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 90^o as shown by the vector diagram in Figure 3.


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 90^o or p/2.

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