PASCO ET-8782 Energy Transfer– Thermoelectric - Manual (Page 30)

Energy Transfer –Thermoelectric Teachers’ Notes–A Model Refrigerator
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Air Flow and Heat Transfer
∆T = 2.6 °C
Q = (0.082 mol) [29.1 J/(mol·°C)] (2.6 °C) = 6.2 J (every second)
(Step 7) This estimate is likely to be high because we are measuring the air that goes straight
through the heat sink. Much of the air from the fan misses the heat sink, so the average
temperature rise for all of the air from the fan would be less than 2.6 °C.
P = (0.60 A) (7.1 V) = 4.3 J/s
(Step 8) The energy supplied to the peltier every second is less than the estimate of energy
transferred to the air by the heat sink. According to conservation of energy, they would be the
same if all of the heat lost by the system were transferred to the air through the heat sink. In fact,
some heat is lost through radiation, and through other parts of the system. It is likely that most of
the discrepancy between Q and P is due to error in the estimate of Q.
Insulator, Fan and Heat Sink
(Step 9) With the cold block at 5 °C (or 18 °C below room temperature) the model represents an
effective refrigerator.
(Step 10) With the insulator removed, the temperature of the cold block increases due to increased
heat flow from the air to the block.
(Step 11) With the fan turned off, the temperature of the hot block increases because the rate of
heat transfer to the air decreases. The temperature of the cold block increases at a similar rate.
The temperature difference between the blocks increases from 41 °C to 47 °C within 3 minutes of
the fan switching off, after which the difference decreases slowly.
As the hot block gets hotter and the temperature difference between the blocks increases, the
tendency for heat to flow from the hot block to the cold block by conduction increases, canceling
the heat-pumping effect of the peltier.
(Step 12) When the fan is turned off the temperature change of the air flowing through the heat
sink increases to about 10 °C. Since the hot block gets hotter, it is evident that the rate of heat
transfer to the air has decreased. The increased temperature change is due to the decreased air
flow.
(Step 13) With the fan turned off, the “cold” block would stabilize at about 40 °C. That is higher
than room temperature, so it would not be a good refrigerator.
(Step 15) The cold block corresponds to the interior of a real refrigerator.
(Step 16) A refrigerator makes the interior cold by pumping heat out of it. It needs insulation to
reduce the rate of heat flow back into it from the surrounding air. It needs a heat sink to transfer
away the heat that it has pumped out of the interior, and the heat resulting from the work that it
does.