Magnitude of magnetic field at distance from central axis

1. Dec 17, 2015

qlzlahs

1. The problem statement, all variables and given/known data
A coaxial cable consists of an inner cylindrical conductor of radius R1 = 0.040 m on the axis of an outer hollow cylindrical conductor of inner radius R2 = 0.080 m and outer radius R3 = 0.090 m. The inner conductor carries current I1 = 4.4 A in one direction, and the outer conductor carries current I2 = 6.9 A in the opposite direction. What is the magnitude of the magnetic field at the following distances from the central axis of the cable?
(μ0 = 4π × 10-7 T · m/A)
(A) At r = 0.060 m (in the gap midway between the two conductors)
(B) At r = 0.150 m (outside the cable)

2. Relevant equations
1. B = (μ_0/(4pi))*(Q)*(v x r)/(r^2)
2. B = ((μ_0/(2pi))*(I)*(r^2 / R^3)

3. The attempt at a solution
I'm just really not sure how to solve this. Do I plug this in the 1st equation or the second? What is r (and R)? Is there a different equation I can use?

2. Dec 17, 2015

Orodruin

Staff Emeritus
This right here is your problem. Physics is not about finding a formula to plug things in to. In order to learn you need to understand in which situations your formulae are applicable. So what do the formulae you quoted eescribe?

3. Dec 17, 2015

qlzlahs

Whoops. So I think I use this equation for magnetic fields in cylindrical conductors?:
B = (µ_0*I*r) / (2π*a^2), where a is the radius and r is the distance.