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First Law of Thermo

  • I2-43: CONVECTION - HOT PLATE

    I2-43
    See convection currents.
    The irregular refraction patterns created by convection currents in air heated from below are easily seen when light from a point source (foreground) shines through the air over a hot plate and onto a screen. This phenomenon is often seen when the sun shines brightly onto surfaces like cars and roads, and is responsible for the twinkling of stars.
    I0, LS1

  • I2-44: CONVECTION - CANDLE IN CYLINDER

    I2-44
    Demonstrate the mechanism of convection.
    A lighted candle lowered into the graduated cylinder goes out quickly because the buildup of gaseous products of combustion at the bottom of the tube prevents it from getting oxygen. Lowering the smaller tube into the larger graduated cylinder just above the candle flame separates the rising hot air from the falling cold air, allowing convection currents to feed oxygen to the candle flame.
    I2

  • I2-45: CONVECTION - HIGH/LOW CANDLES IN CYLINDER

    I2-45
    Brainteaser regarding convection.

    Two candles, one at the level of the table and one raised approximately 30 cm, are lit and then covered by a tube about 50 cm high and 10 cm in diameter. The tube is sealed at the top by a dark plastic cover to prevent air from flowing into the tube as the experiment progresses.

    Engagement Suggestion:
    • Encourage students to predict which candle will go out first, and why.
    • As the demonstration will typically take 2-4 minutes, discuss other related matters and then check in on the demonstration from time to time; ask students if it is behaving as they expected.
    • When both candles have gone out, ask students to discuss what they saw.
    Background:

    As the candles burn, the hot gases composing the products of combustion will be less dense than the cooler original air, and will rise to the top of the tube. The upper candle will therefore be extinguished sooner than the lower one.
    I2

    ii2-45ai2-45bi2-45ci2-45d

     

     

    Two candles, shown in the photograph at the left below, are lit and then covered by a plastic tube (sealed at the top), as seen in the photograph at the right below. In the apparatus as pictured the tube is about 50 cm tall and 10 cm in diameter, and the upper candle is about 30 cm from the bottom.

     

    i2 45 i2 45a

     

    After some period of time, as the oxygen in the tube is consumed by the candle flames, the candles will both cease to burn. The question involves the order in which the candles will go out.

    Which of the following statements is true?

     

     

    • (a) The top candle will go out first, then the bottom candle.
    • (b) The bottom candle will go out first, then the top candle.
    • (c) Both candles will go out at the same time.

     

  • I3-32: ISOBARIC EXPANSION OF AIR

    I3-32
    Demonstrate expansion of air without change in pressure.
    Air is isolated in the can by a plastic bag sealed over the mouth. If the can is heated by the hot plate the air will expand to fill the plastic bag. If the can is removed from the hot plate and the fan blows cool air over the can the plastic bag will again become evacuated.

  • I3-35: SOLAR BAG

    I3-35
    To demonstrate how the density of a gas changes with temperature.
    This is a large bag that will float when the air inside is heated. On a sunny but cool day, unroll the solar bag outside in the shade and fill it with cool air. Tie off the open end. Tie the string to the tied-off end, and move the bag into direct sunlight. The solar bag will soon float as the air inside heats up and expands. Obviously, this demonstration is primarily suited to outreach programs held out-of-doors, not to classroom use.
    I3

  • I4-03: LATENT HEAT - ICE TO WATER TO STEAM

    I4-03
    Show latent heat as ice is transformed to water and then to steam.
    A flask is filled to within one inch of the brim with a mixture of water and ice cubes at the freezing temperature of water. The flask is then heated for about 15 to 20 minutes with the burner on high, with the temperature measured by the dial thermometer. If you were to create a plot of temperature as a function of time, it would clearly show that extra heat is required to produce the ice-water and water-steam phase transitions.
    I0

  • I4-35: LOWERING THE FREEZING POINT OF WATER USING SALT

    I4-35
    Demonstrate that the freezing point of water can be reduced by putting ice in a salt water bath.
    The temperature of an ice and water mixture is measured with a digital thermometer. Adding salt to the icewater mixture reduces its temperature. The thermometer can be used to stir the mixture. By this technique the freezing point can be lowered as much as 15 degrees celcius.
    I4

    i4-35a

  • I5-02: TRANSFORMATION OF MECHANICAL ENERGY INTO HEAT

    I5-02
    Demonstrate transformation of mechanical energy into heat.
    Use an electric drill to spin a wooden dowel rod in a hole on a large wooden beam. Shortly the contact point begins to smoke, indicating generation of heat due to friction.
    I3,I5

    i5-02a

  • I5-03: MECHANICAL EQUIVALENT OF HEAT - JOULE'S METHOD

    I5-03
    Determine the mechanical equivalent of heat.
    Turning the handle with weights hanging from a cord wrapped around the copper cylinder requires a calculable amount of mechanical energy. This energy is converted into heat in the copper cylinder and the water bath, raising their temperature. The constant of proportional, which is the mechanical equivalent of heat, can be calculated using these two measurements. The device counts turns, so you can continue to lecture while cranking, typically 100-150 turns. The result is generally good to better than ten percent. (Note that when you turn the handle the heavy weight should be lifted off the floor, so the net frictional force causing the heating of the cylinder is the difference between the weights on the two ends of the cord.)
    I5, I0

    i5-03a

  • I5-11 ADIABATIC PROCESS - AIR PISTON WITH THERMISTOR

    I5-11
    Demonstrates adiabatic compression and expansion of air
    A thermister is enclosed in a small cylinder of air, the volume of which can be rapidly changed by moving a piston up and down. Pushing the piston down compresses the air, the air heats and the temperature increases, producing an increase in the resistance of the thermistor. Pulling the piston up expands the air adiabatically, the air cools and the temperature decreases, producing a decrease in the resistance of the thermistor. The thermistor is identical to those used in the thermometer probes of the old commercial digital thermometer.
    I5, I0

