Physics · Electric Charges And Fields · NEET
Put the magnitudes of both charges into F = k·q1·q2/r² to get the size of the force. The sign of the charges only tells you the direction. Like charges (both + or both −) push apart (repulsion). Unlike charges (one + and one −) pull together (attraction). NEET expects you to state size and direction separately.
r is the straight-line distance between the two point charges, measured centre to centre. It is not the radius of a sphere and not half the distance. Since force depends on r², getting r wrong squares your error, so it is the most common numerical mistake.
The exact formula F = k·q1·q2/r² is stated for point charges (tiny charged objects). For larger charged bodies you break them into small pieces and add up (superposition). For two uniformly charged spheres, you may treat each sphere's charge as if it sits at the centre, so r is the centre-to-centre distance.
Each charge feels a force set by the field of the other, and that field is proportional to the source charge. So one charge scales the force once and the second charge scales it again — the two effects multiply. This is why doubling one charge doubles F, and doubling both makes F four times larger.
Because F ∝ 1/r², doubling r makes force 1/2² = 1/4 of the original. Tripling r makes it 1/9. Halving r makes it 4 times larger. This inverse-square pattern is tested almost every year in NEET.
Two point charges A and B, having charges +Q and −Q respectively, are placed at a certain distance apart and the force acting between them is F. If 25% of the charge of A is transferred to B, then the force between the charges becomes
Two identical charged conducting spheres A and B have charge q each and a repulsive force F between them. A third identical uncharged conducting sphere is touched to A first, then to B, then removed. The new force of repulsion between A and B (treat as point charges) is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The electrostatic force between two point charges at rest is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. The force acts along the line joining the two charges.
F = k·q1·q2 / r², where k = 1/(4πε₀) = 9 × 10⁹ N·m²/C² in vacuum, q1 and q2 are the charges in coulombs, and r is the distance between them in metres.
The unit of k is N·m²/C² (newton metre squared per coulomb squared). Its value in vacuum is about 8.99 × 10⁹, usually rounded to 9 × 10⁹ N·m²/C² for NEET numericals.
Yes. The force is a vector directed along the line joining the charges. For more than two charges you find each pairwise force by Coulomb's law and add them as vectors using the superposition principle.
Placing the charges in a medium of dielectric constant K reduces the force to F = k·q1·q2 / (K·r²). Water (K ≈ 80) cuts the force to about 1/80 of its vacuum value.
