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The law of inductance:
If a voltage is forced across an inductor, a current will flow through that inductor (and this current will vary with time).

**Switching Fundamentals**

**The law of inductance**

If a
voltage is forced across an inductor, a current will flow through that inductor
(and this current will vary with time). The current flowing in an inductor will
be time-varying even if the forcing voltage is constant. It is equally correct
to say that if a time-varying current is forced to flow in an inductor, a
voltage across the inductor will result. The fundamental law that defines the
relationship between the voltage and current in an inductor is given by the
equation:

v = L
(di/dt)

Two
important characteristics of an inductor that follow directly from the law of
inductance are:

1) A voltage
across an inductor results only from a current that changes with time. A steady
(DC) current flowing in an inductor causes no voltage across it (except for the
tiny voltage drop across the copper used in the windings).

2) A current flowing in an inductor can not change
value instantly (in zero time), as this would require infinite voltage to force
it to happen. However, the faster the current is changed in an inductor, the
larger the resulting voltage will be. Note: Unlike the current flowing in the
inductor, the voltage across it can change instantly (in zero time). The
principles of inductance are illustrated by the information contained in
Figure.

The
important parameter is the di/dt term, which is simply a measure of how the
current changes with time. When the current is plotted versus time, the value
of di/dt is defined as the slope of the current plot at any given point. The
graph on the left shows that current which is constant with time has a di/dt
value of zero, and results in no voltage across the inductor.

The
center graph shows that a current which is increasing with time has a positive
di/dt value, resulting in a positive inductor voltage. Current that decreases
with time (shown in the right-hand graph) gives a negative value for di/dt and
inductor voltage. It is important to note that a linear current ramp in an inductor
(either up or down) occurs only when it has a constant voltage across it.

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