Question

A rod AB moves with a uniform velocity $v$ in a uniform magnetic field as shown in figure.

• A. The rod becomes electrically charged
• B. The end A becomes positively charged
• C. The end B become positively charged
• D. The rod becomes hot because of Joule heating

Answer 1

The correct option is B The end A becomes positively charged

Magnetic force $F=q(\overrightarrow{v}\times \overrightarrow{B})=(-e)(v\hat{i}\times (-B\left)\hat{k}\right)=-ebB\hat{j}$ where we have taken the charge of electron to be (-e) where e is positive. The direction of the magnetic field is taken to be $-B\hat{k}$ i.e into the plane of the paper and velocity is taken along the +ve x-axis.
Thus electrons will move towards the end B and accumulate at end B.
Thus, the end A will become deficient in electrons and will be positively charged.