Physics Concept Question

[kyzoo]

I'm having trouble grasping this concept. Could anyone help me

Why is it that when a conductor moves in a magnetic field, the conductor experiences a force that opposes its motion.

I tried imagining how the current would be created in accordance to the principle that the magnetic field produced by an induced current, will oppose the original magnetic field change that induced that current. But alas, I failed to connect this concept and the previous one. I failed to envisage how the magnetic field changes when you move the conductor.

[superflya]

has it got anything to do lenz's law, wild guess

[Aden]

I believe it has something to do with the magnetic field generating an electromagnetic force in the conductor, which in turn induces small currents (eddy currents). These small currents then create a magnetic field which opposes the changes in the original magnetic field.

[appianway]

This isn't an explanation as to why it occurs (it's a bit beyond VCE), but think about it like this.

When a conductor is moved through a field, according to Maxwell's last equation, an EMF is induced (this IS in the VCE syllabus). EMF is essentially energy per unit charge, so you're hence giving energy to the charges. However, this energy can't come from nowhere - using conservation of energy, you've got to do "work" for the energy to be there. And as work is given as the integral of F dx, by moving it through a force, you're doing work.

As I've said, it doesn't explain why it occurs (if you want, think about the definitions of forces in terms of fields), but it helps to connect things. A little.

[kyzoo]

Ah ok, Thanks guys.

Connections rock.

[kamil9876]

It can basically be worked out just be applying those laws about the forces.

Suppose the magnetic field is into the page, and we have a conductor going in direction --->

Then by one of those hand laws the electrons experience a force down  \/

So now we have electrons moving down \/ and magnetic field into the page and so using the same hand law again you get that the force is in direction <--- As required. QED

Now that was just one situation, you can play around with the others by applying these rules... just watch the motion of electrons (or positive ions if that is less confusing) and the 'chain reaction' and you can derive it all from those laws.

In fact the law is not true in general since for example if the magnetic field was parralel to the motion of the conductor then the force on the electrons/ions would be zero.
                                                             

[kyzoo]

Sweet, another way to understand this - by the concept that the force on electrons in a magnetic field is in the opposite direction to the force on positive charges.

Thanks kamil ^^

[kamil9876]

by the concept that the force on electrons in a magnetic field is in the opposite direction to the force on positive charges.

yeah that's what i used to get the direciton of the force on the electrons.

It's pretty interesting how far you can get in electricity by assuming it's the positive ions that flow in current and not the electrons. Both assumptions yield the same answer in many basic phenomena in electromag, however(for the interested) there are some more complicated cases where it does matter, and you can use it to verify that negative charges are indeed the ones that flow. Check out the Hall effect.