Capacitance
The charge stored per unit potential difference across a conductor or capacitor: C = Q/V, measured in farads (F), where 1 F = 1 C/V. Capacitance of a parallel-plate capacitor is C = ε₀εᵣA/d, where A is plate area, d is separation, ε₀ = 8.85 × 10⁻¹² F/m is permittivity of free space, and εᵣ is relative permittivity of the dielectric. Capacitance is a constant for a given geometry and material.
Formula
C = Q/V = ε₀εᵣA/d
Real World
The regenerative braking system in a Formula E racing car uses supercapacitors with capacitances exceeding 3,000 F to absorb braking energy in milliseconds and release it for acceleration — something conventional batteries cannot match at that rate.
Exam Focus
When asked how changing plate separation affects capacitance and stored energy, use both C = ε₀A/d and E = Q²/2C — and state whether Q or V is held constant.
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