Thermodynamics: The heat is on!admin
Thermodynamics and the thermal properties of matter are inextricably linked to daily living. Most everyone has an innate sense of hot and cold, checks the temperature outdoors before deciding what to wear, knows that spilled water will eventually evaporate and not to expect a snow storm on a hot July day. We all understand that a cold drink will become warmer and a hot drink cooler if left sitting on the counter. On the other hand, we know that both hot and cold liquids retain their respective temperatures for quite some time when placed in a thermos bottle. These everyday experiences of hot and cold, evaporation and freezing, and the transfer of heat are the province of thermal physics.
On a grander scale, the universe and everything in it – heat engines, the weather, cabbages and kings – are subject to rules known as the laws of thermodynamics. In simplest terms, the laws of thermodynamics dictate the specifics for the transfer and transformation of energy. The first law of thermodynamics is an expression of the law of conservation of energy and identifies heat transfer as a form of energy transfer. The second law limits the efficiency of engines. While mechanical energy can always be converted entirely into heat, heat cannot be converted entirely into mechanical energy. Attempts to convert heat completely into energy always produce some waste heat.
Perhaps the British scientist and author C.P. Snow stated the laws of thermodynamics most simply:
- You cannot win (that is, you cannot get something for nothing because matter and energy are conserved).
- You cannot break even (useful energy degenerates into non-useable, disorganized energy, because there is always an increase in disorder).
Research tells us that students learn best when they are allowed to ask questions of nature through exploration and experimentation. With that in mind, this edition of CoolStuff offers several activities that invite students to ask questions and find answers regarding the thermal properties of matter and the laws of thermodynamics.
see the largest Drinking Birds in the world. Artist Daniel Reynolds created these incredible Drinking Birds as an art exhibit in Manhattan NY. The idea came from watching Mr. Wizard…
Heat is transferred in three common ways: convection, conduction and radiation.
The process in fluids, i.e., gases and liquids, in which heat is transferred by the motion of the fluid itself.
Making Convection Visible
To witness convection occurring in a gas, look over the surface of a hot plate or, if possible, above the hot hood of a car. In both instances, the heated surfaces cause the air to expand and rise. The convection currents are visible because cold and hot air have different indices of refraction.
Convection in a liquid may be demonstrated by passing an electrical current through a length of pencil lead which is submerged in a clear tank of water such as an aquarium. Connect two electrical leads to the two ends of a piece of pencil lead approximately three inches in length. After placing the pencil lead into the tank, connect the ends of the other two electrical leads to a six-volt battery or other DC power supply.
Image courtesy of The Exploratorium.
(Note: An inexpensive immersion heater used for heating water for coffee or tea may be used in lieu of the pencil lead heater.) Shine light from a slide projector or flashlight through the tank and onto a white screen. As the pencil lead heats up, convection currents will appear as moving shadows on the screen (See figures above and below).
Image courtesy Montana State University
Conduction: The process whereby heat energy is transmitted through a material as a result of molecular collisions.
Fire Proof Paper
Paul Hewitt suggests the following dramatic demonstration of conduction by solids. Tightly wrap a piece of paper around a thick cylindrical metal rod. When the paper is placed in the flame, it will not catch fire. The metal conducts heat away from the paper, preventing the paper from reaching the temperature required for ignition. (Note: Do not hold the end of the rod in your bare hand. It will rapidly become very hot.)
Fire Proof Balloon
The transfer of heat by conduction and convection may be demonstrated with two similar balloons, one filled with air, the other with water. When a match is brought close to the air-filled balloon, the balloon ruptures. However, when a match is brought within the same distance of the water-filled balloon, the balloon remains in tact. Conduction and convection carry the heat away from the balloon before the temperature required for melting occurs. A second reason the balloon doesn’t burst is related to the heat capacity of water. Water is able to absorb a great deal of heat with little change in temperature.
Ice Melting Blocks
Touch these two black blocks, and one feels cooler. Place an ice cube on each block. One cube completely melts before your eyes, while the other stays frozen! Surprisingly, the “cooler” block melts the ice faster! This discrepant event introduces many concepts, including heat transfer, change of state, and thermal conductivity.
Boiling Water in a Paper Cup
A striking demonstration illustrating thermal conductivity, heat capacity of water and the cooling effect of boiling can be performed with nothing more than a paper cup filled with water and a Bunsen or alcohol burner. When exposed to an open flame, the water in the cup will come to a boil. The cup will not burn until the water has completely evaporated.
The transfer of energy by electromagnetic waves.
