3B Scientific Heat Equivalent Apparatus - Manual (Page 2)

2
2. Description,
The equivalent of heat apparatus can be used to show
the equivalence of mechanical work due to friction
(Nm), electrical energy (Ws) and heat (J). The values
measured in Nm or Ws agree to an accuracy of about
2%. If this equivalence is assumed, the specific heat
capacity of aluminium and copper can be determined.
The stable design with its integrated rotary counter and
a dual ball-bearing mounted shaft make experiments as
simple as possible to perform. To measure temperature
a negative temperature coefficient thermistor (NTC) is
used. This is safely contained inside an aluminium
sleeve. The aluminium sleeve snaps into the friction
cylinder so that it cannot slide out unintentionally.
3. Technical data
Technical data for the friction cylinder (approximate
values):
Diameter D: 48 mm
Height: 50 mm
Aluminum cylinder: mass m
A
= 250 g,
specific heat capacity
c
A
= 0,86 kJ/kg K,
Copper cylinder: m
K
= 750 g, c
K
= 0,41 kJ/kg K
Electrical connection: sockets of 2 mm diameter,
positive pole “+” isolated,
negative pole “–” connected
to ground, reversal of polarity
does not destroy the equip-
ment
4. Operation
• The equivalent of heat apparatus is attached to a
stable workbench using its table clamp. The friction
cord is then wrapped around the friction cylinder
4½ to 5½ times with the counterweight suspended at
the rear and the loose end of the cord hanging
down at the front.
• The bucket provided can be filled with water or
sand etc. (total weight approx. 5 kg) and used as a
weight. The loose end of the friction cord is at-
tached to the weight while the latter is resting on
the ground. It should be observed that the counter-
weight should be no more than about 5 cm above
the ground when the cord is taut. This prevents the
weight being raised by more than about 10 cm dur-
ing the experiment.
• If it is observed that the cord moves to the right
when the crank is turned or fails to remain in its
groove, then the cord should be wrapped around
the cylinder so that the end of the cord with the
weight is on the right and that with the counter-
weight is on the left.
• The temperature sensor should be wetted with a
drop of oil (important!) and inserted into the se-
lected friction cylinder according to Fig. 1 until it is
felt to snap into place and can be turned easily (if it
is inserted too far or not far enough, it is not easy to
turn it). The two connections of the temperature
sensor are attached to a resistance meter (multime-
ter) operating in the range 2 k
Ω to 9 kΩ with a dis-
play accurate to at least three figures. The conver-
sion of the resistance so measured into a corre-
sponding temperature can be performed either with
the help of the conversion table on the last page of
these instructions or by using the following equa-
tion:
151
217
13 0
− =
, R
T (1)
where R must be given in k
Ω to obtain T in °C. This
equation agrees with the table provided by the NTC
thermistor manufacturer in the range from 10 -
40°C to an accuracy of approximately ± 0.05°C.
• Before an experiment the friction cylinder should be
cooled to about 5 - 10°C below the ambient tem-
perature. This can be achieved by putting it in a re-
frigerator or by dipping it in cold water. In the latter
case the hole for the temperature sensor should
point upwards and the cylinder may only be im-
mersed to a depth of about 2/3 the height of the cyl-
inder (tip: if the friction cylinder is dipped in water
inside a plastic bag, it will not need to be dried off
again when it has finished cooling).
• The rise in temperature during an experiment
should continue until the friction cylinder’s tem-
perature has been raised to about 5 - 10°C above
the ambient temperature. The more precisely the
temperature differences for cooling and heating
(with respect to the ambient temperature) are simi-
lar, then the smaller is the net exchange of heat
with the environment.
• For heating the friction cylinder electrically, adapter
cables are provided with plugs of 2 mm diameter at
one end and conventional 4 mm lab plugs at the
other. The power should be provided by a power
supply where voltage and current limiting can be
regulated. The maximum voltage from the power
supply may not exceed 40 V. The positive pole of the
power supply is connected to the isolated socket
(identifiable due to the round, gray plate beneath
the socket) and the negative is connected to the o-
ther socket.
• The heating filaments on the friction cylinders be-
have more or less like normal ohmic resistors with a
resistance of about 11
Ω. Their maximum load ca-
pacity is about 36 W, i.e. for a max. voltage of 20 V