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First Law of Thermo

  • I2-01: CROOKES' RADIOMETER

    I2-01
    Stimulate discussion about radiative heat transfer and conservation of momentum with photons.
    A match or other source of light is brought near the radiometer, resulting in rotation of the vanes. The REAL reason has to do in a very important way with details regarding how molecules interact with each other. The explanation is not nearly as simple as the difference in the momentum of photons when they are absorbed or reflected, or even as simple as the heating effect on the black side, which absorbs more photons, compared with the white side, which reflects more. This appears to be one of those physics devices that is typically explained incorrectly, even in the literature from the supplier that accompanies the radiometer.
    I2

  • I2-03: CROOKE'S RADIOMETER - ROTATION REVERSAL

    I2-03
    Counterintuitive demonstration of Crookes' radiometer designed to make students understand radiation better.
    Heat the Crookes' radiometer with the heater for until it spins very rapidly. When the heater is removed the spinning first ceases then reverses. When the heater is removed, the black side cools faster than the white side. When the temperature of the black side becomes sufficiently below that of the white side the roles are reversed from normal in heating adjacent air, causing more momentum transfer to the white side and rotation in the direction of the black side.
    I2, PS1

  • I2-04 WIEN'S LAW OF THERMAL RADIATION

    I2-04
    Shows that higher temperature blackbodies radiate with shorter wavelengths

    A variable transformer, or Variac, is connected to two identical incandescent light bulbs in parallel. These bulbs are viewed behind red and blue filters respectively. As the voltage is increased by the variac, the lights glow more brightly, and more light is seen through the blue filter relative to that of the red filter. Very little or no blue is seen at low voltages, whereas red is seen to be emitted even at very low voltages.

    Engagement Suggestion:
    • Ask students to compare this to other phenomena that emit light. Where else do you se this change of color with temperature?
    Background:

    Wilhelm Wien postulated in the 1890s that the power curve of blackbody radiation from an object could be computed from its temperature. His original calculations, obviously, did not take quantization into account; in modern practice, the revised calculations are still commonly referred to as Wien's Law.

    Note that this apparatus only works with incandescent lightbulbs. Fluorescent and LED bulbs do not produce their primary light through thermal excitation, and thus don't produce the same kind of blackbody spectrum.
    I2, PS1

  • I2-05: LESLIE'S CUBE

    I2-05
    Illustrate radiation from various surfaces.
    When the cube is heated with boiling water poured into the funnel to fill the cube, each of the six sides of the cube radiates at the same temperature. The amount of radiation emanating from each side is only a function of its surface. Surfaces include (in order of radiative efficiency): black fabric, gray "fabric," white paint, polished stainless steel, flat aluminum, and polished brass.

  • I2-06 THERMOPILE WITH AUDIO OSCILLATOR

    I2-06
    Observe infrared radiation
    The output from a commercial thermopile is connected to an audio oscillator (as in N1-05) such that the frequency of the oscillator is proportional to the temperature observed: the hotter the object the higher the pitch. Use various sources: ice, boiling water, liquid nitrogen, the floor, people, etc. This is only qualitative; the system is not calibrated.
    N1, I2, PS1

  • I2-07: THERMOPILE WITH DVM

    I2-07
    Observe infrared radiation.
    The output from a commercial thermopile is connected to a digital voltmeter where the voltage is proportional to the temperature observed: the hotter the object the higher the voltage. Use various sources: ice, boiling water, liquid nitrogen, the floor, people, etc. This is only qualitative; the system is not calibrated.
    N1, ME2, I2, PW1

  • I2-08 RADIATIVE HEAT TRANSFER

    I2-08
    Shows radiation from a hot object
    As more voltage is applied to the heater it glows more brightly and emits more heat
    I2, PW1

  • I2-09 DEWAR - TRANSPARENT WITH LIQUID NITROGEN

    I2-09
    Demonstrates a dewar
    The dewar contains liquid nitrogen, which can be seen as a clear liquid. Various experiments using liquid nitrogen can also be performed
    I2, I0

  • I2-10: DEWARS - SILVERED AND UNSILVERED

    I2-10
    Illustrate the function of silvering a dewar
    Hot water is put into two dewars, one unsilvered and one silvered. The temperature of both in centigrade degrees is monitored as time passes; the photographs above show the temperatures immediately after the water is poured into the dewars and about thirty minutes later.
    I2, I0

  • I2-11: THERMOS BOTTLE

    I2-11
    Show the parts of a standard thermos bottle.
    An old beat-up thermos bottle can be taken apart to demonstrate what the parts look like for a discussion of how thermos bottles work
    I2

    i2-11a

  • I2-12: RADIATION FROM COLD OBJECT

    I2-12
    Show radiation from a cold object
    If you put a hot object at the focus of one of the concave parabolic mirrors and a thermal probe at the focus of the other mirror, heat from the hot object will heat up the probe, yielding a temperature rise of the thermometer. (Compare the top and center pictures above.) If you put something very cold at the first focus, the temperature will drop. (Compare the top and bottom pictures above.) This demands a rather different explanation - blackbody radiation emitted by all objects - than the rather simple explanation given in the case of the hot object.

    This experiment demands the proper explanation in terms of blackbody radiation emitted by all objects, not just "hot" objects. The historical struggle of physicists to deal with this is documented in an interesting article by Hasok Chang, Lecturer in Philosophy of Science at University College, University of London, entitled Rumford and the Reflection of Radiant Cold: Historical Reflections and Metaphysical Reflexes, in Physics in Perspective Volume 4 Issue 2 (2002), pp 127-169.

