A 1 cm segment of a wire lying along $x$-axis carries current of 0.5 A along $+x$ direction. A magnetic field $\vec{B}=(0.4 \mathrm{mT}) \hat{j}+(0.6 \mathrm{mT}) \hat{k}$ is switched on, in the region. The force acting on the segment is

You are required to design an air-filled solenoid of inductance 0.016 H having a length 0.81 m and radius 0.02 m . The number of turns in the solenoid should be

Assertion (A) : The deflection in a galvanometer is directly proportional to the current passing through it.

Reason (R) : The coil of a galvanometer is suspended in a uniform radial magnetic field.

A particle of mass $m$ and charge $q$ describes a circular path of radius $R$ in a magnetic field. If its mass and charge were $2 m$ and $\frac{q}{2}$ respectively, the radius of its path would be