# Homework Help: Lenz's law

1. Apr 30, 2014

### Bengo

1. The problem statement, all variables and given/known data
Can't seem to upload the figure in my workbook so I'll describe it. A metal bar is placed on 2 rails that slope up with a gradually increasing tangent. The bar has a higher conductivity than the rest of the apparatus. The 2 ends of the rails at the bottom are connected to a power supply. In the absence of any external magnetic field, a voltage was applied to produce a current in the loop. The bar gradually climbed the rails until eventually coming to rest. It was assumed that the gravitational force on the bar was equal to the magnetic force generated by the current.

I just want to know why the bar moves. none of the questions in the passage really address it.

2. Relevant equations

3. The attempt at a solution

The only force I can think of is the one originating from the magnetic field from the current in the loop in which the metal bar is a part of but I don't think it can act on itself.

I tried to apply lenz's law but there is no External magnetic field to oppose

Edit: I guess the attachment did go through

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2. Apr 30, 2014

### SammyS

Staff Emeritus
What is does Lenz's Law state?

3. Apr 30, 2014

### Bengo

It states that when there a change in the magnetic flux within a loop a current will be induced that opposes it.

If the loop starts out with no magnetic flux and then it gains magnetic flux from the emf I would think the bar would move in a way to decrease this flux but I don't know how that translates into it moving upwards

4. Apr 30, 2014

### SammyS

Staff Emeritus
Yes, this is puzzling .

At first I thought that the conductivity might be the key. I don't see that as the case.

5. Apr 30, 2014

### Bengo

Maybe i left out some important info so I'll attach the passage

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6. Apr 30, 2014

### BiGyElLoWhAt

I think you're misinterpreting this question. The bar falls due to gravity. XD

Lentz's law states that $emf = \frac{d\phi}{dt} = \frac{d(\vec{B}\cdot \vec{A})}{dt} = emf = V = IR$

7. Apr 30, 2014

### BiGyElLoWhAt

correction: it's -dphi/dt and the negative sign carrys through =/

8. Apr 30, 2014

### Bengo

I get that the bar falls due to gravity but It's still not clicking with me how lenz's law explains why the bar moves up in the absence of an external magnetic field. Thanks!

9. Apr 30, 2014