AP Physics C 2017

An inductor makes use of induced EMF (see Chapter 20 ) to resist changes in current in a circuit. If part
of a circuit is coiled, then the magnetic field produced by the coils induces a “back EMF” in the rest of
the circuit ... that EMF depends on how fast the current is changing, by Faraday’s law. An inductor in a
circuit is drawn as a little coil, as shown in Figure 19.13 .

Figure  19.13           Symbol  for an  inductor    in  a   circuit.

The voltage drop    across  an  inductor    is

where L is called the inductance of the inductor. Inductance is measured in units of henrys.
What does this equation mean? If the current is changing rapidly, as when a circuit is first turned on or
off, the voltage drop across the inductor is large; if the current is barely changing, as when a circuit has
been on for a long time, the inductor’s voltage drop is small.
We can think of an inductor as storing energy in the magnetic field it creates. When current begins to
flow through the inductor, it stores up as much energy as it can. After a while, it has stored all the energy
it can, so the current just goes through the inductor without trouble. The energy stored in an inductor is
found by this equation.

For the AP Physics C exam, you need to understand circuits with inductors and resistors, as well as
circuits with inductors and capacitors.

Other Circuits

RL Circuits

RL circuits contain just an inductor and a resistor, and perhaps a battery, as shown in Figure 19.14 .