this time?

What To Look For: Notice the area right around the mouths of the two bottles to see how the water moves.

What Happened: Just as hot air rises over cooler air, so a hot liquid will rise over a cooler one.  Warmer liquids are less dense than colder ones and are therefore lighter.  When the warmer liquid was on top, the liquids did not mix, or mixed very slowly.  However, when the warmer water was on the bottom, the water in the two containers mixed very rapidly, since the lighter warm liquid rose up over the cooler liquid.
 
Going Further: What happens when you leave the first two bottles alone for several hours?  Why?  Also, would it make any difference if you colored the cold water instead of the hot?

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Observing Convection Currents In An Aquarium

Materials Needed: Small aquarium or large clear bowl; small jar such as a baby food jar; ice; rock; food coloring.

Procedure: Fill the aquarium with water and allow it to come to room temperature.  Fill a  baby food or similar jar with crushed ice.  Add enough water to the jar to completely fill it and place the cap on the jar.  Weight the jar down with a rock and sink it at one end of the aquarium.  Allow it to sit for a few minutes and then place a few drops of food coloring (or a few grains of instant coffee or powdered drink mix) on top of the water above the jar. Observe how the coloring moves through the water.

What Happened: The food coloring added to the water allowed you to see the movement of the water in the aquarium. The water surrounding the jar of ice was colder than water that was farther away.  The coloring near the cool water tended to drift downward.  As the coloring moved away from cool water to the warmer water, it drifted upward.

With convection, heat moves by warm gas or liquid rising and cooler gas or liquid moving in to take its place.

Heat may also travel by radiation.  When heat moves by radiation, it travels mainly as energy through gases or empty space. It may also travel through some liquids and a few solids such as glass.  Heat moves from the sun through many millions of miles of empty space by means of radiation.  Heat from a fire warms by means of radiation as well. Even heat from a light bulb moves outward by means of radiation.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Electric lamp with the lampshade removed; candle (for “Going Further”).

Procedure:   Turn on the lamp.  Hold your hand about 30 cm (12 in) from the bulb.  Can you feel the heat? 

Next, place your hand 30 cm (12 in) above the light and slowly raise it to about 1 meter (1 yard).  What do you feel? Then, move your hand 30 cm (12 in) underneath the lamp and slowly move it downward about a meter.  What do you feel now?        

What Happened: You were able to feel heat all around the bulb, although it may have felt slightly hotter above the bulb.  When energy moves by means of radiation, it moves outward in all directions.  As the radiant energy comes in contact with matter, such as air or your hand, this energy changes into the heat you feel.

Heat created by radiation may move by other means as well.  You may remember from the paper snake experiment that when air surrounding the bulb is heated, it rises>  This explains why you may have noticed that it was warmer on top of the bulb. You were able to feel heat for some distance above the bulb because, as the air around the bulb was heated, it rose and created a convection current.  Since the only heat you felt below and to the sides of the bulb was due to radiation, you were probably not able to feel quite as much heat below the bulb as you did above.

Going Further: Try this same experiment with a candle.  Be careful!

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Heat and Color

Materials Needed: Three thermometers; three sheets of construction paper - one white, one black, and one another color of your choosing; a sunny day.

Procedure:   Find a sunny location such as a driveway.  Place the three thermometers on the driveway.  After about five minutes, record the temperature of each.

Cover each thermometer with a sheet of paper and leave the paper in place for about 15 minutes.  Remove the paper and record the temperature under each piece.
 
What Happened: At the beginning, the temperature was the same.  At the end of the experiment, the temperature under the white paper was coolest, under the black paper it was hottest, and it was probably somewhere in between under the colored paper.

When heat comes from the sun as radiant energy, it doesn’t actually change to heat energy until it comes in contact with matter.  When the sun’s rays struck the driveway, the radiant energy changed to heat, which could be measured.

Different colors absorb different amounts of radiant energy.  White absorbs the least energy.  Most of the radiant energy is reflected away from the paper.  Since less energy is absorbed, less is changed to heat and the temperature is cooler.

Black is just the opposite.  Most of the sun’s energy is absorbed by black and converted into heat.  This makes the temperature under the black paper much warmer.  The other colors are somewhere in between.  Generally, the darker the color, the more radiant energy it will absorb and change to heat.

Going Further: If you can get several thermometers and several different colors of paper, try to determine which colors absorb the most heat.

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Greenhouse Effect

Materials Needed: Large glass jar with lid; two thermometers (at least one should be able to fit inside the jar); a sunny day.

Procedure:   Find a sunny spot outside.  Place one of the thermometers inside the jar upside down and place the lid on the jar.  Turn the jar upside down so that you can read the temperature.  Place the jar with the thermometer and the other thermometer in sunlight. Record the temperature of each.  Repeat after 30 minutes and one hour.

