EM2_Manual

EM2_Manual - General Physics II Lab EM2 Capacitance and...

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General Physics II Lab EM2 Capacitance and Electrostatic energy General Physics II Lab EM2 Capacitance and Electrostatic Energy Purpose In this experiment, you will examine the relationship between charge, voltage and capacitance of a parallel plate capacitor. Equipment and components Variable capacitor, DC power supply, electrometer with a cable and battery box, switch box, Faraday ice pail, proof plate, charging probe, capacitor (15 pF), ruler, aluminium sphere, grounding wristband,.cables: BNC to spades (unshielded), BNC to crocodile clips (shielded), leads: 4 mm plug to spade, 4 mm plug to plug (for earth grounding). Background Electrostatic energy associated with an electric field can be stored in a capacitor. The storage of such energy requires that one has to do work to move charges from one plate in the capacitor to the other. The charge, Q , on the plates and the voltage, V , between the plates are related according to the equation QC V = , (1) where C is the capacitance which depends upon the geometry and dimensions of the capacitor. For a parallel plate capacitor with plate area A and separation d , its capacitance is A C d ε = , (2) where ε is the permittivity of the medium between the two plates. The permittivity of air is approximately equal to that of vacuum, 0 . The amount of the energy stored in a capacitor is given by 22 V U C == (3) In this experiment, you will carry out measurements on a parallel-plate capacitor to verify the above equations. Procedure Measurement 1: Capacitance of the electrometer and cable It should be noted that whenever you make measurements of charge, voltage or capacitance, you need to consider the effect of the internal capacitance of the electrometer and that of the cables connected to it. As shown in Fig. 1, the electrometer can be thought of as an infinite impedance voltmeter in parallel with a capacitor. The capacitor C 1 represents the internal capacitance of the electrometer, plus the capacitance of the leads. The capacitance of the electrometer and cable C 1 adds to the external capacitor which is connected in parallel . Therefore, it is necessary to know the capacitance of the electrometer and cable in order to have an accurate measurement of the capacitance of an external capacitor. Revised: 24 February 2010 1/9
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General Physics II Lab EM2 Capacitance and Electrostatic energy Figure 1 Schematic of the electrometer 1. Connect the circuit as shown in Fig. 2. Before turning on the electrometer, check that the meter is mechanically zeroed. If not, please ask the technician-in-charge for help. 2. Turn on the electrometer and turn the “FUNCTION” switch to the “3V” position. Flip the “ZERO” switch to the “LOCK” position. Adjust the “ZERO SET” control until the meter reads at center zero.
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This note was uploaded on 03/31/2011 for the course PHYS 1 taught by Professor Nianlin during the Spring '11 term at HKUST.

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EM2_Manual - General Physics II Lab EM2 Capacitance and...

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