# When a magnetic field changes and an emf is produced,

1. May 23, 2014

### jaredvert

Then what gives the energy to create this emf?

2. May 23, 2014

### nsaspook

What makes the magnetic field change from top to bottom of the tube if we drop a magnet in a copper tube?

Last edited by a moderator: Sep 25, 2014
3. May 23, 2014

### jaredvert

It is the induced emf in the copper tube correct? It would make a counterclockwise current. But what I don't understand is where is the energy for this induced emf coming from?

Last edited by a moderator: Sep 25, 2014
4. May 23, 2014

### UltrafastPED

5. May 23, 2014

### jaredvert

Yeah I mean I think I get lenz law but can u explain what I'm missing here? The magnet falling induced a counterclowise current in the copper. Where the energy for this emf is what I don't understand

6. May 23, 2014

### nsaspook

What makes the magnet move?

7. May 23, 2014

### jaredvert

Well gravity but I imagine it's acceleration is slower due to the magnetic force exerted on it

8. May 23, 2014

### nsaspook

9. May 23, 2014

### jaredvert

Oh so I see because it has less velocity it's KE is less and so the energy induced to the emf is equal to mgh - 1/2mvf^2. Where vf is velocity in coil. Correct?

10. May 24, 2014

### nsaspook

Judging by the slow speed of the magnet drop in the tube a good percentage of the KE is in the induced emf and it's magnetic field.

11. May 24, 2014

### jaredvert

How come when he spins it the magnet accelerates faster down? Is it because the area of the plates perpendicular to the sides are smaller and so smaller current induced?

12. May 24, 2014

### nsaspook

At what point in the video do you mean? His spin is mainly to stabilize the magnet during it's drop.