What Happened: The temperature rose inside the jar.  Radiant energy that reached the inside of the jar was converted to heat.  This heat was trapped inside the jar by the glass.  As more radiant energy was changed to heat energy, the temperature rose.  At some point, heat began to escape through the glass by means of conduction and the increase leveled off.  This is sometimes called the "greenhouse effect."  It is what makes the inside of a closed car much hotter than the outside when it has been in the sun, and it is also what makes the inside of a greenhouse much warmer than the outside on a cold day.

Going Further: Other substances besides glass can trap heat.  Carbon dioxide, a gas in the air, is able to trap heat and to keep the earth warm.  Carbon dioxide is formed when  organisms use oxygen or when most fuels burn.  Because of this, some scientists are worried that we may be producing too much carbon dioxide by burning coal, wood, gasoline and other fuels.  They believe that too much carbon dioxide in the atmosphere will raise the temperature of the earth, and may cause the polar ice caps to melt.  If this happens, many areas close to sea level could eventually be flooded by the oceans, and the growth of certain plants and animals might be affected as well.  This heating is called the “greenhouse effect”.  Find out more about it at your school or public library, in an encyclopedia, or on the Internet.

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Boiling Water in a Paper Cup

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Alcohol lamp or candle with safety pan; burner stand; paper cup; water.

Procedure:  IMPORTANT! You need to be sure that the cup you use is paper and not plastic or Styrofoam ®.  Fill the cup 3/4 full of water and place it over your heat source.  Watch what happens.

What Happened: After a few minutes, the water began to boil.  You might have expected the cup to burn, but as long as there was water in the cup, it did not.  This is because the water absorbed the heat before the paper could reach a temperature hot enough to burn. 

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Money to Burn?

CAUTION!  Always be careful to follow all safety precautions when using fire, and use  with adult supervision only!  Keep your alcohol lamp or candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed.

Materials Needed: Candle or alcohol lamp with safety pan; piece of play money (or a small piece of notebook paper about the size of a dollar bill; tin can with smooth sides and no ridges; pair of pliers.

Procedure:   Light your heat source.  Wrap the paper around the can and hold with the pliers as shown.   Place the paper in the flame for a few seconds and remove.  Examine the paper.

What Happened: The paper near the flame may have been covered with soot and possibly a little scorched, but it didn’t burn. As the heat from the flame struck the paper, it was quickly conducted away from the paper by the metal in the can, and the temperature could not rise high enough for the paper to burn.  This result is very similar to the last experiment.

Up to now, we have learned that when matter is heated, it generally expands, and when it cools, it generally contracts.  We have also seen that heat may travel by conduction, convection, or radiation, and we have seen examples of each.  But to really understand why matter behaves as it does, we need to understand what is going on with the individual molecules in matter.  This next series of experiments will help us understand why solids, liquids and gases behave as they do when they are heated, and we will also learn the difference between heat and temperature.

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All Molecules Are Constantly Moving

We have already learned that all matter is made of molecules.  In solids, these molecules are strongly attracted to each other, and they are not free to move around.  In liquids, the molecules are attracted to each other as well, but the attraction is much weaker.  This allows the molecules to “stick” together, but they are still free to move around each other.  Finally, in gases, the molecules have almost no attraction between them, and they are able to spread out and completely fill any container.

Here’s something new.  All molecules, regardless of whether they are in a solid, a liquid, or a gas, are constantly moving.  Even in a solid where the molecules cannot move around, they vibrate, or move back and forth, in place.  Even though we cannot actually see the molecules move, we can see the effects of their moving.
 
Materials Needed: Jar or clear glass; food coloring (see Procedure); water.

Procedure: Fill the container with water.  Allow it to become as still as possible.  Add a drop of food coloring to the water, trying to disturb the water as little as possible.  (If you don’t have food coloring, you can use a pinch of instant coffee or powdered drink mix.)  Observe how the coloring moves through the water.   Allow the water to sit for an hour or so and observe it again.  What do you see now?

What Happened: The food coloring began to swirl in the water.  If you watched closely, you should have seen the food coloring begin to spread throughout the water.  Even though the water appears to be still, the individual molecules of water are constantly moving.  As they come into contact with the molecules of food coloring (which are also moving), they bump each other around.  In this way, the molecules of food coloring are eventually spread throughout the water.

The more molecules are heated, the more energy they have, and the faster they move, as this experiment will show.

Materials Needed: Two clear jars or glasses; food coloring; hot and cold water.
 
