A charged particle moves in a circle of radius $R$ in a region of uniform magnetic field. The magnetic field is at right angles to the velocity of the particle and exerts a force $F$ on the particle. After half a revolution the change in the particle's kinetic energy is:

A negatively charged particle is at rest in a magnetic field $B$. The force on the particle is :

An electron enters a region of magnetic field.

In which case is the initial force on the electron directed towards the bottom of the page?

In which of the following arrangements is the total resistance $6 \mathrm{\Omega }$?

In which of the resistors in the circuit below is the power dissipated the least?

Two long parallel wires carry equal currents in opposite directions. What field do the two wires produce at point M, which is midway between the wires and on the plane of the paper?

A student assigns currents at a junction in a circuit as shown in the diagram.

The student's calculations correctly give that $I_1=3 \mathrm{A}$ and  $I_2=-2 \mathrm{A}$. The value of $I_3$ is: