Short, Easy, Lenz's Law Question

[Sheneron]

Homework Statement

A copper bar is moved to the right while its axis is maintained in a direction perpendicular to a magnetic field as shown in Figure P31.28d (image d, bottom right). If the top of the bar becomes positive relative to the bottom, what is the direction of the magnetic field?

http://img370.imageshack.us/img370/5013/p3126jg2.th.gif By lenz's law if the induced current should be opposing the change in magnetic flux. So if the magnetic flux is decreasing the induced current should be in such a way that the induced magnetic field is in the same direction as the magnetic field. And if the flux is increasing the induced magnetic field should be opposite the magnetic field. So in this problem I can't figure out how we are supposed to know whether the magnetic flux is increasing or decreasing.

Not only that I thought that since the magnetic fields should oppose or contribute with each other then since the bar magnet has a magnetic field downward the answer would be either downward or upward. But neither of those is right. Any help please?

 

[alphysicist]

Hi Sheneron,

Homework Statement

A copper bar is moved to the right while its axis is maintained in a direction perpendicular to a magnetic field as shown in Figure P31.28d (image d, bottom right). If the top of the bar becomes positive relative to the bottom, what is the direction of the magnetic field?

http://img370.imageshack.us/img370/5013/p3126jg2.th.gif


By lenz's law if the induced current should be opposing the change in magnetic flux. So if the magnetic flux is decreasing the induced current should be in such a way that the induced magnetic field is in the same direction as the magnetic field. And if the flux is increasing the induced magnetic field should be opposite the magnetic field. So in this problem I can't figure out how we are supposed to know whether the magnetic flux is increasing or decreasing.

Not only that I thought that since the magnetic fields should oppose or contribute with each other then since the bar magnet has a magnetic field downward the answer would be either downward or upward. But neither of those is right. Any help please?

Which way are the positive charges in the rod moving due to the motion of the rod? Which way is the force on those positive charges? Once you answer those two questions you should be able to find the direction of the magnetic field. What do you get?

 

[Sheneron]

Oh crap, not really sure what I was thinking, but I am still not sure of the answer. The rod is moving to the right, and as it does the positive charges feel a force upward since the positive charges accumulate at the top.

So by right hand rule, if I point towards the velocity and curl my fingers into the page, the force is upwards. So the magnetic field is into the page?

 

[alphysicist]

Oh crap, not really sure what I was thinking, but I am still not sure of the answer. The rod is moving to the right, and as it does the positive charges feel a force upward since the positive charges accumulate at the top.

So by right hand rule, if I point towards the velocity and curl my fingers into the page, the force is upwards. So the magnetic field is into the page?

That sounds right to me.

 

[Sheneron]

Thanks for the help

 

[alphysicist]

Sure, glad to help!