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How does a capacitor work?

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Quick answer

A capacitor works by holding opposite electric charges on two metal plates separated by an insulator. Applying a voltage drives electrons onto one plate and off the other, building an electric field that stores energy. When the circuit needs it, the capacitor discharges quickly, releasing that energy as a burst of current.

A capacitor stores electrical energy in an electric field between two conducting plates separated by an insulator called a dielectric. When voltage is applied, charge builds up on the plates until the capacitor matches the supply voltage, and it releases that energy when connected to a load.

Step by step

  1. A voltage source is connected across the two plates, which are separated by a thin insulating dielectric.
  2. Electrons flow from the source onto one plate while an equal number leave the opposite plate.
  3. Opposite charges build up on the plates, creating an electric field across the dielectric that stores energy.
  4. Charging current falls steadily and stops when the capacitor's voltage equals the supply voltage.
  5. When a load is connected, the stored charge flows back through it, releasing the energy as current.

Key things to understand

  1. A capacitor has two conducting plates or foils kept apart by a thin insulating layer such as ceramic, plastic film or oxide.
  2. Connecting a voltage pushes electrons onto one plate and pulls an equal number off the other, so the plates carry opposite charges.
  3. The separated charges create an electric field across the dielectric, and that field is where the energy is actually stored.
  4. Charging slows as the plate voltage approaches the supply voltage, and current stops once the two are equal.
  5. Because charge cannot cross the dielectric, a capacitor blocks steady direct current but lets changing or alternating current pass through the circuit.

Labelled diagram

  1. Supply

    A voltage source pushes electrons around the circuit toward the capacitor.

  2. Negative plate

    Electrons pile up on this plate, giving it a negative charge.

  3. Dielectric

    An insulating layer stops charge crossing and holds the electric field.

  4. Positive plate

    Electrons are drawn off this plate, leaving it positively charged.

  5. Load

    When connected, the stored charge flows through it and releases the energy.

Charge and field inside a charging capacitor

Real-world examples

  • A ceiling fan in an Indian home uses a run capacitor to create the phase shift that starts and turns its single-phase motor.
  • A camera flash charges a capacitor over a few seconds, then dumps the energy into the flash tube in a split second.
  • Phone chargers use capacitors to smooth the rippled output of a rectifier into steady direct current.
  • A smartphone touchscreen detects a finger because the finger changes the capacitance of a grid of tiny electrodes.

Common mistake

✗ Myth: A charged capacitor is full of extra electrons, like a tank holding fuel.

✓ Fact: A charged capacitor has the same total charge as an uncharged one, because one plate gains exactly the electrons the other loses. The energy is stored in the electric field created by separating those charges.

People also ask

Why does a capacitor block dc but pass ac?
Charge cannot cross the insulating dielectric, so with steady DC a capacitor charges up once and current stops. With AC the voltage keeps reversing, so the capacitor repeatedly charges and discharges in opposite directions. Current therefore keeps flowing in the wires on both sides, and the higher the frequency, the more easily it passes.
What does a capacitor do in a ceiling fan?
A ceiling fan uses a single-phase induction motor, which cannot start turning on its own. The capacitor sits in series with an auxiliary winding and shifts the timing of its current, creating a rotating magnetic field that spins the rotor. A weak or failed capacitor is a common reason a fan hums or turns slowly.
How long can a capacitor hold its charge?
It depends on the capacitor and the circuit. Tiny ceramic capacitors in a working circuit discharge in microseconds, while a large capacitor left disconnected leaks charge slowly through its own dielectric and can stay charged for minutes, hours or longer. Many power supplies add a bleeder resistor to drain them safely after switch-off.
What is capacitance measured in?
Capacitance is measured in farads, named after Michael Faraday. One farad stores one coulomb of charge per volt, which is very large, so everyday capacitors are rated in microfarads, nanofarads or picofarads.
How is a capacitor different from a battery?
A capacitor stores energy directly in an electric field, while a battery stores it in chemical reactions. Capacitors charge and discharge in fractions of a second but hold far less energy for their size, so they suit bursts and smoothing rather than long-term supply.
Why can a capacitor be dangerous after a device is unplugged?
A large capacitor can keep its charge for minutes or longer once power is removed, because nothing may be draining it. Capacitors in microwave ovens, televisions and power supplies can deliver a serious shock, which is why such equipment should be opened only by trained technicians.
What decides how much charge a capacitor can hold?
Larger plate area, a thinner gap and a dielectric with higher permittivity all raise capacitance. Electrolytic capacitors reach high values by using an extremely thin oxide layer as the dielectric, while the voltage rating limits how hard the dielectric can be stressed before it breaks down.

Part of the guide: Energy & Electricity at Home →