PASCO ET-8782 Energy Transfer– Thermoelectric - Manual (Page 27)

Model No. ET-8782 Experiment 3: A Model Refrigerator
27 ®
Procedure
As you follow this procedure take notes of your observations and write down the answers to the
questions.
1. Put the knife switch in the neutral position (straight up). Set the DC Voltage to about 6 volts.
2. Turn on the fan.
3. Start data recording. Set switch to Heat Pump mode. (Check that the current is not more than
1 amp; if it is, the sensor will beep and you should open the switch, decrease the applied
voltage, then close the switch again.)
4. Observe the temperatures of the hot and cold sides of the peltier device. Which side has the
bigger temperature difference from room temperature? Why are they not the same?
5. Let the refrigerator run in this mode for at least 5 minutes while the temperatures reach
equilibrium. Meanwhile, continue on to the next section.
Air Flow and Heat Transfer
6. Observe the air temperatures below and above the heat sink. By how much does the air
temperature increase when it passes through the heat sink? This increase in temperature is
caused by the heat flowing from the heat sink to the air.
You will now estimate the rate of heat transfer from the heat sink to the air. For a gas, we can write
Q = nc
∆T
where, in this experiment:
Q = heat transferred from the heat sink to the air (in joules),
n = number of moles of air (not the mass),
∆T = change in temperature of the air,
c = specific heat of air.
The specific heat of a gas depends on whether it is heated at constant volume or constant pressure.
In this case the air is heated at constant pressure, so the specific heat is c
air
= 29.1 J/(mol·°C).
The manufacturer's specification for the air flow generated by the fan is about 2 liters per second.
At room temperature, one mole of gas occupies about 24.3 liters, so in one second the quantity of
gas is
7. After the temperatures of the hot and cold blocks have stabilized, calculate the heat, Q,
transferred to the air every second. Is your estimate likely too high or too low? Explain your
reasoning.
The power supplied to the heat pump is
n
2 L
24.3 L/mol
--------------------------- 0.082 mol = =