There is no change in the energy of a charged particle moving in a magnetic field although a magnetic force is acting on it . Is this true or false ?

Ans. True . A charged particle experiences a force at right angles to the velocity and so it moves in a circle with a constant speed . So there is no change in the energy of the particle through a magnetic force acts on it .

If an electron is not deflected in passing through a certain space , can we be sure that there is no magnetic field in that region ?

Ans. No, We can not be sure that there is no magnetic field because the force will be zero when the direction of motion is along the direction of the field .

Why is current not a vector though we represent its sense by an arrow in a diagram . Can you name one more physical quantity that , like current , is a scalar , but has a sense represented by an arrow in a diagram ?

Ans. Because current does not get added or subtracted according to vector algebra , it is not a vector . Heat , current , time , etc. , have a sense but they are not vectors .