Physics · Electric Charges And Fields · NEET
Yes. They are two names for the same number. k is just a shorter way to write 1/(4πε₀). So F = k·q₁q₂/r² and F = (1/4πε₀)·q₁q₂/r² are the exact same equation. NEET uses both forms, so learn to switch between them instantly.
k = 1/(4πε₀) = 8.99 × 10⁹ N·m²/C², which we round to 9 × 10⁹ N·m²/C² in almost every NEET numerical. Use 9 × 10⁹ unless the question gives ε₀ and asks you to compute exactly.
ε₀ = 8.854 × 10⁻¹² C²·N⁻¹·m⁻² (also written C²/(N·m²) or F/m). k has units N·m²/C². They are reciprocal-style units because k has ε₀ in its denominator: k = 1/(4πε₀).
Because ε₀ sits in the bottom of k = 1/(4πε₀). A very small number in the denominator gives a very large result. Physically, vacuum has low permittivity, so it resists the field very little, which makes the electric force strong — that strength shows up as the large value of k.
Yes. ε₀ is only for vacuum (or air, nearly). In a medium the permittivity becomes ε = ε₀·K where K is the dielectric constant. This reduces the force, which is the topic of the next page, effect of medium on electrostatic force.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
k = 8.9875 × 10⁹ N·m²/C², commonly rounded to 9 × 10⁹ N·m²/C² for NEET calculations.
ε₀ = 8.854 × 10⁻¹² C²·N⁻¹·m⁻² (equivalently 8.854 × 10⁻¹² F/m), the value given in NCERT.
The 4π comes from the geometry of a sphere. Putting k = 1/(4πε₀) makes later equations, especially Gauss's law, cleaner and free of 4π factors. NCERT calls it 'for later convenience'.
Yes, ε₀ is a fundamental physical constant of free space. It is linked to the speed of light and the permeability μ₀ by c = 1/√(μ₀ε₀).
ε₀ is the basic property of vacuum; k = 1/(4πε₀) is the proportionality constant built from it that appears directly in Coulomb's law. k is large (9×10⁹) while ε₀ is small (8.85×10⁻¹²).
