Electric Field

What these notes cover

These notes cover electric fields for Cambridge International AS & A Level Physics (9702): electric field strength and field lines, uniform fields between parallel plates, the motion of charged particles in a uniform field, Coulomb’s law and the field of a point charge, and electric potential and equipotentials.

Key formulas and definitions

• Electric field strength is the force per unit positive charge: E = F / q (unit N C⁻¹, equivalent to V m⁻¹).
• Uniform field between parallel plates: E = ΔV / d. Field lines run from positive to negative; equally spaced parallel lines show a uniform field.
• A charged particle in a uniform field feels a constant force F = qE, so a particle entering at right angles to the field follows a parabolic path.
• Coulomb’s law: F = Qq / (4πε₀r²) for two point charges a distance r apart.
• Field strength of a point charge: E = Q / (4πε₀r²).
• Electric potential at a point is the work done per unit positive charge in bringing a small test charge from infinity to that point: V = Q / (4πε₀r).
• Electric potential energy of two point charges: Eₚ = Qq / (4πε₀r). Field strength is the negative potential gradient: E = −ΔV / Δx.
• Equipotentials are surfaces of equal potential; no work is done moving a charge along an equipotential.