Otterbein University Department of Physics
Physics Laboratory 1500-12
EXPERIMENT 1500-12
LATENT HEAT, HEAT TRANSFER, AND CALORIMETRY
NAME:
SAFETY
Safety note #1: In this lab you will be working with liquid nitrogen. Liquid nitrogen can cause
severe freeze-burns, so you should handle it with care. Actually, you can come into contact with
liquid nitrogen for a short time without harm, because your body is so hot that the liquid will boil
when you touch it, forming an insulating layer of vapor (called a Leidenfrost layer after the
physicist who first studied it). You can demonstrate this effect by spilling a few drops of liquid
nitrogen  drops of liquid will float across the table on the layer of vapor. (You can do the same
thing with water and a hot pancake griddle.) What you should not do is touch solid objects that
have been immersed in liquid nitrogen, because such objects will not produce an insulating vapor
layer if you touch them. They may even freeze to your skin. Similarly, avoid getting liquid
nitrogen on rings or other jewelry or on your clothing. Don’t handle LN2 if you are wearing
shorts or sandals, and avoid sloshing or spilling it.
Safety note #2: Thermos bottles are designed to withstand rapid changes in temperature, but not
as extreme as those you will get with liquid nitrogen. Occasionally, one will break. You should
wear glasses or safety goggles when you work with an open thermos of liquid nitrogen.
INTRODUCTION
In this lab you will measure the latent heat of vaporization of liquid nitrogen and the heat
capacity of a metal object. You will also test the theory of heat transfer.
Liquid nitrogen we have in the lab sits at a temperature of 77 K, for the same reason that boiling
water will be at 100ºC – during a phase transition, putting heat into the substance doesn’t
increase the temperature, it changes the phase.
To measure the latent heat of liquid nitrogen, you will supply a known amount of heat to the
liquid nitrogen in your thermos bottle, and measure the change in mass due to the evaporation of
the nitrogen. Because the thermos bottle is not perfectly thermally insulating, some liquid
nitrogen will evaporate due to heat that leaks through the sides and top. To correct for this
effect, you will measure the rate at which nitrogen evaporates due to the heat leak.
The thermos bottle was invented by Sir James Dewar in the late nineteenth century. (For that
reason it is more formally known as a Dewar flask, or simply a Dewar.) The walls of the Dewar
flask are made of two layers of glass with vacuum in between, to reduce heat leaks due to
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Otterbein University Department of Physics
Physics Laboratory 1500-12
conduction. To reduce the heat leak due to radiation, the surface of the glass is coated with
silver. Since you have measured the heat leak, you can estimate how good the silver coating is at
reflecting radiation.
Finally, using the known latent heat of liquid nitrogen, you can measure the heat capacity of a
metal sample by measuring the amount of liquid boiled off when you put the metal in the
nitrogen.
HEAT GAINED BY NITROGEN + HEAT GAINED BY METAL = 0
PART 1 - RESIDUAL LOSS
Fill your thermos with liquid nitrogen. Put it on your scale. Once the reading is stable, measure
the mass of the thermos with nitrogen. Wait about 5 minutes, measuring the time with your
stopwatch, then measure the mass again. Read ahead while you wait for this.
Mass of thermos + nitrogen at start: ________________
Mass of thermos + nitrogen after: _________________
Duration of wait: _____________
Use this to find how much nitrogen boils off due to stray heat getting into the thermos.
Residual loss of nitrogen = __________ g/min
PART 2 - LATENT HEAT OF VAPORIZATION
In this section, we will find how much heat it takes to turn a
gram of nitrogen into a gram of vapor.
Set up the circuit shown below, using one multimeter as a
voltmeter, and the other as an ammeter. Be sure not to touch any
bare wires when the power is on!
The resistor is the large brown tube with orange wires soldered
to it. This is a “power resistor” that can dissipate a lot of heat. If
you are unsure of your circuit, have the instructor check it
before you turn it on.
Once your circuit is set up, switch it on. Turn up the voltage and
current knobs on the power supply until you get about 0.3 to
0.4A of current. Be careful of the resistor, as it will get hot.
Turn off the circuit.
Carefully lower the resistor into the liquid nitrogen. Be sure
that the resistor is completely submerged (ideally on the
bottom). It may be convenient to tape the wires to the thermos
bottle.
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Otterbein University Department of Physics
Physics Laboratory 1500-12
Wait until the nitrogen stops boiling.
Measure the mass of the nitrogen plus resistor: _______________
Now, turn on the power and start a stopwatch. Nitrogen should start to boil off. While it is
boiling, note the current and voltage. Boil off nitrogen for about 3 minutes. When finished, turn
off the power and wait a few seconds for the boil to stop. Record the time the power was on and
the final mass.
Current: __________
Voltage: __________
Duration of boil: __________
Record the mass of the nitrogen plus resistor again: ______________
Find the mass of nitrogen that boiled away, Δm. Correct this for the residual losses measured in
part 1. (That is, some of the nitrogen boiled away was NOT due to the resistor, but air currents,
etc. Find out how much and subtract it off.)
Remember that the power dissipated by a resistor is P = IV. Find the total heat ΔQ that was put
into the nitrogen. Use this to find the latent heat of vaporization, defined as LV = ΔQ/Δm. Check
with your instructor that the result is reasonable.
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Otterbein University Department of Physics
Physics Laboratory 1500-12
PART 3 - SPECIFIC HEAT CAPACITY
In this section, we will find the heat capacity of the metal object - how much heat we need to
apply to one gram of metal to change its temperature by one degree.
Take out the resistor. If required, top up your thermos bottle.
Measure the mass of the metal slug provided. Slug mass = ________________
Measure the mass of the thermos bottle: _____________
Now, carefully and slowly lower the metal slug into the bottle. Use a stopwatch to see how long
nitrogen boils off.
Boil duration: _______________
Now measure the final mass of the thermos bottle and slug: _______________
(Notice that the nitrogen boils most violently just before it stops. Can you explain this?)
Find the mass of nitrogen that boiled off. Correct for residual losses.
Find how much heat the nitrogen took from the metal slug, by using the LV you find in Part 2.
Find the specific heat capacity of the metal, cp, by using the expression ΔQ = cp m ΔT
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Otterbein University Department of Physics
Physics Laboratory 1500-11
Identify the material of the metal slug from the table.
How much does your answer deviate from the one in the table?
Metal
Specific Heat
Capacity cp - J/g K
Metal
Specific Heat
Capacity cp - J/g K
Aluminum
0.91
Osmium
0.13
Antimony
0.21
Platinum
0.13
Beryllium
1.83
Plutonium
0.13
Bismuth
0.13
Potassium
0.75
Cadmium
0.23
Rhodium
0.24
Carbon Steel
0.49
Selenium
0.32
Cast Iron
0.46
Silicon
0.71
Chromium
0.46
Silver
0.23
Cobalt
0.42
Sodium
1.21
Copper
0.39
Tantalum
0.14
Gold
0.13
Thorium
0.13
Iridium
0.13
Tin
0.21
Iron
0.46
Titanium
0.54
Lead
0.13
Tungsten
0.13
Magnesium
1.05
Uranium
0.12
Manganese
0.48
Vanadium
0.39
Mercury
0.14
Zinc
0.39
Molybdenum
0.25
Wrought Iron
0.50
Nickel
0.54
Niobium
(Columbium)
0.27
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