How Things Work - Chapter 8 Demonstrations
Section 8.1 Air Conditioners
Demonstration 8.1.1: A Bean Illustration of Heat Flow from Hot to Cold
Description: A glass dish is initially divided into two sides. One side contains mostly black beans (representing fast moving atoms) while the other side contains mostly white beans (representing slow moving atoms). When the division is removed and the beans are stirred randomly, the result is an even mixture of beans in each side, not an accumulation of pure black beans on one side and pure white beans on the other.
Purpose: To show how statistical issues affect physical problems.
1 bag of black beans
1 bag of white beans
1 shallow glass dish (a rectangular baking pan)
1 cardboard or wooden divider for the pan
Procedure: Divide the pan in half and partially fill the two halves of the pan with beans. On the left, put mostly black beans and on the right, mostly white beans. Announce that the black beans represent fast moving and thus energetic atoms and that the white beans represent slow moving and thus less-energetic atoms. Because of how the beans are distributed, the left side of the pan is hot (mostly fast atoms) while the right side of the pan is cold (mostly slow atoms).
Now remove the divider and stir the beans randomly. After a few seconds, reinsert the divider and examine the beans. Note that each side now contains a roughly equal mixtures of the two beans. Each side is now at an intermediate temperature, neither hot nor cold. Point out that there is no fundamental law that makes it impossible for all the white beans to end up on the right (extra cold) and all the black beans to end up on the left (extra hot). That outcome is not forbidden, it's just incredibly unlikely.
The same result holds true for temperature—if a hot object and a cold object touch, there's no mechanical law that forbids heat from flowing from the cold object to the hot object so that the hot object becomes hotter and the cold object becomes colder. That outcome is not forbidden by the laws of motion, it's just incredibly unlikely.
Explanation: The laws of thermodynamics incorporate statistical issues that are not contained in the basic laws of motion. They predict the dynamics of large assemblies of particles that are exhibiting thermal behaviors. This illustration with beans shows how statistical issues similarly contribute to more visible situations.
Demonstration 8.1.2: Animation of Natural and Unnatural Heat Flows
Description: Two open boxes are placed on the table, each with a thermal energy meter and an entropy meter. The boxes are initially given unequal amounts of thermal energy, as indicated by the different readings of their thermal energy meters and their entropy meters. Transferring thermal energy from the box with more thermal energy to the box with less thermal energy causes the total entropy of the two boxes to increases, but only until the two boxes contain equal amounts of thermal energy. If the transfer continues beyond that equilibrium, the total entropy decreases.
Purpose: To show that entropy and the second law cause heat to flow from hotter to colder and that transferring heat the other way requires the production of additional entropy.
2 open boxes, each with two "meters", a thermal energy meter that reads from 0 to 4 and an entropy meter that reads from 0 to 15.
10 units of thermal energy (I use red paper disks).
5 units of ordered energy (I use white paper disks).
Start with both boxes empty of any thermal energy, so the thermal energy meters read 0 and the entropy meters read 0. Add the first thermal energy unit to the left box and change its thermal energy meter to 1. Note that this unit of thermal energy is extremely disordering to the highly ordered (cold and empty) box, so that you must change its entropy meter to 8. In this animation, 1 unit of thermal energy added to the cold box causes its entropy to increase by 8 units. Now add a second unit of thermal energy to the left box. Change its thermal energy to 2 and its entropy to 12. This time, 1 unit of thermal energy added to the cool box causes its entropy to increase by just 4 units. The box started at a hotter temperature, so 1 unit of thermal energy wasn't as disordering as before. Repeat the addition to the left box two more times, increasing the thermal energy of the left box by 1 unit each time and the entropy by 2 units, then 1 unit. Note that as the box gets hotter, each additional unit of thermal energy produces a smaller increase in the box's entropy. You should now have 4 units of thermal energy and 15 units of entropy in the left box. The right box still has 0 and 0.
Now touch the two boxes together and begin transferring thermal energy units from the left box to the right box. As the first unit of thermal energy leaves the left box, its thermal energy meter drops to 3 and its entropy drops to 14. When you put that thermal energy unit in the right box, its thermal energy meter increases to 1 and its entropy increases to 8. By moving thermal energy from the hot left box to the cold right box, you've caused the total entropy of the two boxes to increase from 15 to 22 unit!
