PASCO ET-8782 Energy Transfer– Thermoelectric - Manual (Page 13)

Model No. ET-8782 Experiment 1: Conservation of Energy and the First Law of Thermodynamics
13 ®
W = Q
hot
– Q
cold
4) Compare your observed value of work, W
observed
(which is the area under the Power vs. Time
plot) to the quantity Q
hot
– Q
cold
. Are they equal?
5) In a real heat engine, only part of the heat that flows out of the two-reservoir system
(Q
hot
– Q
cold
) is converted to useful work. In this experiment, the work that you observed (the
useful work) was the work done on the load resistor. Can you account for all of the energy
that flowed out of the hot reservoir with your values of W
observed
, Q
hot
and Q
cold
? If not, where
did the “lost energy” go?
6) Calculate the proportion of net heat flow from the aluminum blocks that was converted to
useful work;
7) Write an equation in terms of the “lost energy”, E
lost
, and your observed data, W
observed
, Q
hot
and Q
cold
.
8) In this experiment the “useful work” was the work done on the load resistor. What was the
result of doing work on the resistor? How could you modify the circuit in order to make better
use of the work done by the heat engine?
Conservation of Energy
In the Heat Pump phase of the cycle the power supply put energy into the system. Then, in the
Heat Engine phase heat flowed out of the hot reservoir and part of it was converted into electrical
energy, which was supplied to the load resistor.
9) Calculate the percentage of energy put in during the Heat Pump phase that was recovered as
useful work during the Heat Engine phase;
10) Is this a good way to store energy?
Conduction and Heat Flow Through the Insulators
One of the losses of energy in this experiment has to do with heat flow by conduction through the
polyethylene foam insulators. The rate of heat flow through the insulator is
where:
% of useful work
W
observed
Q
hot
Q
cold
–
---------------------------- 100 %
×
=
% recovered
energy generated
energy put in
----------------------------------------- 100 %
×
=
Q
i
t
⁄
kA
T
∆
x
------- =