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Inductive circuits

Like a capacitative circuit, a pure inductive circuit behaves quite differently from one containing resistance alone. Consider the circuit in Figure 1.

When the switch is closed the lamp (L1) in series with the inductance lights more slowly than lamp L2 suggesting that the inductance resists a changing current.

A coil with an iron core has a much greater inductance than one with an air core, and using the circuit in Figure 2 you can show that the lamp will dim markedly or even go out when the iron core is placed in the coil. If d.c. were used, however, the lamp would remain alight whether there was an iron core or not.

In the inductive circuit the current and voltage are not in phase; in fact, the current lags behind the voltage by 90o as shown by the vector diagram in Figure 3.

In inductive circuits the voltage leads the current by 90o (p/2)

This can be shown mathematically as follows.

Let the current flowing through an inductance L be i= I0sin(wt)

Now the back e.m.f. produced is E = - Ldi/dt = -wLi0 cos(wt)

The applied p.d. is therefore: -E = v = wLi0cos(wt) = wLi0sinw(t + p/2)

This shows that the voltage leads the current by 90o or p/2.

An easy way of remembering the lead and lag is to use the word CIVIL.

Capacitance current (I) leads voltage; Inductance voltage leads current (I)
© Keith Gibbs 2011