A very long coaxial cable consists of an inner thick wire (radius R) carrying a
ID: 1480631 • Letter: 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?
A. Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R
B. Magnetic field increases linearly from the centre of the cable, until we reach r=2R.
C. Magnetic field is 0 everywhere inside the cable.
D. Magnetic field is zero for r=0 to R, then increases linearly as we go from r=R to 2R
A thick current carrying wire has radius R. The current density (current/area) is not constant, but increases proportional to the square of the distance from the centre of the wire, r. The total current is I. What statement is true about the magnetic field inside (r<R) and outside (r>R) this wire?
Inside the wire, magnetic field is proportional to r; outside it behaves the same as for a wire with constant current density.
A.Inside the wire, magnetic field is proportional to ; outside it is the same as for a wire with constant current density.
B.Inside the wire, magnetic field is proportional to ; outside it behaves the same as for a wire with constant current density.
C.Inside the wire, magnetic field is proportional to r; outside it behaves the same as for a wire with constant current density.
D.Inside the wire, magnetic field is proportional to ; outside it increases proportional to r.
Explanation / Answer
1) A. Magnetic field increases going from r=0 to R, then decreases from r=R to 2R becoming 0 at r=2R
2) B. Inside the wire, magnetic field is proportional to r cubed ; outside it behaves the same as for a wire with constant current density.
we can explain the above two statements by using Ampere's law