THE IDEA

Two plates and a gap

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Plates

Two conductors that hold equal and opposite charge

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Dielectric

An insulator between them that stops current and holds the field

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Field

The energy lives in the electric field inside the dielectric

A capacitor stores energy in an electric field, not in a chemical reaction, so it can give it back almost instantly.

WHAT SETS CAPACITANCE

Big area, thin gap

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C = ε₀ε_rA / d Capacitance C, in farads, for two flat plates

Area A Bigger plates hold more charge at the same voltage

Gap d A thinner dielectric gives more capacitance

ε_r The dielectric constant: about 2.2 for polypropylene film

ε₀ A constant of nature, 8.85 × 10⁻¹² F/m

Thin film and lots of area: that's why film capacitors are long strips rolled up tight.

STORED ENERGY

Energy grows with voltage squared

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Q = CV Charge stored, in coulombs

E = ½CV² Energy stored, in joules

100 µF at 2 kV ½ × 100 × 10⁻⁶ × 2,000² = 200 J

100 µF at 4 kV 800 J: double the voltage, four times the energy

Because energy goes as V², pushing the voltage up is the cheapest way to store more: that's why high-voltage capacitors exist.

THE UNITS

Reading a capacitor's label

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µF, nF Capacitance: microfarads and nanofarads

kV Rated voltage: what it can hold for its rated life

J Stored energy at rated voltage

J/cm³ Energy density: how much energy fits in each cubic centimetre

±10% Capacitance tolerance, typical for high-voltage film parts

Voltage and capacitance on the label tell you the energy; energy density tells you how hard the design is working.

A SHORT HISTORY

From jars to films

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1745–46 The Leyden jar: a glass jar coated with metal foil inside and out, the first capacitor

Early 1900s Wax- and oil-impregnated paper between metal foils for radio and power

Mid 1900s Plastic films such as polystyrene and polyester replace much of the paper

Later 1900s Polypropylene film becomes the workhorse for high-voltage, AC and pulse capacitors

The idea hasn't changed since the Leyden jar: two conductors, a good insulator, and as little gap as you can get away with.

UNIT 1 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how a capacitor stores energy, what sets its capacitance, and why voltage is the big lever.