# Induction question

1. Feb 5, 2009

### likephysics

A moving electron causes a magnetic field. So any wire with current flow has mag field around it.
When a Permanent magnet is bought near a coil, current flows initially and then the induced mag field opposes the PM field.
For the induced field to exist, electrons must be flowing through the wire. correct?

2. Feb 6, 2009

### KnowPhysics

yes. the current must be flowing the wire to keep induced field active.

3. Feb 6, 2009

### likephysics

well then, we can make use of that current to light a bulb or charge a small battery?

4. Feb 6, 2009

### vanesch

Staff Emeritus
Yes, of course.

5. Feb 7, 2009

### likephysics

If current is already flowing in the coil, then why do we need changing magnetic field?

6. Feb 7, 2009

### Staff: Mentor

So you were asking whether a current has to exist previously in the coil, in order for induction to occur when you move the permanent magnet? In that case, the answer is "no."

7. Feb 7, 2009

### likephysics

All I am asking is why do we require a changing Magnetic field for induction to occur?
Current is already flowing when the PM is close to the coil.

8. Feb 9, 2009

### cabraham

The fact that a time-changing ac magnetic field produces induction, and a steady dc magnetic field does not, is an axiom. There is no formal proof in terms of anything more fundamental. This is a postulated fact based on empirical observation which has been consistent under all known conditions since day one with no exceptions ever found. Does this help?

Claude

9. Feb 9, 2009

### KnowPhysics

Don't get confused. The induction in the coil is completely different. it is because of the current flowing through the wire it is nothing to do with your PM. if you want to produce induction with PM then the magnet must move (the magnetic field will cut by wire it will produce induction in wire).

10. Feb 9, 2009

### Gnosis

When one magnetic field remains motionless relative to another, voltage induction cannot occur. Voltage induction requires a change in the physical movement or change of intensity of a magnetic field or the movement of a conductor cutting through a magnetic field. For a continual voltage to be induced, a continually change in these factors must occur on a continual basis.

11. Feb 9, 2009

### zoobyshoe

Current does not flow because of the PM's proximity to the coil, it flows because of the relative motion between the PM's field and the inductor. Current will only flow while you are moving the PM toward or away from the coil.

If you put a magnet near a coil and then let them both sit still, there will be no current whatever while they are sitting still.

The energy in electricity comes from the energy expended in moving the magnet or the coil, not from anything else. The special property of a magnetic field is that it can interact with, and put pressure on, the free electrons in the conductor: if we move the magnetic field relative to the conductor it will cause electrons to move. If we don't move the field, no electrons will move. A flow of current requires a constant relative motion of the magnetic field and the inductor.

12. Feb 28, 2009

### alfaphysics

The induction is not just lightning a bulb it is used in devices like transformer to step up or step down the voltage