Sliding conductor on a metal rail, with perpendicular B-field

[serverxeon]

Homework Statement

In the following diagram, the brown metallic conductor rod is given a slight push to the left.
The black lines are all conducting wires, with the entire setup placed in a perpendicular B-field. Ignore friction.

What will happen?

1) The rod slowly come to a stop?
2) The rod continues to move at constant speed?
3) The rod comes to a stop and reverses direction?

[PLAIN]http://img52.imageshack.us/img52/9687/magqj.png

The Attempt at a Solution

1) I think this should be the answer, as kinetic energy has to be converted to electrical current (heating of the wires). The motion has got to die down.
However, I cannot identify the retardation force that acts on the rod which is necessary for the motion to stop.

2) I don't think it's going to move forever as argued above.

3) I seriously doubt this man. Intuition, somehow.

 

[PeterO]

Homework Statement

In the following diagram, the brown metallic conductor rod is given a slight push to the left.
The black lines are all conducting wires, with the entire setup placed in a perpendicular B-field. Ignore friction.

What will happen?

1) The rod slowly come to a stop?
2) The rod continues to move at constant speed?
3) The rod comes to a stop and reverses direction?

[PLAIN]http://img52.imageshack.us/img52/9687/magqj.png

The Attempt at a Solution

1) I think this should be the answer, as kinetic energy has to be converted to electrical current (heating of the wires). The motion has got to die down.
However, I cannot identify the retardation force that acts on the rod which is necessary for the motion to stop.

2) I don't think it's going to move forever as argued above.

3) I seriously doubt this man. Intuition, somehow.

As the conductor moves to the left, the area of the loop reduces. That means the amount of flux threading the loop reduces. That reduction in flux means a current is induced in the loop. Once that current is flowing through the moving wire, a force is induced on that wire [F = B.I.l ] that is the force that stops the wire.

 

[serverxeon]

Hmm, but,

the direction of the induced current in this case is anti-clockwise.
Therefore, the direction of the current in the rod, is upwards.
Wont the force on the rod be to the left?! that's not retardation?

 

[PeterO]

Hmm, but,

the direction of the induced current in this case is anti-clockwise.
Therefore, the direction of the current in the rod, is upwards.
Wont the force on the rod be to the left?! that's not retardation?

Induced Forces ALWAYS oppose change so one of those direction reckonings is backwards. See if you can work out which one.

 

[serverxeon]

oh! sorry a slipped on my part.
so it's clockwise induced current. B-force is therefore on the right.
I see. thanks.