If a wire of length ' $L$ ' carrying a current ' $i$ ' is bent in the shape of a semi-circular arc as shown in the figure, then the magnetic field at centre of the arc is

A galvanometer having 30 divisions has a current sensitivity of $0.0625 \frac{d i \nu}{\mu A}$. If it is converted into a voltmeter to read a maximum of 6 V , then the resistance of that voltmeter is

The magnetic field at the centre of a long solenoid having 400 turns per unit length and carrying a current ' $i$ ' is $6.24 \times 10^{-2} \mathrm{~T}$. The magnetic field at the centre of another long solenoid having 200 turns per unit length and carrying a current $\frac{i}{2}$ is

If a proton of kinetic energy 8.35 MeV enters a uniform magnetic field of 10 T at right angles to the direction of the field, then the force acting on the proton is

(Mass of proton $=1.67 \times 10^{-27} \mathrm{~kg}$ and Charge of proton $=1.6 \times 10^{-19} \mathrm{C}$ )