# Qquestion on a faraday's induction law problem

1. May 6, 2007

### O.J.

we hav a conducting coil entering a magnetic field. now my question is

for a moving charge in a B, the force given is F=I LxB

now when we move a coil (that has charges) we're technically moving charges through a B so does that equation still apply? if not, y?

2. May 6, 2007

### O.J.

sorry for reposting. but to make my point clear. F is givin by q vxB

and a moving conducting coil means a moving set of charges so doesnt that equation apply? it boild down to this: wats the diifference between moving charges in a conductor and moving charges in space?

3. May 6, 2007

4. May 6, 2007

### Hootenanny

Staff Emeritus
5. May 6, 2007

### O.J.

if so (which my intuition tells me too) then, i would like an explanation ont hsi:

today we had a problem where a coild is moved with velocity v into a magnetic field B into the page. The professor said while the coil is moving inside the field it experiences no force (it only experiences force when its entering/exiting the field). can u explain why?

6. May 6, 2007

### lpfr

If there is no current in the coil there is no force.
But, even if there is current, if the field is uniform and the axis of the coil is parallel to the magnetic field, all the forces in the coil compensate and there is no net force.

7. May 6, 2007

### Hootenanny

Staff Emeritus
Your professor is indeed correct, with one caveat; the magnetic field must be constant. The reason the coil experiences a force when it is entering and exiting the field is because you are changing the flux through the coil, which according to Faraday's law induces a emf and hence current in the coil. This emf causes the electrons to 'drift' with some velocity around the loop, roughly in the same direction. This causes a force to be exerted in accordance with F = q(vxB). However, once the coil is completely inside the magnetic field, the magnetic flux through the coil is constant (since you are not changing the area of the coil in the magnetic field); as such, no current is induced, therefore, the electrons no longer have a drift velocity.

That said, you are still moving the coil through the magnetic field with some non-zero velocity; but don't forget, the coil isn't completely composed of electrons, there are also positively charged nuclei in the coil. And moving positive charges in a magnetic field experience a force that is opposite in direction to negative charges. So the short answer is the coil is uncharged.

I hope that makes sense

8. May 6, 2007

### O.J.

thanks a lot. it makes perfect sense. i forgot to consider that fact. thanx again man.

9. May 6, 2007

### Hootenanny

Staff Emeritus
Twas a pleasure...