# Homework Help: A strip of copper placed in a magnetic field

1. Oct 18, 2012

### 1MileCrash

1. The problem statement, all variables and given/known data

A strip of copper 10 mm wide and 700 micrometers thick is placed in a uniform mag. field B magnitude 0.9 T, with B perpendicular to the strip. A current i = 45 A is then sent through the strip, such that a Hall Potential difference V appears across the width of the strip. Calculate V.

2. Relevant equations

3. The attempt at a solution

I know that V = Ed, so I think my first step is to find E.

However, I'm running into problems because it seems like they have given me a two dimensional object...

For example, I assume E = i/pA would work here to find E, but what exactly is cross sectional area if the object is a "two dimensional" strip?

I don't understand how they want me to work this. Any ideas?

2. Oct 18, 2012

### grzz

The strip has the given width and the given thickness (i.e. the cross-sectional area) and some lenght which is not given.

3. Oct 18, 2012

### 1MileCrash

I see, but then how do I use V=Ed?

I assume this d is length?

Or should I rethink the approach entirely?

thanks.

4. Oct 18, 2012

### grzz

Are you familiar with how a Hall voltage is generated?

5. Oct 18, 2012

### 1MileCrash

Yes, the magnetic field is perpendicular to the strip which gives a force perpendicular to itself and the velocity of the charge carriers which tends to pull the charge carriers to one side of the strip.

6. Oct 18, 2012

### grzz

As the charge cariers are passing along the lenght of the strip and through the cross-section of the strip, they are pulled one way by the magnetic field and so they pile up on one face of the strip. This seperation of charges creates an electric field which will force the charge cariers in the opposite direction.
So the Hall voltage stops growing when the
magnetic force = electric force
Why not start from this step?

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