  • I5-12: ADIABATIC EXPANSION OF AIR - FOG IN BOTTLE

    I5-12
    Demonstrate adiabatic cooling of air
    A bottle containing a small amount of water is sealed by a stopper. A tube passes through the stopper and is attached to a bicycle tire pump which is pumped to pressurize the bottle. When the pressure rises to a sufficient level it blows the stopper off the bottle, allowing the air to expand adiabatically and cool. The cooling process condenses water vapor in the bottle, producing fog. If the stopper is quickly replaced and the pressure in the bottle increased, the air becomes warmer and the fog disappears. A small lamp illuminates the bottle to make the fog very visible, even in a large room.
    I7, LS2

  • I5-13: ADIABATIC EXPANSION OF AIR - GRAPH OF TEMP

    I5-13
    Demonstrate adiabatic expansion of air by plotting the temperature.
    A tightly corked bottle containing air with a small amount of water is pressurized using a bicycle tire pump. When the cork pops the air expands adiabatically, cooling sufficiently to form fog in the bottle. The computer plots a graph of the air temperature in the jar using a thermistor probe, as seen in detail below. As air is pumped into the jar the temperature slowly rises; when the cork blows the temperature suddenly falls to below its initial level. The graph can be displayed on video projectors in the lecture hall.

    i5-13a

  • I5-15: ADIABATIC EXPANSION OF CARBON DIOXIDE

    I5-15
    Illustrate adiabatic cooling by producing dry ice
    Carbon dioxide, leaked slowly out of the fire extinguisher onto a black felt cloth, produces dry ice, which can be easily seen. Adiabatic expansion and cooling occur when the CO2 gas comes out of the nozzle under high pressure and expands in the atmosphere. Enough is produced to pass the cloth around the class so that students can feel that it is actually cold. This experiment is a bit more complicated than simple adiabatic expansion. The carbon dioxide actually exists in the fire extinguisher as a liquid, so that much of the cooling is due to the evaporation of the liquid CO2 before it is ejected from the nozzle.
    FS1

  • I5-21: HEATING AIR BY COMPRESSION

    I5-21
    Demonstrate heating air by compression.
    A few pumps of the tire pump into a mostly filled basketball warms the end of the pump noticeably. You can show that this is not due to friction by moving the pump handle back and forth in the same style with no load. You can simultaneously demonstrate cooling by expansion by observing that while the pump and the needle get rather warm, the air inside the ball actually cools.
    I5

  • I5-22 FIRE SYRINGE

    I5-22
    Demonstrates heating air by compression

    This demonstration consists of a transparent cylinder with a flared base, and a plunger that can be pushed into it. A small (very small) piece of cotton is pushed into the bottom of the tube using the wire provided, and the plunger is sealed into the tube. The plunger is pushed down sharply, compressing and thereby heating the air within. The temperature rises high enough to ignite the cotton with a flash, which can be readily seen through the plastic tube.
    Engagement Suggestion
    • Consider inviting a volunteer from the audience to try the demonstration. This will require careful supervision, but is safe. Just ensure that the syringe isn't knocked off the table by an overenthusiastic student!
    • This demonstration works best with a very small amount of cotton to ignite, no more than a few millimeters at most. Consider showing the device with different amounts of cotton, and how the results change. Encourage students to discuss reasons for this.
    Background
    This demonstration illustrates that an essentially fixed mass of air will increase in temperature when its volume is reduced, i.e. it is heated when compressed. The fire syringe is a simple piston, and can be used to introduce a discussion of the use of pistons in engines.

    Consider using this demonstration in conjunction with both other thermodynamics demonstrations from section I5, and relating it back to general gas behaviour with demonstrations from section I3.
    I5

  • I5-51: SPECIFIC HEAT - ALUMINUM AND COPPER

    I5-51
    Illustrate calorimitry and to determine experimentally the specific heats of aluminum and of copper.
    Boil the aluminum and copper blocks in a pan of water and place the metal blocks at 100 degrees C into equal amounts of water at room temperature in two styrofoam containers. Stir the water in the containers with the digital thermometer probe, measuring the temperature when equilibrium is reached. From these measurements the specific heats can be determined.

    Multiply the specific heat by the molar mass to find the molar specific heat. The values for both materials are nearly the same and equal to 3R=6(1/2)kT.
    I5, I0, ME1

  • K6-22: ENERGY CONVERSION - IMMERSION HEATER

    K6-22
    Demonstrate quantitatively the conversion of electrical energy into heat.
    This 300-watt immersion heater is used to heat approximately 300 ml of water in a borosilicate beaker. Measure the initial water temperature with a digital thermometer, allow it to heat for a fixed time, then measure the final temperature. Compare the temperature change calculated for the energy conversion (as per Q=mcT where ! is the energy transferredm m is the mass of water, c is the specific heat, and T is the change in temperature) to that measured, and invite students to talk about the meaning of the difference (heat loss through the sides of the beaker, etc.).

    Note that the heater will (obviously) get hot! Do not allow it to burn your hand or the power cord.
    K6, I0