Question

A magnetic field exerts no force on
(A) A stream of electrons
(B) A stream of protons
(C) Unmagnetized piece of iron
(D) Stationary charge

Answer 1

Hint: We all know that an electric charge experiences a force when placed in an electric field. Similarly, a magnetic substance experiences force when placed in a magnetic field. But there are a lot of ways to produce magnetic fields, and hence we will have to analyse the conditions and check if it will experience a force or not.

Complete answer:
The force exerted on a charged particle is given by the Lorentz equation which is as follows
\[F=q(v\times B)\] where \[F\] is the force experienced, \[q\] is the amount of electric charge, \[v\] is the velocity of the charge and \[B\] is the magnetic flux in the field
The above equation can be simplified as \[F=qvB\sin \theta \]
Hence the force experienced will be zero if any of the terms in the above equation is zero. The force experienced will be zero if the charge on the particle is zero, that is, the particle is uncharged or if the velocity of the particle is zero, that is the particle is at rest. The force experienced is zero when the angle between the velocity of the charge and the magnetic flux is zero.
Now analysing the given options, we can say that a magnetic field exerts no force on a stationary charge.

So, the correct answer is “Option D”.

Note:
In the options (A) and (B) given to us in the question, there is a stream of electrons and protons. A stream of electrons and protons implies that a current is flowing in the region and the particle will experience a force on it. When an unmagnetized piece of iron will be placed in the magnetic field, the molecular magnetic dipoles will align in a manner to magnetise the bar.