Week9_2 - AC Generators Electric generators take in energy...

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1 AC Generators ± Electric generators take in energy by work and transfer it out by electrical transmission ± The AC generator consists of a loop of wire rotated by some external means in a magnetic field
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2 Induced emf In an AC Generator ± The induced emf in the loops is ± This is sinusoidal, with ε max = N A B ω sin B d N dt NAB t ε ω Φ =− =
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3 Lenz’ Law ± Faraday’s Law indicates the induced emf and the change in flux have opposite algebraic signs ± This has a physical interpretation that has come to be known as Lenz’ Law ± It was developed by a German physicist, Heinrich Lenz
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4 Lenz’ Law, cont ± Lenz’ Law states the polarity of the induced emf in a loop is such that it produces a current whose magnetic field opposes the change in magnetic flux through the loop ± The induced current tends to keep the original magnetic flux through the circuit from changing
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5 Lenz’ Law – Example 1 ± When the magnet is moved toward the stationary loop, a current is induced as shown in a ± This induced current produces its own magnetic field that is directed as shown in b to counteract the increasing external flux
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6 Lenz’ Law – Example 2 ± When the magnet is moved away the stationary loop, a current is induced as shown in c ± This induced current produces its own magnetic field that is directed as shown in d to counteract the decreasing external flux
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7 In equal-arm balances from the early 20th century (Fig. 23.18), it is sometimes observed that an aluminum sheet hangs from one of the arms and passes between the poles of a magnet, which causes the oscillations of the equal arm balance to decay rapidly. In the absence of such magnetic braking, the oscillation might continue for a very long time and the experimenter would have to wait to take a reading. Why do the oscillations decay? T he aluminum sheet i. .. Cu rrents i n the alum. . Al u minum is fe rro m a 33% 33% 33% 1. The aluminum sheet is attracted to the magnet. 2. Currents in the aluminum sheet set up a magnetic field that opposes the oscillations. 3. Aluminum is ferromagnetic. 10
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8 Problem 23.22. ± A rectangular coil with resistance R has N turns, each of length and width w as shown in Figure P23.22. The coil moves into a uniform magnetic field with constant velocity . What are the magnitude and direction of the total magnetic force on the coil ± (a) as it enters the magnetic field, ± (b) as it moves within the field, and ± (c) as it leaves the field? v r B r
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9 Problem 23.21. ± A conducting rectangular loop of mass M , resistance R , and dimensions w by falls from rest into a magnetic field as shown in Figure P23.21. During the time interval before the top edge of the loop reaches the field, the loop approaches a terminal speed v T .
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This note was uploaded on 06/02/2008 for the course PHYS 102 taught by Professor N/a during the Spring '08 term at Drexel.

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Week9_2 - AC Generators Electric generators take in energy...

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