In a coaxial, straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero.

Options

A. zero at a point on axis of inner cable
B. zero at a point outside the cable
C. maximum at the surface of inner cable
D. zero in between inner and outer cable

Answer

a, b, c

Explanation

If we apply Ampere.s law, then the magnetic field inside the inner cable is same as that due to a cylindrical wire. Hence, the field will be zero at a point on the axis of cable and it becomes maximum at the surface of theinner cable. Outside the inner cable, the field starts decreasing and when the outer cable is also being included in the amperes loop then the magnetic field further goes down due to decrease in enclosed current because the outer cable carries current in the opposite direction. Finally for points outside the co-axial cable, the net current enclosed in amperes loop becomes zero (equal and opposite currents) and hence the magnetic field becomes zero for all points outside the cable. There is a magnetic field in the gap between the inner and outer cables. Hence options (a), (b) and (c) are correct.