PASCO ET-8782 Energy Transfer– Thermoelectric - Manual (Page 14)

Energy Transfer –Thermoelectric Conservation of Energy and the First Law of Thermodynamics
14 ®
Q
i
/t = heat flow rate through the insulator,
k = thermal conductivity of the polyethylene foam = 0.036 W/(m·°C),
A = area through which the heat flows,
∆T = temperature difference across the insulator,
x = thickness of the insulating material.
You will estimate the amount of heat that flowed through the foam in contact with the front face
of the cold block.
Measure the height and width of the cavity in the insulator that surrounds the aluminum
block. Calculate the cross-sectional area, A in m
2
.
Measure the thickness, x, of the foam that covers the front face of the block. Do not include the
sides of the foam (you are only calculating the heat flow through the front face). Record your
measurement in meters.
From the temperature graph, determine the difference,
∆T, between the temperature of the cold
block and room temperature. This value changed during the experiment, so record the maximum
difference, when the cold block was at its coldest. This will give you an estimate of the maximum
heat flow rate through the insulator.
11) Calculate the heat flow rate through the foam, Q
i
/t. This is the heat flow rate in joules/second.
To find the total amount of heat in joules, multiply this number by the total time in seconds
that the experiment ran; Q
i
= (heat flow rate) × (time).
12) How does your estimate of Q
i
compare to the heat, Q
cold
, that was pumped out of the cold
block in the Heat Pump phase? Is it much larger, much smaller, or similar?
13) Is your estimate of heat flow through the insulator too high or too low? Remember that you
ignored the sides in your estimate, and that you used the maximum temperature difference for
∆T.
14) How would the flow of heat through the insulator on the hot side compare to heat flow
through the insulator on the cold side? Consider both the magnitude and direction of heat
flow.
15) Is heat flow through the insulators (on the hot and cold sides) a significant factor in this
experiment? Could the heat flow through the insulators account for the discrepancy between
your observed results and the first law of thermodynamics?
16) How would your results have differed if you had not used the insulators?
Further Investigation
What are some factors that you could vary in the experimental apparatus and procedure? Predict
how changing those factors would affect the results. Do an experiment to test one of your
predictions.