Download W9e „Heat Capacity of Solids and Liquids“

Fakultät für Physik und Geowissenschaften
Physikalisches Grundpraktikum
W9e „Heat Capacity of Solids and Liquids“
Tasks
1 Determine the heat capacity of a water-filled calorimeter equipped with an electrical heater.
2 Measure the heat capacity of two different metal cylinders.
3 Calculate the molar heat capacity of the metal samples and compare with the law of Dulong and
Petit.
4 Measure the heat capacity of a liquid.
Literature
Physikalisches Praktikum, 13. Auflage, Hrsg. W. Schenk, F. Kremer, Wärmelehre, 1.0.1, 1.0.3, 3.0, 3.1,
3.2
Physics, P. A. Tipler, 3rd Edition, Vol. 1, 16-1, 16-4, 16-7
University Physics, H. Benson, Chap. 19
Accessories
Calorimeter with electrical heating and magnetic stirring device, laboratory power supply, digital
multimeters, digital thermometer, metal samples, test liquids, balance, notebook with measurement
program
Keywords for preparation
- Calorimeter, principle, heat capacity, heat loss, temperature-time diagram
- Experimental determination of heat capacities by heat exchange
- First law of thermodynamics
- Specific and molar heat capacity, law of Dulong and Petit
- Temperature dependence of the specific heat capacity
- Temperature measurement with electrical temperature sensors
- Voltage-correct circuit
1
Remarks
General. In case of a constant electrical power Pel and under the condition that the temperature rise
above room temperature is not too large, the temperature rises linearly in a time interval t. The
energy supplied by the electrical heater is Q  Ctot T  Pel t . This is absorbed in form of heat by
the calorimeter (heat capacity CK) and the test fluid (heat capacity Cfl = mfl cfl ). The total heat capacity
is given by the sum Ctot= CK+ Cfl. By measurement of the electrical power Pel  UI and the
determination of the slope b=T/t of the time dependent temperature rise, the heat capacity Ctot
can be calculated from Ctot  UI / b . The temperature is measured with a digital thermometer. The
measurement of the heat capacity of a test liquid is made analogously to the measurement of the
calorimeter’s heat capacity.
Specific. In case of the calorimeter with electrical heater (heating resistance) the circuit has to be set
up as a voltage correct circuit. (see Fig. 1 below). In case of a voltage-correct circuit the voltage drop
U is directly measured at the resistance heater using a digital multimeter in the ACV range. The
current I is measured with a digital multimeter in the ACA range. The correct choice of the
measurement range is made according to the expected values of voltage and current. The
characteristic data of the heating resistance are U = 12 V at a power P = 12 W. The electrical circuit
should be checked by the supervisor.
The calorimeter vessel is filled with distilled water, such that the resistance heater can be safely
immersed. The water mass is measured with a balance. Switch on the magnetic stirring device.
Measure the pre-period for some minutes to control the constant initial temperature; switch on the
power supply and measure the temperature at appropriately chosen time intervals up to a
temperature rise of maximally 5 K above the actual room temperature; measure the post-period for
a few minutes. Subsequently a second measurement might be done under identical conditions to
check for the reproducibility. Note that the calorimeter has to be cooled down and filled with fresh
distilled water for this. Plot the data in a temperature-time diagram, determine the slope b  T / t
and calculate the total heat capacity. Subtracting the heat capacity of water (Cw = m cw ) the heat
capacity of the calorimeter vessel can be determined as CK = Ctot - Cw.
The measurement of a solid or a fluid is done with the same technique.
Pay attention to the continuous rotation of the magnetic bar to ensure a good mixing of the
calorimeter fluid. Before the start of a new measurement the calorimeter has to be cooled down to
room temperature and new fluid has to be filled in, in order to provide for reproducible initial
conditions.
2
Fig 1. Electric circuit
(voltage correct circuit)
V
A
U
K
M
F
H
T
Voltmeter (digital multimeter)
Amperemeter (digital multimeter)
Laboratory power supply
Calorimeter
Magnet stirring device
Stirring fish
Heater
Temperature sensor (Pt-100) with Ohmmeter
Fig. 2 Digital multimeter
1
2
Front panel
Measuring range selector (grey: resistance, blue: DCV
– direct voltage and ACV – ac voltage, red: DCA –
direct current and ACA – alternating current, yellow:
capacitance, frequency, ...)
3
Terminals (COM: common terminal, V /  voltage
against COM and resistance, mA: current for small current
values, 20A: current for measurements up to 20 A)
VC220 Data
Accuracy: ACA 2%+5dgts, ACV 1%+5dgts
Table 1: Masses of test bodies
Nr.
Material
m/g
1
Copper
126,667(±0,005)
1
Steel
111,729(±0,005)
1
Aluminium
38,453(±0,005)
2
Copper
128,895(±0,005)
2
Steel
111,868(±0,005)
2
Aluminium
38,474(±0,005)
3
Copper
127,875(±0,005)
3
Steel
112,310(±0,005)
3
Aluminium
38,433(±0,005)
4
Copper
126,643(±0,005)
4
Steel
111,960(±0,005)
4
Aluminium
38,448(±0,005)
Steel: 72% Fe, 18% Cr, 10% Ni
3