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

3
and corresponding current of roughly 1.8 A. To set
an operating point, it is recommended that the cur-
rent limit be set to exactly 1 A with voltage limited
to about 15 V. These settings cannot be altered
thereafter. Power is disconnected simply by remov-
ing the power lead until needed for the experiment.
5. Maintenance
• The equivalent of heat apparatus in principle re-
quires no maintenance. It can be wiped clean with
soap and water. Solvents should not be used. Im-
mersion in water should also be avoided.
• The friction cylinders should be plain naked metal.
If a coating has formed on them, this can be re-
moved using metal cleaner.
• The friction cord can be washed if necessary. For a
good value alternative, woven nylon cord can be u-
sed as a replacement.
6. Experiment procedure and evaluation
6.1 Conversion of mechanical work into heat
6.1.1 Experiment procedure
• First the various masses are measured:
Primary weight (e.g. bucket with water) m
H
= 5.22 kg
Counterweight (at friction cord) m
G
= 0.019 kg
Aluminium cylinder m
A
= 0.249 kg
• Other values to be measured in advance:
Ambient temperature T
U
= 23.2°C
Diameter of cylinder where friction occurs
D
R
= 45.75 mm
• After cooling the cylinder, it should be screwed to
the base, the temperature sensor should be inserted
and the friction cord should be wrapped around it.
(cf. Section 4). After a few minutes, that should be
ignored for the sake of a homogenous temperature
distribution, the resistance of the temperature sen-
sor is R
1
= 8,00 k
Ω (corresponding to T
1
= 14,60°C by
Eq.1).
• After zeroing the counter, the experiment is begun
by turning the crank and thus lifting the primary
weight from the ground. This slightly loosens the
cord so that it causes less friction on the cylinder.
The primary weight remains at the same height and
should remain there for the rest of the experiment.
• After n = 460 turns the experiment is halted and the
resistance value read off: R
2
= 3.99 k
Ω (T
2
= 30.26°C).
Since the temperature continues to rise for a short
time after the experiment is completed (homogeniz-
ing the temperature distribution), the minimum
value of the resistance is noted as the measured
value. This is reached a few seconds after the end of
the experiment. After that the resistance increases
again since heat is exchanged with the environment
to cool the cylinder down to a lower temperature.
6.1.2 Experiment evaluation
Work W is defined as the product of force F and dis-
placement s
s F W
⋅ =
(2)
The force of friction acting is
g m F
A
⋅ =
(3)
(g is the acceleration due to gravity) in the direction of
the displacement
R
D n s
⋅ π ⋅ =
(4)
• Placing Equations 3 and 4 into Equation 2 gives:
R A
D n g m W
⋅ π ⋅ ⋅ ⋅ = =
= ⋅ ⋅ ⋅ ⋅
04575 0 1416 3 460 81 9 22 5 , . . . 3386 Nm (5)
The heat stored in the friction cylinder
ΔQ is determined
from the temperature difference (T
2
– T
1
) and the specific
heat capacity given in Section 3:
( )
= − ⋅ ⋅ = Δ
1 2
T T m c Q
A A
( )
= − ⋅ ⋅
KJ 60 14 26 30 249 0 86 0 . . . . 3353 J (6)
In this example the disagreement between the mechani-
cal work and the heat energy is found to be no more
than about 1%. Due to unavoidable tolerances relating
to the composition of materials (aluminium is very soft
and almost impossible to work mechanically, so that it is
always alloyed), the specific heat capacity can fluctuate
quite noticeably. The specific heat capacity is most easily
calculated by heating it electrically using the equiva-
lence between heat and electrical energy.
6.2 Conversion of electrical energy into heat
6.2.1 Experiment procedure
• After cooling the friction cylinder it should be
screwed into the base (the same experimental con-
ditions as for the friction experiment) and the tem-
perature sensor inserted. After a few minutes that
should be ignored for the sake of homogenous dis-
tribution of temperature, the resistance of the tem-
perature sensor is R
1
= 8.00 k Ω (corresponding to
T
1
= 14.60°C by Eq. 1).
• Now the power supply that has been configured in
advance (see Section 4) should be connected to the
heating element and a stopwatch started. Voltage
and current (as displayed by the power supply)
should be noted:
U = 11.4 V; I = 1.0 A
• After t = 300 s the experiment is halted and the
resistance of the sensor is read off:
R
2
= 3.98 k
Ω (T
2
= 30.32°C)