Module 3.9 Capacitance and capacitors

Knowledge levels

Sub-module	A	B1 / B2 / B2L	B3
3.9 Capacitance/capacitor	—	2	1

Content: operation and function; factors affecting capacitance; working voltage; types, construction and colour coding; series and parallel calculations; exponential charge/discharge and time constants; testing.

What a capacitor does

• Two conducting plates separated by an insulator, the dielectric.
• When connected to a DC supply, electrons pile up on one plate and leave the other; the capacitor stores energy in an electric field between the plates.
• Once charged it blocks steady DC; it passes AC (continuously charging and discharging). Uses: smoothing ripple, coupling AC signals while blocking DC, timing circuits, tuned circuits, motor start, suppression of interference.

Capacitance

C = Q / V, measured in farads (F). One farad stores 1 coulomb per volt. Practical values: µF, nF, pF.

Factors (parallel-plate capacitor): C = ε₀ εᵣ A / d (× (n − 1) for n plates)

Change	Effect on C
Larger plate area A	increases
Smaller distance d between plates	increases
More plates (interleaved)	increases
Higher dielectric constant εᵣ	increases

Relative permittivity (dielectric constant) εᵣ: air/vacuum 1, paper about 2-3, mica 5-7, ceramics 10 to several thousand.

Energy stored: W = ½ C V².

Key fact: Capacitors can hold a lethal charge long after the power is removed. Discharge them through a suitable resistor before handling (e.g. strobe-light power units).