Move another thermal energy unit from left to right. This time the left box reads 2 units of thermal energy and 12 units of entropy. The right box reads 2 units of thermal energy and 12 units of entropy. Again, the total entropy has increased, this time to 24 units!
But now move another thermal energy unit from left to right and see what happens. The left box has 1 unit of thermal energy and 8 units of entropy, while the right box has 3 units of thermal energy and 14 units of entropy. The total entropy is only 22 units; so you've voliated the second law! To pay for this movement of heat against its natural direction of flow (i.e., from colder to hotter), you must create some extra entropy. Pick up some ordered energy units and exchange them for some thermal energy units. Toss a couple of these thermal energy units into the right box and increase the box's two meters -- they'll both go off scale. Point out that the relationships between thermal energy and entropy used in this animation are only approximations chosen for easy illustration. The point here is that by converting enough ordered energy into thermal energy and adding it to the right box, the overall entropy can be made to increase even as the thermal energy content and temperature of the left box decrease. While heat flows naturally from hotter to colder, it must be pumped in order to go from colder to hotter.
Explanation: The 2nd law of thermodynamics encourages the flow of heat from hotter to colder, but prevents it from flowing from colder to hotter without assistance.
Demonstration 8.1.3: A Simple Heat Pump Using Air
Description: As you pump air into a sealed jug, its temperature rises and heat flows out of it into the room. When you let the air in the jug expand, its temperature drops and heat flows into it from the room.
Purpose: To show how compression and expansion of a gas can be used to move heat around.
1 strong narrow mouth jug (plastic or glass)
1 one-hole rubber stopper for the jug with a pipe in the hole
1 small electronic temperature sensor
1 hand air pump (a bicycle pump, for example)
Procedure: Insert the temperature sensor into the jug, so that its sensor is hanging freely in the air inside the jug. Now insert the stopper into the jug. Use the hose to connect the pipe in the stopper to the air pump. Note the temperature of the air inside the jug.
Now begin to pump air into the jug. As you do, the temperature inside the jug will rise. You'll have to go fast enough that the heat won't have time to flow out into the room, or you won't see much of a temperature rise.
While the air inside the jug is still pressurized, wait for it cool down to room temperature. Then suddenly release the pressure, either by popping the stopper out of the jug or removing the hose from the pipe. As the air expands and flows out of the jug, the temperature of the remaining air will fall well below room temperature.
Explanation: When you compress air to pack it into the jug, you're doing work on that air and its energy is increasing. This increased energy takes the form of a rise in the air's thermal energy, and is thus accompanied by a rise in temperature. When you allow the air to expand out of the jug, it must lift the surrounding air out of its way and thus does work on the surrounding air. Its energy is decreasing. This decreased energy takes the form of a drop in the air's thermal energy and is thus accompanied by a drop in temperature.
Follow-up: Discuss how you would use this process to move heat from one room to another. At what times should you move the apparatus between the rooms?
Demonstration 8.1.4: A Simple Heat Pump Using a Condensable Liquid
Description: You fill a clear glass tube with gas and insert a piston into the tube. When you push the piston deep into the tube, the gas inside it turns into a liquid. When you pull the piston out of the tube, the liquid turns back into a gas.
Purpose: To show that compressing some gases can actually cause them to liquefy at room temperature and that allowing some liquids to decompress can actually cause them to become gaseous at room temperature.
1 fire syringe (available from a scientific supply company—normally used to show that compressing air suddenly can cause it to become hot enough to ignite cotton)
1 gas duster (an aerosol canister that's used to blow dust off optics—it contains a hydrofluorocarbon that's liquid at high pressure but gaseous at low pressure, even at room temperature)
Procedure: Remove the piston from the fire syringe and spray the gas duster into the cylinder until all the air has been displaced and replaced by the hydrofluorocarbon (HFC) gas. Quickly insert the piston so as to trap the HFC gas. Now push the piston deep into the cylinder. When the pressure in the gas becomes high enough, it will liquefy. You should obtain a small fraction of a milliliter of liquid at the very bottom of the cylinder when the piston is almost at the bottom. When you then let the piston move back away from the bottom of the cylinder, the liquid will boil away into a gas. While in principle you should be able to feel the cylinder become hot during the compression process and become cold during the decompression process, the effect is so small that I've been unable to feel it.
