Electric Fields · revision map · A-Level 9702, Topic 18

TOPIC 18 · ELECTRIC FIELDS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
thelucidstem.com
ELECTRIC FIELDS18
Field Strength & Uniform Fields

A field is the force per unit positive charge on a small test charge.

E = F / q
unit N C−¹ or V m−¹
E is a vector
  • Field lines run from + to −; their density shows strength.
  • Between parallel plates the field is uniform.
E = V / d
uniform field between plates a distance d apart
+ + + + + + − − − − − − E = V / d, same everywhere
Coulomb's Law

Two point charges push or pull along the line joining them, an inverse-square law.

F = Q₁Q₂ / (4πε₀r²)
ε₀ = 8.85 × 10−¹² F m−¹ · r centre to centre
  • Force ∝ each charge, and ∝ 1 / r².
  • Like charges repel; unlike charges attract.
  • A charged sphere acts as a point charge at its centre.
+ + r F F like charges: equal and opposite forces, pushed apart
Field of a Point Charge

A point charge has a radial, inverse-square field.

E = Q / (4πε₀r²)
  • Divide Coulomb's force by the test charge: E = F / q.
  • Lines point outward from +Q, inward toward −Q.
  • Same inverse-square shape as the gravitational field.
+ Q E falls as 1/r²
Electric Potential

Work done per unit positive charge to bring it from infinity, zero at infinity.

  • Field is the negative potential gradient: E = −dV/dr.
  • Positive near +Q, negative near −Q.
  • Energy of a pair of charges: Ep = Qq / (4πε₀r).
V = Q / (4πε₀r)
V r V = Q/4πε₀r V → 0 as r → ∞ (+Q)
BUILDS ON · Charge & current · IGCSE electricity → ELECTRIC FIELDS → LEADS TO · Capacitance · Gravitational fields · Magnetic fields
thelucidstem.com · Cambridge AS & A Level Physics 9702 · TheLucidSTEM revision map