# Homework Help: Cancelling the Earths magnetic field

1. Nov 12, 2012

### deborahcurrie

1. The problem statement, all variables and given/known data
A researcher would like to perform an experiment in a zero magnetic field, which means that the field of the earth must be cancelled. Suppose the experiment is done inside a solenoid of diameter 1.0{\rm m} , length 5.0{\rm m} , with a total of 9000 turns of wire. The solenoid is oriented to produce a field that opposes and exactly cancels the field of the earth.
What current is needed in the solenoid's wires?

2. Relevant equations
We use this for B=μ0*I*(N/L) for solenoids.
Bearth=5E-5T
μ0=1.257E-6
N=9000
L=5.0 m

3. The attempt at a solution
I=(B*L)/μu0*N)=(5E-5*5)/(9000*5) = 0.02209=2.2E-2
Mastering Physics says this is incorrect. What did I miss?

2. Nov 12, 2012

### Delphi51

The formula for I looks good. Trouble with the substitution, I think. Looks like the number for u0 didn't go in and there is an extra 5.

3. Nov 15, 2012

### deborahcurrie

That was my bad. My actual equation looks like:

(5E-5*5)/(1.257E-6*9000)=0.02209 which is wrong according to Mastering physics. Doesn't like 2.2E-2 either.

4. Nov 15, 2012

### haruspex

Earth's field varies greatly from place to place. Is this the value you were told to use?

5. Nov 15, 2012

### deborahcurrie

I talked with the instructor and found my error -- I just needed to adjust for units the question required. I usually catch this, but not this time. Thanks for your replies anyways. It's always good to know there is a safety line!

6. Nov 15, 2012

### haruspex

OK, I did wonder about the units. In your post it says e.g. "diameter 1.0{\rm m}". LaTex problem?
PS - I guess you mean the units the answer was required in, like mA?

7. Nov 16, 2012

### deborahcurrie

Not sure what a La Tex problem is -- but you are correct on the PS