Question

There are two point charges +1 μC and –2 μC. The ratio of magnitude of electrostatic forces acting on them respectively will be

Answer 1

Given:

There are two point charges +1 μC and –2 μC.

To find:

The ratio of the magnitude of electrostatic forces acting on them respectively will be

Solution:

Coulomb's law states that the magnitude of the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.

F=k Q1Q2/r^2

where Q1 and Q2 are the magnitudes of the two charges respectively and r is the distance between them.  

The proportionality constant k is called the electrostatic constant.

The force exerted by charge Q1 = +1mC on charge Q2 is,

F1 = kQQ2/r^2

The force exerted by charge Q2 = -2mC on charge Q1 is,

F2 = kQ1Q/r^2

So the ratio of the force exerted by both charges on each other will be,

F1/F2 = kQQ2/r^2 / kQ1Q/r^2

F1/F2 = Q2/Q1 = -2/1

∴ F1 : F2 = -2 : 1