Question

A long straight wire carries $10 A \,d.c$. current an electron travels perpendicular to the plane of this wire at a distance $0.1\, m$ with velocity $5.0 \times10^{6} m \,s^{-1}$. Force acting on the electron due to current in wire is

• A. zero N
• B. $0.6 \times10^{-17} N$
• C. $2.4 \times10^{-17} N$
• D. $2.2 \times10^{-17} N$

Answer 1

Answer: (A)

Force on the electron $\vec{F}=-e(\vec{v} \times \vec{B})$. As electron is travelling in a plane perpendicular to the plane of wire, so angle $\theta$ between $\vec{v}$ and $\vec{B}$ is either $0^{\circ}$ or $180^{\circ}$ and $\sin 0^{\circ}$ or $\sin 180^{\circ}=0$.