# Role of inductors in A.C and D.C circuits

#### circuit2000

Joined Jul 6, 2006
33
1)A bulb is connected in series with an inductor and a 6 volt D.C supply. A soft iron core is inserted into the coil quickly. It is said that there is a momentary increase in the intensity of the bulb because at that particular instant the induced e.m.f is greater than the e.m.f of the source. Is it true?
2)Now, the D.C source is replaced by an A.C source having an r.m.s voltage of 6 volt and 50 Hz frequency. When compared to the intensity of the bulb in first question when steady state is reached, how will be the intensity of the bulb?
The book answer says that the intensity of the bulb is greater when steady state is reached(A.C voltage circuit) than in the D.C circuit. Is it true? If so, is it because the r.m.s value of voltage being 6 volts, the peak value will be greater than that and so the intensity of bulb increases?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

For case 1 ,I don't see it. A soft iron core is not a magnet, so how can it induce EMF? A magnet would have such an effect.

In case 2, I also disagree. The RMS measurement for AC was specifically to let AC ab DC power relate to one another. A bulb with 6 volts DC should have the same power usage as whan it is on 6 volts AC RMS. The AC peak voltage is higher, but there is also the time when the waveform goes to zero and produces no output. The instantaneous output is greater when at peak, but the average over time is the same.

#### circuit2000

Joined Jul 6, 2006
33
The first one seems right to me because when a soft iron core is introduced the self inductance of the coil increases and hence there is an increase in induced e.m.f. I may be wrong.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

It's a DC circuit. You induce current in a coil when magnetic force lines cut the conductors. The iron core is not magnetic, so no induced EMF. The instantaneous effect would be to dim the bulb, as the coil inductance would increase, and some energy would be used to increase the field around the coil.

One of us is probably right - it shoud be easy to set up an experiment to demonstrate the effect.

#### kinyo

Joined Jun 6, 2005
13
The usual equation for voltage across an inductor is L(di/dt). However, it can also be I(dL/dt) which is the basis for inductive sensors. So yes, it is possible to have induced voltage as long as there is an existing current on the inductor. The existing current creates a magnetic field which can be disturbed by an iron core.