# I Is energy needed to maintain an electromagnetic field?

1. Mar 1, 2017

### physea

Hello

Is we apply a voltage at the ends of a coil, a magnetic field will be generated? Is that correct?

Will that empty the battery that is responsible for the voltage or a partial energy from the battery will be stored in the magnetic field of the coil and after that the battery will not empty forever?

If that's the case, can we get back the energy from the magnetic field, to charge the battery in a reversible way?

Thanks!

2. Mar 1, 2017

### BvU

Hi,

The battery goes empty: once the magnetic field is built up, all the energy the battery delivers is dissipated in the resistance the wire represents, to the tune of V x I.

At the instant the battery is disconnected, the self-inductance of the coil generates an electric potential that can recover a small amount of energy from the decaying field -- but again the resistance of the wire dissipates most of that.

3. Mar 1, 2017

### Staff: Mentor

As BvU mentioned, if there is any resistance then that will drain the energy. But if you use superconductors then you can store the energy for later retrieval.

https://en.m.wikipedia.org/wiki/Superconducting_magnetic_energy_storage

4. Mar 1, 2017

### physea

Isn't there a material that will become magnetic due to the voltage applied to it and not due to electron flow that produces energy loss due to resistance?

5. Mar 1, 2017

### Staff: Mentor

I am not aware of such a material, but materials isn't really my field.

6. Mar 1, 2017

### BvU

Note that the material in your post #1 isn't magnetic: it conducts a current and the current is the source of the magnetic field.

7. Mar 1, 2017

### CWatters

If you apply a DC voltage and the coil is "ideal" then the current increases indefinitely. If it's a non-ideal coil the final value of the current is limited by the resistance.

You can do more interesting things using an AC voltage or a voltage that's switched on an off rapidly. For example when the voltage is ON the current in the coil (aka Inductor) increases storing energy in the coil. When the applied voltage is switched off you can recover that energy and you can do it at a different voltage to the applied voltage. This approach or a similar approach is used in DC to DC converters that transform say 9V to -9V or +12V.

PS: This is a very simplified explanation of how they work!

8. Mar 1, 2017

### CWatters

You could but you can never achieve an overall gain in energy. For example if you use the energy stored in the coil to recharge the battery you can't also use it to do anything else. So it would be a rather pointless exercise.