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Unformatted text preview: Experiment #5 Measuring the Magnetic Field Using a Flip Coil Introduction In this experiment we measure a magnetic field by using Faraday's law of induction. We assume that you understood the derivation in your first year lecture course, or, if not, that you will consult a textbook. In MKS units, Faraday's law is: dt BdA d dt d V - =- = The voltage, V , is the integral of the electric field around a closed contour, dl E . In practice, V is the voltage between the ends of a nearly closed loop of wire and is the total magnetic flux passing through the surface contained by the loop. This expression is independent of the cause of the time variation of . That means, it can result from changes in the magnetic field or from moving the coil between regions with different magnetic fields. The minus sign came from Lenz's Law, since the induced currents oppose the magnetic flux change. That is, if the flux decreases through the surface enclosed by the loop of wire, a current will flow in the circuit, in a direction so as to try to maintain the flux constant. Flip-Coil Technique Faraday's law shows that we can detect the presence of a magnetic field by moving a loop of wire through the region where it is present. For example, the space between the poles of a permanent magnet. However, in order to make a quantitative measurement of the flux or the field, the equation above shows that we need to integrate the voltage over time during the motion: (1) The easiest way to perform an integration is to use the familiar RC circuit shown in Figure 2 . Figure 1. The red ring represents a magnet placed on the lab bench. The coil, with its leads attached, is moved from the vertical orientation to the horizontal one so that the magnetic...
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- Fall '08