We experience the sensation of heat via radiant energy from the sun, a light bulb, or a fireplace.
Absorbers & Reflectors
Good absorbers of radiant energy are also good emitters. To demonstrate how surface color affects the emission and absorption of radiant energy, you will need two or three containers of the same size and shape. One container should be white or shiny; the other container should be black.
Radiation cans, such as those shown here, work well. Soup cans with labels removed may be also be used. After painting one can black, fill both cans with warm water. Thermometers placed in the cans will reveal that the water in the black can cools faster. If ice water is used instead and the cans are exposed to direct sunlight or light from a heat lamp, the dark can will warm faster.
The radiometer, invented in 1873 by the chemist Sir William Crookes, consists of a rotating shaft with four vanes. The shaft and vanes are sealed in a glass container which has over 99% of the air removed. The vanes are painted black on one side, silver on the other. As we have seen, when exposed to radiant energy, the black surfaces will become warmer than the silver surfaces.
In the presence of light, the air molecules that remain inside the bulb begin to move faster as they absorb energy from the light. At the same time, the light warms the vanes, the black surfaces more so than the silver. As the randomly moving molecules strike the vanes from all sides, the ones striking the cooler, reflective silver vanes take on very little additional energy. However, the molecules coming in contact with the warmer, dark vanes do gain energy and leave with considerably higher speeds. As a consequence, the molecules push on the dark vanes harder than they do on the silver vanes, producing rotation.
You may wish to try these demos!
- In the absence of bright light, the radiometer can be made to turn by directing a hair dryer at the vanes. The infrared radiation from the heating element will produce results similar to those obtained with visible light. This demonstration never ceases to amaze those that witness it.
- If you cool the glass quickly in the absence of a bright light source by placing ice on the glass, the vanes will turn backwards (i.e. the silver sides are trailing).
Work may be completely converted into heat, as briskly rubbing your hands together demonstrates, but it is not possible to completely convert heat into useful work.
Large ball bearings may be used to demonstrate the eventual fate of most forms of energy. In this case, mechanical energy is converted into internal energy.
- Place a piece of paper between the two large spheres. Now smash them together. After the spheres collide, smell the area on the paper where they came into contact. What do you detect? What do you suppose caused this odor?
- Did the paper get hot? That is, where did the energy that was responsible for the heating come from? As best you can, describe the energy transformation that took place as a result of the collision.
- Examine the point of contact. Do you observe any signs of scorching on the paper? What can you conclude about the increase in the paper’s temperature? In theory, what could you do to cause combustion?
Smelling a piece of paper placed between colliding spheres reveals that the kinetic energy possessed by the moving spheres has been converted into heat.
Heating up a Hanger
The conversion of mechanical energy into heat may be dramatically demonstrated by simply bending a coat hanger. First cut a 30-cm length of coat hanger with wire cutters. Grab the ends of the wire in each hand and rapidly bend it back and forth several times. Now touch the point on the wire where the bending occurred. (Caution! the coat hanger can sometimes get surprisingly hot, so only touch the hot spot briefly.)
An adiabatic process is one in which no heat is added to or removed from a system. This does not mean that the temperature of the system necessarily remains constant, for even in the absence of external interactions with the surroundings, the system is free to exchange energy between thermal (internal energy) and mechanical forms. Adiabatic conditions are closely approximated when the process happens so quickly that there is no time to transfer heat, or if the system is very well insulated from its surroundings.
Place a rubber band loosely looped over the index fingers in contact with skin just above your upper lip. Now quickly stretch the rubber band. What do you experience? Now let the rubber band relax quickly. What do you feel now?
When the rubber band is stretched quickly, work is done on it, causing its internal energy to rise. This rise reveals itself as a small increase in temperature. When the rubber band is allowed to quickly contract, it performs work and suffers a reduction in internal energy which produces a cooling sensation.
Fire in the Hole!
As everyone who has pumped up a bicycle tire knows, if you rapidly compress a gas, it gets hot. The fire syringe is a device that dramatically demonstrates just how much heat may be produced when a volume of air is rapidly compressed. Temperatures of over 260 degrees C (500 degrees F) may be produced with this simple device. Since paper burns at 235 degrees C (454 degrees F), a small piece of tissue paper is easily ignited when the plunger is rapidly inserted into a glass cylinder.
It is believed that Rudolf Diesel knew of this demonstration when he began work on his compression engine.
