214
Electricity and Basic Electronics
In Figure 12-5, the switch 1 (S
1
) has just
been closed. The circuit current is increasing,
so the magnetic fi eld around the coil is
expanding. This induces emf in the coil. The
induced emf opposes the voltage of the power
source, so it opposes the change in circuit
current. This means that it slows the increase
of circuit current. In other words, a circuit
with a coil takes longer to reach its maximum
circuit current than the same circuit without
a coil.
Figure 12-6. When the magnetic fi eld is decreasing, the current produced by
the induced emf fl ows with the circuit current. This slows the circuit current’s
decrease to zero.
When current in a dc circuit reaches its
maximum and becomes constant, the magnetic
fi eld stops expanding and becomes constant,
too. Since there is no change in the magnetic
fi eld, there is no induced voltage and no
opposing current. Circuit current is allowed to
fl ow freely through the circuit. The magnetic
fi eld surrounding the coil is constant.
When the power source is removed from the
circuit (S
1
opened and S
2
closed), Figure 12-6,
the circuit current starts to decrease toward
S2
Collapsing
magnetic field
Current produced
by induced emf
S1
Figure 12-5. The induced emf produces current that opposes the circuit current,
slowing down the circuit current’s increase.
S2
S1
Expanding
magnetic field
Current produced
by induced emf
Increasing circuit current