Question

A magnetic field exerts no force on:
A. A magnet
B. An unmagnetized iron bar
C. A moving charge
D. Stationary charges

Answer 1

In this question, we need to comment where a magnetic field exerts no force. For this, we will use the definition of the magnetic field and follow the properties of the magnetic fields as well.

Complete step by step answer:
Lorentz equation for magnetic force is given as $F = q\left( {V \times B} \right)$, where $q$ is the amount of electric charge, $V$ is the velocity of charge, $B$ is magnetic flux and $F$ is the Magnetic force.
We know that the magnetic force on a charged particle is given as
$F = q\left( {V \times B} \right)$ where velocity of the charge is perpendicular to the magnetic field. When we pace a stationary charge in the magnetic field it experiences no force on it since the velocity of the particle is zero.
We know magnetic force on a charged particle is given as
$F = q\left( {V \times B} \right) - - (i)$
Since the stationary charges has no velocity to it so $V = 0$,
Now by substituting the velocity $V = 0$ in equation (i), we get
$F = q\left( {0 \times B} \right) \\\ \therefore F= 0 \\\$
Hence we can say when stationary charges are placed in the magnetic field they experience no force on them.

So, the correct answer is “Option D”.

Note:
Students must note that the work done in moving a charged particle between two points is equal to zero in the magnetic field because the magnetic force is always perpendicular to the velocity of that charged particle.