Electric Field Mapping October 2009

Electric Field Mapping October 2009 - HB Electric Field...

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HB 09-25-07 Electric Field Mapping Lab 3 1 Electric Field Mapping Lab 3 Equipment mapping board, U-probe, resistive boards, templates, dc voltmeter (431B), 4 long leads, 16 V dc for wall strip Reading Your textbook. Precautions 1. Before turning mapping board over, remove voltage leads from board. 2. Turn mapping board over by grasping two diagonal corners of board. Please do not grasp edges of mapping board, which may break a wire underneath the board near the edge. 3. Handle resistive boards carefully by edges. It is easy to scratch the resistive coating. 4. During the experiment, keep checking that the knurled nuts, particularly the one to the U-probe, are tight. 1 Description and Theory The force between charged particles is attributed to an electric field ~ E which has the di- mensions of force per unit charge. A charge q in the presence of an electric field produced by other charges experiences a force q ~ E . Another useful quantity is the voltage or potential (these two words are equivalent), which have the dimensions of work per unit charge, or in S.I. units, joules/coulomb (J/C). This last combination of units, J/ C), is called the volt (V). The potential between two points a and b, or the potential at a with respect to b, is given by V a - V b = V ab = R b a ~ E · d ~ , where d ~ is a differential displacement. If a surface is constructed so that the electric field ~ E is always perpendicular to it, the potential difference between any two points on the surface will be zero. The surface has a constant voltage and is called an equipotential surface. If several charged conductors exist in a region of vacuum or air, electric field lines and equipotential surfaces can be constructed. A voltmeter is an instrument that has two metal probes. If the two probes are put in contact with any two conductors, the voltmeter will give the voltage or potential between the two conductors. But the voltmeter will not give the voltage between two points in vacuum or air because vacuum or air cannot supply the necessary charge or electrical current necessary to make the voltmeter work. It can be shown that if perfect conductors are embedded in a homogeneous resistive medium, the electric field and equipotentials are the same as in vacuum. This is the technique used in this experiment. Highly conducting paint is used to paint “electrodes” on a flat resistive board. Voltage differences are applied to the electrodes and the equipotential surfaces in the resistive medium are found by using a voltmeter. This experiment is two, not three, dimensional, as the conducting board is a flat thin sheet. The equipotentials that you measure will be lines, not surfaces. The equipotentials obtained in this experiment are equivalent to those that would be obtained if the electrodes
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