Question

Work done on an electron moving in a solenoid along its axis is equal to

• A. zero
• B. -evB
• C. ilB
• D. None of these

Answer 1

Answer: (A)

A magnetic field is produced around and within the solenoid. The magnetic field within the solenoid is uniform and parallel to the axis of solenoid. $F_m=q(\overrightarrow{\mathbf{v}}\times \overrightarrow{\mathbf{B}})=Bqv\sin \theta$ Since, angle between $\overrightarrow{\mathbf{v}}$ and $\overrightarrow{\mathbf{B}}=0^o$ , therefore $F_m=0$ . Now, $W=\overrightarrow{\mathbf{F}}\times \overrightarrow{\mathbf{d}}\cos \varphi$ Since $F=0$ $\Rightarrow$ $W=0$