# Homework Help: Force between two bar magnets.

1. Mar 24, 2014

### Mutaja

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

You're given two bar magnets as shown below. In the gap between the two bar magnets, there's a bit of copper that's 1mm thick. The area of the bar magnet is 2.5cm2, and the flux density is 0.5T. Compute the force between the two bar magnets.

2. Relevant equations

3. The attempt at a solution

So the force is given by this formula:

F = B * H * A

H = $\frac{B}{µ0}$ = $\frac{0.5T}{4∏ * 10^-7}$ = 397887

F = 0.5T * 397887 * $2.5cm^2$ = 497385.

The answer I get is about 20 000 times bigger than what I'm expecting.

Where am I going wrong?

F is the force.
B is the flux density in the air gap.
H is the field strength in the air gap.
A is the area in the air gap.

I'm sorry for the use of the wrong symbol, but ∂ was the closest I could get.

Also, I realized that I have a gap filled with copper instead of air, just as I was about to post this. I have no examples involving gaps with anything else than air as far as I'm aware. I will keep looking for this and update this post if I find anything.

Any input is appreciated.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### bar magnets.PNG
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2. Mar 24, 2014

### SteamKing

Staff Emeritus
Well, for one thing, you are sloppy and inconsistent with the units of your calculation. Is area supposed to be in cm^2 or m^2? It makes a difference.

3. Mar 24, 2014

### Mutaja

I'm sorry for this. 2.5cm2 should be 0.00025m2.

Also, I've overlooked the fact that my equation should contain 1/2.

I therefore get 24.87N, and that looks alright.

Thanks for your input, even if it's just to let me know I'm not doing my work properly.