# Inductor and capacitor as current and voltage sources

1. Jun 8, 2013

### anhnha

Hi, I have just seen this statement in another forum:
An inductor is a constant current source for a limited time until its energy is expended. A capacitor is the counterpart being constant voltage until its energy is spent.

I am not quite understand it. Can you help me? This is not a homework. I am learning by myself.

2. Jun 8, 2013

### Simon Bridge

It's easiest to see for the capacitor .... a capacitor stores energy which is released as it discharges.
But it is not true that it acts as a constant voltage source.

3. Jun 9, 2013

### rude man

It is a true statement for an ideal inductor if the inductor is shorted. Then the current is constant forever! But in reality the inductor will have some internal resistance R, so the current decays as exp(-Rt/L).

Similarly, a capacitor is a source of constant voltage if there is no internal resistance in shunt with it. But any internal (or external) shunt resistance causes voltage decay as exp(-t/RC).

4. Jun 9, 2013

5. Jun 9, 2013

### anhnha

Then these are only current and voltage sources if no load are connected?

6. Jun 9, 2013

### Simon Bridge

Well OK - but in that idealized case of R=0, the energy is discharged in zero time isn't it?

7. Jun 9, 2013

### anhnha

When inductor is connected to a resistor, its energy decreses with time. Finally the current is zero.
Can we call it a current source?
I don't see it fit the definition:
A Current source provides a constant current through a load, with varying voltage.

8. Jun 9, 2013

### Staff: Mentor

The idea is that, for a sufficiently small time interval, the inductor behaves like an ideal current source. This does not mean that it is an ideal current source for all time.

If you consider circuit conditions at a particular instant in time (such as time t = 0+ after a switch closes or opens, thus changing the circuit layout in some way), then the current through the inductor at that instant will be the same as it was in the previous instant (t = 0). The inductor will manifest any required EMF to maintain the current flow, just as a current source would.

We use a similar concept when we analyze collisions in the presence of friction or changing potential energy; for a sufficiently brief time of collision momentum can be considered to be constant, even though we know that over longer time periods the momentum changes due to external forces acting on the system.

9. Jun 9, 2013

### anhnha

Thanks, get it.
But if so, consider inductor as a current source seems useless to me.
Is there any real application for this?

10. Jun 9, 2013

### Staff: Mentor

Sure! Besides its use for analyzing initial conditions for sudden circuit changes, the property can be used to smooth the current of a power supply when the load presents sudden brief changes or for "killing" current spikes (noise) on power or signal lines, for example. Cars use interrupted coil current to generate high voltages for ignition sparks (the current forces a path across the spark plug gap).