Friday 7-26-96
The relevant section in the book is 20.3
An inductor is simply a coil of wire. If we now look at a circuit composed only of a coil of wire and an AC power source, we will again find that there is a 90 degree phase difference between the voltage and the current in the inductor. This time, however, the current lags the voltage by 90 degrees, so it reaches its peak 1/4 cycle after the voltage peaks.
The reason for this has to do with the law of induction. As the voltage increases from zero, the flux through the coil (inductor) changes. A negative current is induced in the coil. When the voltage peaks, the flux goes from increasing to decreasing, so momentarily the current reaches zero. As the voltage decreases, the flux decreases, giving rise to a positive increased current in the coil...the current is maximum when the voltage (and flux) pass through zero, and then begins decreasing again as the voltage and flux head for a negative peak. When they reach that negative peak, the current is again zero, but it goes negative again as the voltage rises, reaching a negative peak when the voltage passes through zero. (Again, repeat the cycle)
As with the capacitor, an inductor exhibits a resistance called inductive reactance. This also depends on the frequency, and is given by:
X = wL, where L is the inductance of the loop (this depends on the geometry of the loop). The unit of inductance is the henry.
As with capacitive reactance, the voltage across the inductor is given by V = IX.