Adiabatic cooling may be easily demonstrated with an inflated bicycle tire. When the air is released from the tire, the escaping air will be noticeably cooler than the tire. After all the air has escaped, the valve stem will be cold to the touch. The effect can also be demonstrated with using any aerosol spray can or CO2 fire extinguisher. When gas escapes from either container, it cools dramatically. In the case of the aerosol spray can, the dispensed gas is cold. The escaping CO2 from a fire extinguisher is so cold that it solidifies and forms dry ice. Students love to see this solid form of CO2 falling like snow.
Cloud in a Bottle
Adiabatic processes are very important in the atmosphere. In fact, adiabatic cooling of rising air is the dominant cause of cloud formation. Using a gallon jar, a rubber glove, some water and a match, you can demonstrate adiabatic expansion and in the process, produce your own cloud.First cover the bottom of the jar with a thin layer of water. Drop a lit match into the jar. Quickly place the fingers of the glove inside the jar and stretch the open end of the glove over the mouth of the jar. Put your fingers in the glove and pull the glove outside the jar.
This rapid expansion of the gas inside the bottle results in adiabatic cooling. Presto! You should see a wispy cloud inside the jar.
Little Heat Engines That Can
A heat engine is any device that converts internal energy into mechanical work. All heat engines – steam engines, jet engines, and internal combustion engines – extract useful energy as heat flows from a higher temperature to a lower temperature. As the second law of thermodynamics states, while it is possible to convert work completely into heat, it is not possible to convert heat completely into useful work.
You may demonstrate heat engine essentials with several simple devices. Some of these devices may be easily constructed; others may be purchased at modest cost. The devices described here may not seem like heat engines since they do not resemble the engines that power our cars, lawnmowers, or airplanes. However, analysis will show that all take in heat, convert some of this energy into mechanical work, and expel the rest.
In about 100 BC, Hero of Alexandria invented a heat engine that used steam for propulsion (see figure right). Water in the lower vessel is converting into steam. After passing through two supporting pipes, the steam is expelled through ports on opposite sides of the sphere. Unbeknownst to Hero, he had produced the first steam turbine, versions of which are still used to this day.
Make Your Own…
A very simple version of Hero’s engine may be constructed with an empty soda can and a heat source such as a Bunsen burner or a can of Sterno.
- Place an empty soda can, with its opener lever still intact, on its side. Using a nail or pin, make two holes opposite each other in the side of the can, about one half inch above the bottom. Before removing the nail or pin from each hole, bend it to the right and parallel to the rim. The slanted holes will insure that the escaping steam will leave tangentially with respect to the sides of the can.
- Bend the can’s opener lever so that it is perpendicular to the top of the can and tie a short length of fish line through the hole in the lever. After adding water so that it just covers the bottom of the can, tie the other end of the fish line to a support such as a ring stand.
- Place the bottom of the can over the heat source. When the water starts boiling, the can will begin to spin. If the fish line offers too much resistance and impedes the motion of the can, place a snap swivel, used for fishing and available in any sporting goods store, between the fish line and the opener lever.
Watch a demonstration of Hero’s engine
The Drinking Bird
The classic toy the drinking bird exemplifies a thermal engine described by the Rankine cycle. (Rankine cycles describe the operation of steam heat engines commonly found in power generation plants.) The toy consists of two hollow glass spheres connected by a hollow tube. The upper sphere, the head of the bird, is coated with felt. The bottom sphere is partially filled with a liquid, methylene chloride, which evaporates very rapidly. The tube extends into the liquid and provides a passageway between the two spheres.
The engine is set into operation by moistening the head with water. As the water evaporates, the head is cooled. The vapor pressure of the methylene chloride within the head decreases as it is cooled. The greater vapor pressure in the lower sphere pushes the liquid up the tube, thus shifting the bird’s center of gravity. Tipping downward, the bird’s felt-covered beak enters water. As the bird dunks, the head is re-moistened, the liquid returns to its tail, and the process begins again.
The Mysterious Rubber Band Heat Engine
Most materials expand when heated. Rubber, being a rather peculiar substance, contracts when heat is applied. As the figure below illustrates, this may be demonstrated by heating a rubber band that supports a mass.
Exploratorium Science Snacks http://www.exploratorium.edu/snacks/
Let’s Build a Can Stirling Engine http://www.bobblick.com/techref/projects/stirling/can/can.html
Thermodynamics: Building Simple Heat Engines http://sci-toys.com/scitoys/scitoys/thermo/thermo.html#hero
Thermodynamics Demonstrations http://buphy.bu.edu/~duffy/thermodynamics.html
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