PASCO TD-8551A MECHANICAL EQUIVALENT OF HEAT - Manual (Page 5)

012-04331E Mechanical Equivalent of Heat
1
Introduction
Crank
Counter
Mass
(
≅
10 kg)
Aluminum Cylinder
with embedded
Thermistor
Nylon Rope
Figure 1 Mechanical Equivalent of Heat Apparatus
Equipment
Instruction
Manual
Nylon Rope
Mechanical
Equivalent
of Heat
Apparatus
Powdered
Graphite
Rubber Band
Mass
Container
Figure 2 Equipment
MANUAL
The principle of the conservation of energy tells us that if a
given amount of work is transformed completely into heat,
the resulting thermal energy must be equivalent to the
amount of work that was performed. Of course, since work
is normally measured in units of Joules and thermal energy
is normally measured in units of Calories, the equivalence is
not immediately obvious. A quantitative relationship is
needed that equates Joules and Calories. This relationship is
called the Mechanical Equivalent of Heat.
The PASCO scientific Model TD-8551A Mechanical
Equivalent of Heat apparatus allows accurate determination
of the Mechanical Equivalent of Heat (to within 5%). The
apparatus is shown in Figure 1. A measurable amount of
work is performed by turning the crank, which turns the
aluminum cylinder. A nylon rope is wrapped several times
around the cylinder so that, as the crank is turned, the
friction between the rope and the cylinder is just enough to
support a mass hanging from the other end of the rope. This
insures that the torque acting on the cylinder is constant and
measurable. A counter keeps track of the number of turns.
As the cylinder turns, the friction between the cylinder and
the rope converts the work into thermal energy, which raises
the temperature of the aluminum cylinder. A thermistor is
embedded in the aluminum so that, by measuring the
resistance of the thermistor, the temperature of the cylinder
can be determined. By monitoring the temperature change of
the cylinder, the thermal energy transferred into the cylinder
can be calculated. Finally, the ratio between the work
performed and the thermal energy transferred into the
cylinder determines J, the mechanical equivalent of heat.
The TD-8551A Mechanical Equivalent of Heat apparatus
includes the items shown in Figure 2.
➤ IMPORTANT: In addition to the Mechanical
Equivalent of Heat apparatus, several other items are
needed to measure the mechanical equivalent of heat.
These items include:
• Digital Ohmmeter for measuring the resistance of the ther-
mistor in the aluminum cylinder. (An analog meter can be
used, but accuracy will be significantly sacrificed.)
• Refrigerator (or some ice), for cooling the aluminum cyl-
inder below room temperature.
• known Mass of approximately 10 kg which can be sus-
pended from the nylon rope. (The apparatus comes with a
container which can be filled with sand or dirt for the 10 kg
mass; if this is done, you will need an accurate balance for
measuring this mass. Of course, you can fill the container
by adding sand in measured increments of 1-2 kg.)
• Thermometer for measuring room temperature is conven-
ient, though the thermistor can be used for this purpose.
• Calipers and a Balance for measuring the mass and diame-
ter of the aluminum cylinder if you wish these measure-
ments to be part of the experimental process. (Approximate
values are Mass: 200 ± 1.5 grams; Diameter: 4.763 ± 0.02
cm; Diameter including thickness of nylon rope:
4.94 ± 0.05 cm. These values can be used, but there is
some variation, so your results will be more accurate if you
make the measurements yourself.)