# Magnetic Fields and the Biot-Savart Law

Hi, I'm new here but been looking to you guys for solutions for a while now. However, I cannot find the kind of solution on here already, so I thought I'd put it out here.

My main problem with physics is always just finding the formulas I need. If you can at least point me in the right direction with the right formula(s), I'll be most grateful.

## Homework Statement

The magnetic field 35.0 cm away from a long, straight wire carrying current 7.00 A is 4.00 µT.

(a) At what distance is it 0.300 µT?
466.6667 cm

(b) At one instant, the two conductors in a long household extension cord carry equal 7.00 A currents in opposite directions. The two wires are 3.00 mm apart. Find the magnetic field 35.0 cm away from the middle of the straight cord, in the plane of the two wires.

??? nT

(c) At what distance is it one tenth as large?

??? cm

(d) The center wire in a coaxial cable carries current 7.00 A in one direction, and the sheath around it carries current 7.00 A in the opposite direction. What magnetic field does the cable create at points outside?

??? nT

## Homework Equations

B= µ0*I / 2*pi*r, where µ0 is 4*pi x 10^-7, I is current in A, and r is distance in meters.

I'm sure there are others, but if I knew them, I'd wouldn't be here.

## The Attempt at a Solution

Part A)
B = µ0*I / 2*pi*r
(3e-7 T) = (12.56637e-7)*(7 A) / 2*(3.14159...)*r
r = (12.56637e-7)*(7 A) / 2*(3.14159...)*(3e-7 T) = 14/3 m = 4.6667 m = 466.6667 cm

Part B) ??? No idea where to start... I thought the 2 opposite forces would cancel each other out, but not totally it seems. It asks the answer in nano-tesla, so I assume that there is some canceling out here.

Part C) Extension of part B, asking where the field is 1/10th the field is in part B.

Part D) Seems similar to part B, but you tell me.