Explanation: At room temperature, the HFC compound forms a liquid when its pressure is high and a gas when its pressure is low. Compressing the gas first causes it to become a hot gas. After most of its excess heat has flowed into the cylinder walls, this cooling gas becomes a liquid. As that condensation occurs, still more heat flows out of the material and into the cylinder walls. Now decompressing the liquid first causes it to become a cold gas. After heat has flowed into it from the cylinder walls, this warming liquid becomes a gas. As that evaporation occurs, still more heat flows into the material from the cylinder walls.
Demonstration 8.1.5: Examining an Air Conditioner (or Refrigerator or Dehumidifier)
Description: You examine the three major components of an air conditioner—the compressor, the evaporator, and the condenser.
Purpose: To show how a real heat pump works.
1 air conditioner (or a refrigerator or a dehumidifier)
Procedure: Follow the path of the working fluid from the compressor, through the condenser, through the evaporator, and back to the compressor.
Explanation: The working fluid in the unit enters the compressor as a room temperature, low-pressure gas and leaves as a hot, high-pressure gas. It then enters the condenser and leaves as a room temperature, high-pressure liquid. It then enters the evaporator and leaves as a cool, low-pressure gas. Finally it returns to the compressor.
Demonstration 8.1.6: A Peltier Junction - An Electronic Heat Pump
Description: An electric current is run through a thermoelectric cooler, causing one of its surfaces to become hot and the other surface to become cold.
Purpose: To display a thermoelectric effect in which electric power is used to pump heat from a colder surface to a hotter surface.
1 thermoelectric cooler, based on Peltier junctions (available from a scientific supply company)
1 power supply or hand-powered generator
Procedure: Attach the two terminals of the thermoelectric device to the power source and send current through it. One surface of the device will become quite hot and the other quite cold. If you attach the hot surface to a good room temperature heat sink and place a drop of water on the cold surface, the drop of water will soon freeze to form ice. Show that reversing the direction of current flow through the device reverses the direction in which heat is pumped between its surfaces.
Explanation: The thermoelectric device is using the Peltier effect, the reverse of the Seebeck effect, to pump heat from a colder surface to a hotter surface. The device contains a number of individual Peltier junctions, formed by touching two dissimilar semiconductors. Just as a thermocouple can power an electric current when it has a temperature difference across its wires, a thermoelectric device can produce a temperature difference across its junctions when it's powered by an electric current.
Section 8.2 Automobiles
Demonstration 8.2.1: A Simple Heat Engine - A Steam Engine
Description: The boiler of a toy steam engine is heated by a gas flame. When steam from the boiler is delivered to its cylinder, the steam engine's piston begins to move back and forth and a flywheel spins rapidly.
Purpose: To show that heat flowing from a hot region to a cold region can be used to do "useful" work.
1 toy steam engine (available from a scientific supply company)
natural gas (or alcohol, if appropriate)
Procedure: Fill the boiler with water and connect the steam engine to the gas supply. Light the engine's burner and allow the water to begin to boil. When pressurized steam begins to flow to the cylinder, the piston will begin to move in and out and the steam engine's flywheel will begin to turn.
Explanation: Heat from gas flame enters the colder water and heats it to its boiling temperature. The hot, high-pressure steam then flows to the lower pressure in the cylinder and pushes the piston out of the cylinder. The piston's movement opens a valve that vents the steam from the cylinder and allows the piston to return into the cylinder. Once the piston reaches the bottom of the cylinder, the valve closes and steam once again pushes the piston out. This motion of the piston repeats over and over, while the piston's reciprocating motion causes the flywheel to turn. The flywheel stores energy and helps the piston move smoothly through its cycle. Overall, heat is flowing from the boiler to the outside air and a small fraction of that heat is diverted by the steam engine and converted into mechanical work.
