Physics · Electric Charges And Fields · NEET
It is kq/r² (r squared). The 1/r term with k is the electric POTENTIAL V = kq/r, not the field. Field is E = kq/r². A quick check: field has units N/C (or V/m), and only kq/r² gives those units. Mixing these two is the most common NEET slip.
Electric field is a property of the source charge q alone at every point in space, even when no other charge is there. We define it as E = F/q₀, the force per unit positive test charge q₀. When you compute F = kqq₀/r² and divide by q₀, the test charge cancels, leaving E = kq/r². So the field belongs to q; the second charge only appears when you later ask about force.
The sign of the source charge. For a positive charge the field points radially OUTWARD (away from q) at every point. For a negative charge it points radially INWARD (toward q). Always drop a test charge and imagine the force on it if it were positive.
Force needs two charges: F = kqq₀/r² acts on q₀. Field needs only one: E = kq/r² exists whether or not a test charge is present. Link them with F = q₀E. So field is 'force waiting to happen' per coulomb of test charge.
The magnitude formula stays E = k|q|/r² (use the size of the charge). Only the direction flips to inward. In vector form E = kq r̂/r² already handles the sign automatically: a negative q makes the vector point opposite to r̂ (inward).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
E = kq/r² = q/(4πε₀r²), with k = 1/(4πε₀) = 9 × 10⁹ N·m²/C². Here q is the source charge and r is the distance from it to the point.
Newton per coulomb (N/C), which is the same as volt per metre (V/m). Both come from E = F/q₀ and E = V/r.
It follows an inverse-square law: E ∝ 1/r². Doubling the distance makes the field one-fourth; tripling it makes the field one-ninth.
E = kq/r² = (9 × 10⁹ × 2 × 10⁻⁶) / (1)² = 1.8 × 10⁴ N/C, directed radially outward since q is positive.
It is a vector. It has both magnitude (kq/r²) and direction (radially outward for +q, inward for −q). This is why fields from many charges are added by the vector superposition principle.
