Physics · Electric Charges And Fields · NEET
It DECREASES. In vacuum the force is F0 = (1/4πε0) q1q2/r². In a medium the permittivity becomes ε = K·ε0, and since K is always greater than 1, the denominator gets bigger, so the force gets smaller. F_medium = F0 / K. Air is a special case: K for air is almost 1 (about 1.0006), so the force in air is basically the same as in vacuum.
A dielectric medium has molecules that get polarised by the field of the charges. These polarised molecules create their own small field pointing opposite to the original field. NCERT calls this the 'opposing field' that 'only reduces' the external field. So the net field between the two charges, and therefore the force, is reduced. A bigger K means stronger polarisation and more reduction.
Permittivity ε is the actual property of the medium and has units (C²N⁻¹m⁻²). Dielectric constant K (also called relative permittivity εr) is just a ratio: K = ε/ε0. It has NO units. K tells you how many times bigger the medium's permittivity is compared to vacuum. So ε = K·ε0. Use K in the force formula, use ε when the question gives you the absolute value.
For vacuum, K = 1 exactly, so F = F0 (no change). For air, K ≈ 1.0006, which is so close to 1 that in NEET problems air is treated as vacuum unless told otherwise. Only when the medium is water (K ≈ 80), glass, oil, or a numbered dielectric slab does K matter.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
F = (1/4πε) q1q2/r² where ε = Kε0. This equals F0/K, where F0 is the vacuum force and K is the dielectric constant of the medium.
Yes. NCERT states K is always greater than 1 for any real medium. Only vacuum has K exactly equal to 1. This is why any medium reduces the force.
About 80. So two charges in water feel a force 80 times weaker than the same charges the same distance apart in vacuum. This is why salts (ionic compounds) dissolve easily in water - the pull between opposite ions drops sharply.
No. The separation r stays the same. Only the permittivity in the formula changes from ε0 to Kε0. If both distance and medium change, apply both effects: multiply the r² change and divide by K.
Same idea. A dielectric between capacitor plates reduces the field, which is why capacitance increases by factor K: C = K·C0. The force page and the capacitor page both use the same dielectric constant K.