Demonstration 8.2.2: A Simple Heat Engine - a Dipping Duck
Description: A glass duck toy is placed so that it can dip its bill into a glass of water. It repeatedly leans forward and appears to drink from the glass of water. This tipping behavior is powered by a heat engine in the duck.
Purpose: To demonstrate an interesting form of heat engine.
1 dipping duck heat engine toy (from a scientific supply company)
1 very full glass of water for the duck to "drink"
Procedure: Place the dipping duck next to the glass of water so that it can lean forward until it's horizontal and just dips its bill into the water. Tip the duck forward so that its bill gets wet and then allow it to return to upright. In a few seconds, evaporative cooling will lower the temperature of its head and fluid will begin to rise up inside of the duck's body. The duck will tip over and take a drink of water, while the fluid returns to the bottom of its body. The duck will return to its upright position and the process will begin again.
Explanation: Evaporative cooling keeps the duck's wet head colder than its dry tail. Because of its cold head, gas inside the duck's body evaporates in the tail area and condenses in the head area. This process creates a pressure imbalance between the duck's head and tail that pushes the liquid upward from the tail to the head. Each time this transfer of liquid occurs, the duck's center of gravity rises until the duck becomes unstable and tips forward. When the duck reaches a horizontal orientation and wets its bill in the water, the liquid is able to flow back toward its tail. Overall the duck is a heat engine, using the flow of heat from its warmer tail to its colder head to make the duck dip repeatedly.
Another way to look at the thermodynamics of Dipping Duck is to realize that there's order in the separation of liquid water from dry air. The evaporation of water into the dry air increases the overall entropy of the situation and Dipping Duck uses that rising entropy to get some work done.
Demonstration 8.2.3: A Simple Heat Engine – A Candle Carousel
Description: A lit candle causes a small carousel to turn, probably ringing several tiny chimes as it turns.
Purpose: To demonstrate an interesting form of heat engine.
1 holiday candle carousel, such as are sold in holiday gift shops.
1 candle (or more if required)
Procedure: Place the candle(s) in the holder at the base of the carousel and light it. As heat begins to rise upward from the flame, the turbine blades of the carousel will experience a torque and the carousel will begin to turn.
Explanation: The convection caused by the hot flames and the colder surrounding air can do work on the turbine blades of the carousel and make it turn. As heat flows from the hot flames to the cold room, some of that heat is converted into useful work.
Demonstration 8.2.4: A Simple Heat Engine – A Lava Lamp
Description: A Lava Lamp uses heat from a bulb to keep blobs of molten wax rising and falling in its container.
Purpose: To demonstrate an interesting form of heat engine.
1 Lava Lamp
Procedure: Simply operate the Lava Lamp. Blobs of molten wax will be heated from below, rise upward in the liquid, cool until their density drops, and then sink.
Explanation: The lamp uses buoyancy changes to do work. It heats the wax, causing its density to drop below that of the surrounding liquid. The wax then floats upward and cools. Its density rises above that of the surrounding liquid. The wax then sinks and begins its trip all over again.
Demonstration 8.2.5: Burning a Fuel and Air Mixture - an Exploding Milk Container
Description: A gallon plastic milk jug, containing air and a small amount of alcohol, is exposed to a high voltage spark. With a bang and a bright flash of light, the container leaps up into the air.
Purpose: To illustrate how much energy is contained in even a small amount of liquid organic fuel.
1 gallon polyethylene milk container, with a screw top (clean and completely dry)
2 medium-sized nails
1 board, about 20 cm on a side and about 1 cm thick
a spark coil or tesla coil
methyl alcohol (methanol)
1 small beaker, marked at 0.5 ml volume intervals
Procedure: Pound the nails through the middle of the board, about 1 cm apart, so that their points emerge from the board. Bend these sharp points slightly toward one another and attach wires to them from the other side of the board. Clamp the board to the edge of a table, with the nail points sticking upward. Connect one of these wires to an earth ground and put the other wire where you can reach it easily with the spark generator. The wire should be long enough that you will be 2 m or more away from the milk container when it explodes.
