A very long coaxial cable consists of an inner thick wire (radius R) carrying a

Question

A very long coaxial cable consists of an inner thick wire (radius R) carrying a current of 5A, and an outer thick wire encasing the inner wire (starting at R and going out to 2R) and carrying 5A in the opposite direction. What statement is true about how the magnetic field changes as a function of distance from the cable centre, r?

Also, please explain why.

Magnetic field is zero for r=0 to R, then increases linearly as we go from r=R to 2R

Magnetic field is 0 everywhere inside the cable.

Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R.

Magnetic field increases linearly from the centre of the cable, until we reach r=2R.

Magnetic field is zero for r=0 to R, then increases linearly as we go from r=R to 2R

Magnetic field is 0 everywhere inside the cable.

Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R.

Magnetic field increases linearly from the centre of the cable, until we reach r=2R.

Explanation / Answer

The correct answer is c.

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