Electric Field Intensity — Definition, Formula and SI Unit

Definition: The electric field intensity at a point is the electrostatic force experienced per unit positive test charge placed at that point.

Formula: E = F/q

Where: • E = Electric field intensity (vector) • F = Electrostatic force on test charge (N) • q = Test charge (C) [must be small enough not to disturb the field]

SI Unit: • E = F/q → [N/C] or equivalently [V/m] • 1 N/C = 1 V/m

Dimensional formula of E: E = F/q = [MLT⁻²] / [AT] = [MLT⁻³A⁻¹]

Electric field due to point charge (Coulomb's Law): E = kq/r² = q/(4πε₀r²)

Where: • k = 9 × 10⁹ N·m²/C² (Coulomb's constant) • ε₀ = 8.854 × 10⁻¹² C²/N·m² (permittivity of free space) • q = source charge (C) • r = distance from charge (m)

Key properties of electric field:

1. Vector quantity (has direction) • For positive charge: E points away from the charge • For negative charge: E points toward the charge

2. Superposition principle: • Net E = vector sum of individual fields due to each charge

3. Electric field lines: • Start from positive charges, end at negative charges • The density of field lines indicates field strength • Field lines never intersect

4. Uniform electric field: • Between parallel plates: E = V/d (V = voltage, d = distance) • Direction: from +ve plate to −ve plate

Solved Example 1: A charge of 2 μC experiences a force of 0.1 N. Find E. E = F/q = 0.1 / (2 × 10⁻⁶) = 5 × 10⁴ N/C

Solved Example 2: Find E due to a 4 μC charge at 0.3 m distance. E = kq/r² = (9×10⁹ × 4×10⁻⁶) / (0.3)² = 36000 / 0.09 = 4 × 10⁵ N/C

Relation: E = V/d (uniform field) • If a 100 V potential difference across 2 m gap: E = 100/2 = 50 V/m = 50 N/C