Now measure 1.5 ml of alcohol in the beaker and pour this into the milk container. Put the top on the container and swirl the alcohol around the inside of the container to help it evaporate. Give it about twenty seconds to evaporate completely. Invert the milk container and push the cap down over the two upward-pointing nails so that the nails pierce the cap.
Step back to a safe distance, put on the safety glasses, and touch the spark generator to the exposed wire. A spark will occur inside the milk container, igniting the alcohol and air mixture. The container will explode with a flash and a loud report, and it will fly up into the air. In most cases, the milk container will tear open and will not be reusable.
Explanation: The spark provides the initial activation energy needed to start the chemical reaction between the alcohol and air. Once ignited, the burning mixture does lots of work on its environment and on your ears.
Demonstration 8.2.6: Knocking in an Automobile Engine
Description: A tiny piece of cotton or paper towel is dropped to the bottom of a clear glass cylinder. A narrow piston is inserted into that cylinder and shoved suddenly to the bottom, compressing the air inside the cylinder. The air becomes so hot during the compression that it ignites the cotton or paper, which burns with a bright flash of light.
Purpose: To show just how hot air can become when it's compressed tightly and to show how knocking occurs in an automobile engine.
1 fire syringe (from a scientific supply company)
1 tiny piece of cotton or paper towel
Procedure: Make sure that the inside of the glass fire syringe is clean and dry, and that the air it contains is fresh. Make sure that the piston travels smoothly through the cylinder and lubricate the O-rings very lightly with salad oil if it doesn't. Drop or push a tiny piece of cotton, just a dozen fibers or so, to the bottom of the cylinder and insert the piston. Install the plastic guard around the glass tube and place the whole assembly on a firm rubber pad that is itself on a solid table. When you're ready, push the piston suddenly and vigorously to the bottom of the cylinder. The cotton will burst into flames.
Explanation: You do work on the air as you push the piston into the cylinder. The air's energy increases and this energy increase takes the form of thermal energy—the air becomes hot. Since you compress the air very suddenly, heat has little time to flow out of the air to the walls of the tube and the air becomes so hot that it ignites the cotton. Since the cotton is surrounded by hot, compressed air, it burns with a bright flash of light.
Demonstration 8.2.7: Hero's Engine
Description: Steam produced by water boiling in a spherical vessel emerges from that vessel through two arms that are arranged in a Z shape. As the arms push the steam in one direction, the steam pushes back and the vessel experiences a torque. It begins to spin rapidly.
Purpose: To show how steam can be used to create rotational motion (a primitive turbine-like heat engine).
1 Hero's engine (available from a scientific supply company)
1 suspension for the Hero's engine (preferably with a swivel clip)
1 gas burner
Procedure: Partly fill the Hero's engine with water and install the cap. Suspend the Hero's engine from its support and place the burner beneath it. Ignite the burner and allow the water to boil. When steam begins to emerge from the arms of the Hero's engine, the reaction forces on the arms will produce a torque on the engine and it will begin to spin rapidly. Turn off the burner so that it doesn't get out of control.
Explanation: The ejected steam exerts a torque on the engine, which undergoes angular acceleration. The steam is doing work on the engine, converting a small amount of its thermal energy into work as heat flows from the hot steam to the colder room air. The Hero's engine is a simple heat engine.
Demonstration 8.2.8: An Air Turbine or Windmill
Description: You blow air from a compressed air line or tank at a turbine or fan and it begins to spin. With the turbine or fan attached to a generator, it produces electric power.
Purpose: To show that a high-pressure (or high-speed) fluid can be used to generate electricity.
1 turbine or fan assembly, attached to a generator
1 light bulb
1 light bulb holder
compressed air or a tank of high pressure gas
Procedure: Insert the bulb in the holder and use the two wires to connect it to the generator. Allow the air or gas to flow through hose and direct the stream of air or gas toward the turbine blades. The blades will begin to spin, turning the generator and generating electricity. The light bulb will illuminate.
Explanation: As the air or gas flows through the turbine blades, they experience lift forces. These lift forces produce torques on the blades about their central pivot and the blades begin to turn. They turn the generator, which produces electricity.