JEE Main · 2015 · 04 Apr 2015 · Offline · Magnetic Effects of Current

Two coaxial solenoids of different radii carry current I in the same direction. Let

F_{1}

be the magnetic force on the inner solenoid due to the outer one and

F_{2}

be the magnetic force on the outer solenoid due to the inner one. Then:

(A) $F_{1}$ is radially inwards and $F_{2}$ is radially outwards
(B) $F_{1}$ is radially inwards and $F_{2} = 0$
(C) $F_{1}$ is radially outwards and $F_{2} = 0$
(D) $F_{1} = F_{2} = 0$ ✓ Correct

Answer: (D)

Solution

Inside a long solenoid the field is uniform and runs along the axis, while the current in the windings circles the circumference, perpendicular to that axis. The force on each element of winding is

d F = I d ℓ \times B

. For the inner solenoid sitting in the outer one's axial field this gives a purely radial force that, summed around the full circle, cancels to zero. The outer solenoid lies in the inner one's external field, which is essentially nil, so it feels nothing either. Neither coil experiences any net magnetic force.

JEE Main · 2015 · 04 Apr 2015 · Offline · Single correct option

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JEE Main · 2015 · 04 Apr 2015 · Offline · Magnetic Effects of Current

Solution

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