Procedure:   Fill one of the containers with cold water from the refrigerator.  Fill the other container with hot water from the tap.  Allow both to settle.

Place a drop of food coloring (or other coloring agent - see the last experiment) in each container, being careful to disturb the water as little as possible.  Notice how quickly the coloring moves through each container.

What Happened: The coloring moved through the hot water considerably faster than through the cold.  This is because the molecules of hot water were moving much faster than the molecules of cold water.  This caused them to collide with the molecules of coloring much more rapidly and with more force.  As a result, the molecules of coloring in the warm water spread out much quicker.

Materials Needed: A thermometer.

Procedure: Examine the thermometer.  Most thermometers are made of a glass tube with a very thin hollow center.  This tube is sealed at both ends, and has a bulb at the bottom. This bulb is filled with a liquid.  If the liquid is colored, it is probably colored alcohol. If it is a silver liquid, it is probably mercury. The liquid extends up into the tube for some distance through the hollow tube.  The tube is also usually flattened on one side to make it easier to see the colored liquid.

Read the temperature on the thermometer.  If you don’t know how to do this, have a teacher or other adult to help you.  Now place the thermometer inside a refrigerator for a few minutes.  Read the thermometer again.  Remove the thermometer and place your thumb over the bulb.  Hold it in place for a few minutes and watch what happens to the level of the liquid.

What Happened: The thermometer probably read somewhere between 18-24 ºC (65-75º F) in the room.  When you placed it in the refrigerator, the temperature dropped, probably to near freezing.  When you placed your thumb on the thermometer bulb, you saw the liquid rise in the tube as your thumb warmed the bulb.

You have already learned that a liquid will expand when heated and will contract when cooled.  When the liquid in the bulb got hotter, it expanded, and some of the liquid was forced up a narrow tube in the glass.  When the liquid cooled, it contracted, and the liquid fell back down the tube.  You saw this as a rise and fall in the temperature.

Going Further: O.K., this seems too simple.  You learned this in second grade, right?  Well then, can you use what you know about molecules in a liquid to explain why the liquid expands and contracts?  Maybe this next experiment will help.

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Making A Model Thermometer

Materials Needed: Glass bottle with screw top; modeling clay; clear soda straw; food coloring.

Procedure: Fill the glass bottle to the top with cool water.  Put a drop of food coloring in the water and shake or stir to mix it well.  Punch a hole in the top of the cap just large enough for the straw to fit through.  Place the straw in the top of the cap and allow about 3 cm (1 in) to stick out the bottom. Place a lump of clay around the top of the straw and cap to seal it. Screw the cap on the bottle tightly.  A little water may go up the straw, but that’s O.K.

Place your hands around the bottle to warm the water, but don't squeeze.  Watch what happens to the water level in the straw.
 
What Happened: The water rose up into the straw as your hands warmed the water.  In your model thermometer, just as in a real thermometer, the heating of the liquid causes the liquid to expand and forces it up in the straw or tube.

You already know that water expands when heated.  The reason it does is because the heat energy makes the individual water molecules vibrate faster.  As they do, they tend to bump into each other more and more, and push each other farther apart, even though they are still attracted to one another.  As they push each other farther apart, they take up more space and the water expands.

Going Further: Why did you need to use a glass bottle instead of a plastic one?

By now, we have done many experiments that show how solids, liquids and gases all expand when heated and contract when cooled.  We have also seen two exceptions to that rule.  First, we have seen that water will actually expand at the instant it freezes, but only because of the shape of it’s molecules.  Likewise, we have seen that solid rubber will contract when it is heated, but again, it is only because of the springlike shape of its molecules.  These are special cases.

In the last experiment, we saw that heated water expands because heat makes the molecules vibrate faster and causes them to push each other farther apart.  This also explains the general rule that matter almost always expands when heated and contracts when cooled.  The more heat you add to matter, the more you cause it’s molecules to vibrate.  As heat is taken away from matter, its molecules vibrate less.  If the vibrations get fast enough, they can overcome the attraction the molecules have for each other.  This experiment will show you how.

Materials Needed: A group of your friends or classmates (16 is a good number); thin thread; an open area.

Procedure: Line up in four rows of four people each.  (If you have more, line up in as near the same number of rows and people in each row as you can, but it doesn’t have to be exact.) Each person should tie themselves to the person in front, behind, and on either side with a 1 meter piece of string.  (Only those in the middle will be tied to four other people.  The ones on the outside will only be tied to two or three.)

Now pretend that your group is an ice cube, and each of you is a single water molecule in that ice cube.

Move in as close as you can to one another and remain as still as possible.  At this point there is no heat at all in the ice cube.  The molecules are not moving. Next, pretend that you are being heated.  This causes you to vibrate, so you should begin to move back and forth and side to side, but be careful not to break any of the strings.  As you begin to move, you will notice that you are moving away from one another and the “ice cube” is expanding.

