Question

Two coaxial solenoids of different radii carry current $I$ in the same direction. Let $\overrightarrow{F_1}$ be the magnetic force on the inner solenoid due to the outer one and $\overrightarrow{F_2}$ be the magnetic force on the outer solenoid due to the inner one. Then

• A. $\overrightarrow{F_1}$ = $\overrightarrow{F_2}=0$
• B. $\overrightarrow{F_1}$ is radially inwards and $\overrightarrow{F_2}$ is radially outwards
• C. $\overrightarrow{F_1}$ is radially inwards and $\overrightarrow{F_2}=0$
• D. $\overrightarrow{F_1}$ radially outwards and $\overrightarrow{F_2}=0$

Answer 1

The correct option is A $\overrightarrow{F_1}$ = $\overrightarrow{F_2}=0$

Consider the two coaxial solenoids. Due to one of the solenoids magnetic field at the centre of the other can be assumed to be constant.

Due to symmetry, forces on upper and lower part of the solenoid will be equal and opposite and hence resultant is zero.