# BR physics--magnetic flux passage

#### fluff

This is from BR physics p. 132 on E&M. The figure in the passage shows a bar connected to a circuit with a resistor in a magnetic field pointing into the page. The bar is moving to the left.

The question asks for the direction of the induced current flowclockwise or counterclockwise. The way that I solved this problem is to use the right hand rule with F B and v, determined that the force pointed up and so the current must also point in that direction. Therefore, current flows in a counterclockwise direction.

However, can you also use the right-hand rule with your thumb pointing in the direction of the current, and then figuring out which way magnetic field should go (into or out of the page) to minimize flux?

I feel like the above two are the same thing, but just want to make sure.

#### BerkReviewTeach

##### Company Rep & Bad Singer
Vendor
10+ Year Member
This is from BR physics p. 132 on E&M. The figure in the passage shows a bar connected to a circuit with a resistor in a magnetic field pointing into the page. The bar is moving to the left.

The question asks for the direction of the induced current flowclockwise or counterclockwise. The way that I solved this problem is to use the right hand rule with F B and v, determined that the force pointed up and so the current must also point in that direction. Therefore, current flows in a counterclockwise direction.

However, can you also use the right-hand rule with your thumb pointing in the direction of the current, and then figuring out which way magnetic field should go (into or out of the page) to minimize flux?

I feel like the above two are the same thing, but just want to make sure.
Exactly! the first method is the standard right hand rule to determine the direction of the force causing the induced force and the second way is via Lenz's law.

If the system is gaining Xs as the bar moves, then the current is being induced to create dots to offset the increase in X-flux. Putting your right thumb out in the direction of the "dots" results in the curling of your fingers in a counterclockwise fashion, so the current must be counterclockwise to generate a B-field coming out from the interior of the loop.