More heat is being added, so you should begin to move faster.  At some point, the strings will begin to break.  Now you can move around, but you should hold two other people’s hands and stay within arm’s length of each other.  As you move, you may change the hands you are holding.  The “ice cube” has now melted, and you are water in the liquid state.  Your attraction for each other as molecules has weakened with the breaking of the strings, but you hands represent the attraction the molecules in a liquid still have for each other.

Even more heat is now being added, so you should begin to move even faster.  You will soon be unable to hold onto anyone else’s hand, and you will begin moving apart.  As you keep moving randomly, you will actually move farther and farther apart.  Enough heat has now been added to cause the water to boil and evaporate.
 
What Happened: You have just seen how molecules move in solids, liquids and gases when they are heated.  When the molecules are not moving at all, there is no heat present. This is the point scientists call “absolute zero” and it is - 273.15 ºC (- 459.67 º F).  Because the molecules cannot move any slower, this is the coldest temperature possible, and that is why it is called absolute zero.

As heat was added, the molecules (you) moved faster and faster.  When there was enough heat to cause you to melt, the heat energy overcame the strong attraction (the strings) you had to each other.  As more heat was added, you moved faster still, and moved farther apart, but you were still moving slow enough to hold on to each other, just like molecules in a liquid.  However, enough heat was finally added that you could no longer hold on to each other, and you went off in all directions like molecules in a gas.

All molecules of matter behave pretty much the same way.

Materials Needed: Small container; container about twice as large as the small container; waterproof thermometer; hot tap water.

Procedure: Fill both containers with the hot water.  Use the thermometer to measure the temperature of each. 

Now answer this question: If you were to spill both of the containers of water on yourself, which would be more likely to hurt you and why?

What Happened: The temperature of each container should have been the same. However, if you were to spill both containers on yourself, the container with more water would have hurt you more.

Temperature is a measure of the average energy of the moving molecules.  The molecules are moving around at the same average rate or speed in both containers, so about the same number of molecules are hitting the thermometer bulb in both containers.  As a result, the same amount of heat is being transferred to the liquid inside the thermometer in each container, and so the liquid expands at the same rate in both.  Because of this, the temperature is the same. 

However, if there is twice as much water in the larger container, there are twice as many molecules moving around, and there is twice as much total heat energy.

When we measure temperature, we are measuring the average energy of the molecules, but when we measure heat, we are measuring the total amount of heat present.  Temperature is measured in degrees Fahrenheit or Celsius, while heat energy is measured in calories.  So another way of stating our result is that the temperature of the two containers is equal, but there are twice as many calories of heat in the larger container as in the smaller one.

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Weight of Hot Air

   

Materials Needed: Homemade balance; two large paper grocery bags; lamp; tape; paper clips.

Procedure: Bend two paper clips into an “S” shape.  Tape a piece of string to each bag on the bottom center and tie the other end of each string to a paper clip.  Place the balance on the corner of a table as shown.  Because the grocery bags are fairly large, you will probably want to use a meter or yard stick or a long dowel for the balance arm. Hang each bag on opposite ends of the balance by the paper clip and adjust the balance so that it is level.  Remove the lampshade from the lamp and place it underneath one of the bags.  Turn the lamp on and observe what happens.  After a few minutes, turn the lamp off, and again watch what happens.

What To Look For: You should see the bag over the lamp begin to rise when you turn on the lamp.  When you turn the lamp off, the bag should return to where it was at the beginning, but it won’t do so as fast as it rose.

What Happened: You already know that heated air rises, and this creates a convection current, so it should not surprise you, then, that the hot air from the lamp “pushed” the bag above it up in much the same way as if you were to blow upward into the bag.  But, if you just blew into the bag, when you stooped blowing, the bag would immediately return to where it was before.  That didn’t happen here.

As this warm air from the bulb moved upward, it filled the bag with warm air.  Warm air is lighter than cold air, since the molecules are moving faster and are spread farther apart. In this case, the warm air rose and lifted the bag up with it.  When you turned the light off, the warm air didn’t come out of the bottom of the bag because it was lighter than the cooler air below it.  Eventually, the warmer air in the bag was cooled by the surrounding air, and as the air in the bag got cooler, it  became heavier, and it sank.

You have now learned two very important facts about air.  First, air rises when it is heated, and second, heated air rises because hot air is lighter than the cooler air around it.  Knowing these two very basic facts can help you understand a lot about how air behaves in nature.  Lets apply what we’ve learned in this last experiment.

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Making a Hot Air Balloon