# Homework Help: Compute ΔB seen by a magnetometer flown on a satellite

1. Apr 24, 2012

### harlowz2

1. Sample calculation - If a satellite carrying a magnetometer flies at 300km over a current that is flowing at 100km w/ a magnitude of 150 μA/m2 (over a cross-section of ~5 km2), how big a ΔB will be seen by the magnetometer?
Assumptions (based on my understanding):
-Satellite altitude remains constant
-Current altitude and magnitude remains constant
-Speed of the satellite does not matter

Knowns:
Current density (Amps/m2) = J = 150
Distance between magnetometer and current = r = 200 km
Current area = Area = 5 km2

2. I have very limited experience with magnetic field calculations, but I believe the equation that should be used is as follows (note: instructor did not provide any equation):
A form of the Biot-Savart Law ⇔ B= (μ0/4$\pi$) * $\int$(J x r)/r2 d$\upsilon$

3. I do not have an attempt at a solution, since I have no idea where to start. Please help!!

UPDATE: Here is what I have tried so far, and the units appear to check out.
Using a scalar form of the Biot-Savart Law, and assuming a unit depth for the volume integral, I get...
B = (μ0/4$\pi$) * (J * Area)/r2

After plugging numbers in, I arrive at: ΔB = 1.1111*10-12 T = 1.110 pT

Is this anywhere close, or even sound right? I have the tendency to pull ideas out of thin air.

Thanks,
Zack

Last edited: Apr 24, 2012
2. Apr 25, 2012

### diazona

First thought:
Why do you do that?

3. Apr 26, 2012

### harlowz2

I was assuming a scalar form because:
1) All values given to me in the problem statement are scalar, not vector.
2) I really have no idea how to do the problem. Doing that is my best attempt at the problem.

4. Apr 28, 2012

### diazona

Sorry for the delayed response, I had some trouble getting to the site yesterday.
OK, well, it's a good thought, but that's not a correct assumption. Even if you're only given scalar values in the question, you will still generally have to use the vector form of the equation. The scalar version you used applies only under certain very specific conditions.

In this case, you'll have to use the geometry of the problem to determine the vectors you need to use. I would suggest starting by drawing a diagram, and if you can, upload it as an attachment here.