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Inductance in an AC circuit
An inductor is simply a coil of wire (often wrapped around a piece of ferromagnet). If
we now look at a circuit composed only of an inductor and an AC power source, we
will again find that there is a 90° phase difference between the voltage and the current
in the inductor. This time, however, the current lags the voltage by 90°, so it reaches
its peak 1/4 cycle after the voltage peaks.
The reason for this has to do with the law of induction:
Applying Kirchoff's loop rule to the circuit above gives:
As the voltage from the power source increases from zero, the voltage on the inductor
matches it. With the capacitor, the voltage came from the charge stored on the
capacitor plates (or, equivalently, from the electric field between the plates). With the
inductor, the voltage comes from changing the flux through the coil, or, equivalently,
changing the current through the coil, which changes the magnetic field in the coil.
To produce a large positive voltage, a large increase in current is required. When the
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This note was uploaded on 11/22/2011 for the course PHY PHY2053 taught by Professor Davidjudd during the Fall '10 term at Broward College.
 Fall '10
 DavidJudd
 Physics, Inductance, Power

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