# What if field disappears?

1. Dec 15, 2015

### Moazin Khatri

I always have trouble understanding potential energy. Here's a realistic question which bothers me a lot.
Let's say I have a horse shoe electromagnet on a table. And I also have a permanent horse shoe magnet. I turn on the circuit(the electromagnet is functioning) And I bring the electromagnet and the permanent magnet closer to each other( I'm doing work because like poles are facing each other and repelling). Now the system seems to have some kind of energy because if I let go they will move apart with some velocity. But what if I turn off the electromagnetic? Will that energy disappear?

2. Dec 15, 2015

### DaveC426913

No. It will be converted into kinetic energy. That energy could be harnessed to do work, such as denting your floor.

3. Dec 15, 2015

### Moazin Khatri

But how? If I turn off the electromagnet won't they stay where they are?

4. Dec 15, 2015

### DaveC426913

I misunderstood. I thought you were supposing that turning off the electromagnet would cause them to "move apart with some velocity".

Rereading, I see what you're saying.

5. Dec 15, 2015

### Moazin Khatri

Yes sir. I'm sorry if I didn't write that correctly. My English is weak.

6. Dec 15, 2015

### DaveC426913

Not at all. I was entirely my fault.

7. Dec 15, 2015

### Moazin Khatri

Do you have an answer to that question sir?

8. Dec 15, 2015

### DaveC426913

I gotta think it through.

9. Dec 15, 2015

### Moazin Khatri

I'll be waiting! :-)

10. Dec 15, 2015

### nasu

You don't need the top magnet to analyse the problem. The field of the electromagnet has some energy when it is powered.
When you turn off a coil the current does not just drop down to zero instantaneously. You just introduce a capacitor (the gap) in series with the coil. The current will oscillate back and forth. like in any LC oscillator. The energy will be dissipated eventually by resistive effects and radiation.
The time to dissipate may be quite short but it is there.

11. Dec 15, 2015

### Moazin Khatri

Is the energy stored in the field related to the work I did to bring the two magnets closer to each other? If I change the size of the permanent magnet or its strength and do the same experiment again. Will it store more energy this time?

12. Dec 15, 2015

### nasu

Yes. Again, consider the case without battery. If you try to approach the coil with a magnet you will fill some repulsion, You will have to do some work to approach the magnet to the coil. This is due to the current induced in the coil, which current creates a field opposite to that of your magnet. The energy is stored in this current (or its field). This energy will be eventually dissipated into heat and maybe radiation.

In your case you have the two phenomena overlapped. You already have some current flowing and then induce some extra.

13. Dec 15, 2015

### Moazin Khatri

Got it. Thanks a lot. I'm very grateful to you for clearing this big confusion I had.

14. Dec 15, 2015

### Khashishi

An electromagnetic is an inductor https://en.wikipedia.org/wiki/Inductor. The stored energy in an inductor is $1/2 L I^2$. This stored energy is actually located in the magnetic field surrounding the inductor.