TWO WAYS TO MAKE THE PLATES

Foil or a whisper of metal

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Separate aluminium foil, about 5–10 µm thick, laid against plain film

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A few tens of nanometres of aluminium or zinc evaporated onto the film in vacuum

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Metallization is hundreds of times thinner than foil, and that changes everything

Same film, same physics: the difference is how thick the metal is.

SELF-HEALING

A capacitor that repairs itself

Film/foil Molten metal from a breakdown bridges the hole: a dead short

Metallized The thin metal around the fault vaporizes, isolating it in microseconds

The cost A tiny patch of electrode, so the part loses a sliver of capacitance

In metallized film a defect costs a little capacitance; in film/foil it usually costs the part.

FILM/FOIL STRENGTHS

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Peak current Thick foil carries the highest currents

Voltage The highest voltage ratings, into the hundreds of kilovolts

Stability Capacitance stays put over life

Weakness No self-healing: a breakdown is normally the end, so cleanliness is everything

Film/foil is the choice when current and voltage are extreme and the process can keep defects out.

METALLIZED STRENGTHS

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Energy density Thin electrodes leave more room for dielectric

Graceful aging Parts lose capacitance slowly instead of failing suddenly

Segmented patterns Some films are metallized in patches joined by tiny fuses that blow and isolate a bad area

Weakness The thin metal limits peak current

Metallized film is the choice for defibrillators, DC link and power-factor capacitors, where density and graceful aging matter.

GAME

Each job below favours one construction. Decide which, then check the reason.

Ask two questions: how extreme is the current, and how much does compactness matter?

UNIT 4 STUDY COMPLETE

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You've compared film/foil with metallized film and seen how self-healing turns a fatal defect into a tiny loss.