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Transcript
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