Physics / A2 level
Distinguish the direction of force from scalar electric potential.
Electric field strength is force per unit positive test charge. A negative charge experiences a force opposite to the field direction. Field lines describe a vector field; they are not paths that every moving charge must follow.
Potential is energy per unit charge. Point-charge field strength falls as inverse distance squared, while potential varies as inverse distance. Between closely spaced parallel plates away from edges, a uniform-field approximation can be useful.
Electric field strength is force per unit positive test charge. A positive charge accelerates along the field if no other force acts, while a negative charge feels force in the opposite direction. Field lines represent the field, not guaranteed particle paths: a charge may already have transverse velocity. Around a point charge, field strength magnitude varies as 1/r².
Electric potential is potential energy per unit charge, usually measured relative to a stated zero. For an isolated point charge with zero at infinity, V = Q/(4πε₀r). Potential contributions add as scalars; field contributions add as vectors. At the midpoint of equal positive and negative charges, potentials cancel while their fields reinforce. Zero potential therefore does not imply zero field.
Between closely spaced parallel plates, away from edges, E ≈ ΔV/d. This is not the general expression for every electric field. A 200 V difference across 0.010 m corresponds to about 20000 V/m. Edge regions curve the field, so a real deflection experiment must consider the region traversed. The field direction points towards decreasing potential, opposite the potential gradient.
A charge moved through a potential difference changes electric potential energy by qΔV. A freely accelerated charge can convert that change into kinetic energy when other transfers are negligible. Track the sign of q and the initial and final potentials; an electron need not move towards lower potential energy by moving towards lower electric potential. Potential and potential energy are related but not interchangeable.
Its charge is negative, so increasing electric potential decreases its electric potential energy. That decrease can supply kinetic energy as the electric force accelerates it.
The sign of q determines the force direction.
Uniform-field magnitude between plates separated by d.
Potential of an isolated point charge with zero at infinity.
A +2.0 μC charge is in a field of 3000 N/C. What force magnitude acts?
F = qE = 2.0 × 10⁻⁶ × 3000 = 0.006 N.
Place equal positive and negative simulated charges symmetrically. Investigate potential and field at their midpoint.
Compare field arrows with potential contours, then inspect a parallel-plate field. A static charging demonstration does not replace quantitative field calculations or represent every particle trajectory.
Opposite to the electric field
F = qE reverses direction when q is negative.
0.006 N
F = qE = 2.0 × 10⁻⁶ × 3000 = 0.006 N.
Potential energy per unit charge
Potential has units J/C, equivalent to volts.