# Homework Help: How do you find the magnetic field?

1. Jan 21, 2008

### oxjenpenaxo

1. The problem statement, all variables and given/known data
A 61 mg mass just balances the balance arm of a current balance when the strip current is 3.0 A. If the strip is 2.2 cm long, what is the magnetic field strength inside the solenoid in which the current balance is located?

2. Relevant equations
F = BIl (Magnetic Field x Current x Length)
F = mg (Mass x Gravity)

3. The attempt at a solution
I was thinking that the weight of the mass is equal to the force of the magnetic field so I made the forces equal to each other.

BIl = mg

and solved for B

B = mg/Il
= 9.06 T

but it turns out the answer is wrong...

2. Jan 21, 2008

### Kurdt

Staff Emeritus
At a glance it looks as though you've simply forgotten to convert the units to SI units.

3. Jan 21, 2008

### rock.freak667

Well I got what you got using g=9.81ms$^{-2}$. Could it be that you have to use a different value for g? (Or my calculator's battery is dying...)

EDIT: What is the correct answer btw?

Last edited: Jan 21, 2008
4. Jan 21, 2008

### oxjenpenaxo

It says that the correct answer is 9.1 x 10^-3 T

Oh... I had another problem like this and I was also off by like some 10^-3
but I got the 9.1 part right... ehe....

Last edited: Jan 21, 2008
5. Jan 22, 2008

### Kurdt

Staff Emeritus
Like I said before its a units thing. I get the correct answer.

6. Jan 22, 2008

### oxjenpenaxo

yeah haha alrighty thanks