    Note that this experiment uses materials from I5-51 and L3-16. If you want to use those demonstrations in the same class, be sure to discuss logistics with Lecture-Demonstration staff in advance.
    I2, I0, I5, L3

    I2-12A

  • I2-21 THERMAL CONDUCTIVITY IN METALS

    I2-21
    Demonstrates thermal conductivity in various metals
    Heat from a gas burner at the center is conducted along rods of copper, aluminum, and brass. Wax blocks at the ends of the rods melt and drop off the rods due to the conduction of heat, in the following order: copper (3.98 Watts/cm deg C), aluminum (2.37 Watts/cm deg C), and brass (1.23 Watts/cm deg C).
    I2, I0

  • I2-22 THERMODYNAMICS BY TOUCH

    I2-22
    Demonstrates that touching a material tells something about its conductivity, not necessarily its temperature
    Various materials, all at room temperature, are arranged on a cart, and students are invited to touch them. The materials in order of increasing conductivity, are: styrofoam, wood, plastic, slate, steel, aluminum, and copper.
    I2

  • I2-24: THERMAL CONDUCTIVITY IN WATER

    I2-24
    Demonstrate that water is a poor conductor of heat.
    An immersion heater placed at the top of a dewar of water causes the water near the top of the dewar to boil. However, heat is not readily conducted through the water to the bottom, and the bottom remains at a much lower temperature even 15-20 minutes after the water on top begins to boil.

  • I2-26: LEIDENFROST PHENOMENON

    I2-26
    Demonstrate the Leidenfrost effect.
    Turn on the hot plate to high for about two minutes to pre-heat the aluminum sheet/skillet. Then squirt a few large drops of water (with green food coloring to make it visible) onto the aluminum skillet. The water forms drops which skitter around on the hot plate for an unexpectedly long duration, because of an insulating layer of water vapor (steam). Big drops can be created which will persist for a minute or longer, while executing interesting oscillations. This is known as the Leidenfrost phenomenon.

    i2-26a

  • I2-27: THERMAL EQUILIBRIUM BETWEEN ALUMINUM AND COPPER

    I2-27
    Show process of thermal equilibrium happening between touching aluminum and copper cylinders.
    Pieces of copper and aluminum are held together by a large C-clamp. Small holes are drilled into the pieces to a allow a digital thermometer probe to be inserted to measure the temperature of each block, showing that the blocks are initially the same temperature, at equilibrium. Remove the thermometer probes and put a flame under one block to create a temperature difference. Remove the flame, reinsert the probes, and watch as the blocks come to thermal equilibrium.
    I2, I0, tools

  • I2-28: WATER BALLOON AND CANDLE

    I2-28
    To demonstrate the transfer of heat by water
    A balloon filled with water is held above a candle flame. Contrary to most students' expectations, the balloon does not burst. The water in the balloon conducts heat away from the rubber before it can melt.
    I0

  • I2-29: Thermal Conductivity - Metal Bars and Liquid Crystals

    I2-29
    To show the different rates of heat conduction in several metals
    This demonstration consists of a series of bars of different metals, with temperature-sensitive liquid crystal strips attached to each. When the tips of the bars (and only the tips) are lowered into a beaker of hot water, the liquid crystal strips will change colour at different rats, showing the different rates of heat conduction of the different metals.

    Note: Use water no hotter than 90C. Do not expose to open flame, and do not apply water or heat to the liquid crystal strips, and keep them out of direct sunlight.
    I2

  • I2-41: CONVECTION - POWDER IN WATER

    I2-41
    Illustrate convection.

    Heat one side of the tube, and the water will rise on that side by convection, carrying the powder, which makes the convection visible. To avoid overheating and destroying the apparatus, heat only for about ten seconds in a blast. Before beginning, rotate the entire apparatus so that the powder is uniformly distributed.

    A video camera may used to enlarge the action in the lecture halls.

    Background:

    The isolated heat source produces convection currents through the apparatus. Heated water rises up one side of the loop, drawing powder with it, while cooler water is drawn down the other side.
    I2

    i2-41a

  • I2-42: FALLING CANDLE

    I2-42
    Demonstrate how a flame burns in the absence of normal convection.
    A candle, attached to the lid of a one-gallon jug, is lit and the lid screwed onto the upside-down jug. Throw the upside-down jug into the air and catch it or hold the upside-down jug high and drop it and catch it as it falls. While it is falling, the system inside the jar is in a weightless environment, so convection currents cease. In normal burning, the hot air rises by convection, allowing cooler air containing more oxygen to continuously feed the fire. Without these convection currents the candle should immediately go out, BUT IT DOES NOT.
    I2

    A candle mounted on the lid of a gallon jug is lit, and the lid quickly affixed to the jug. In this configuration the candle will remain lit for over one minute before the oxygen in the jug is sufficiently used up by the combustion process and the flame is extinguished.

    Now suppose that the candle flame is lit and the lid again quickly affixed to the jug. However, the bottle is now dropped about six feet starting from the orientation shown in the photograph below.

    i2 42

    What will happen? In particular, by the time the jug falls six feet the candle flame will:

    • (a) burn more brightly.(b) remain at about the same brightness.
    • (c) burn less brightly.
    • (d) go out.