What is a capacitor?
A capacitor is a component that stores energy in an electric field. In its simplest form it is just two conducting plates with an insulating layer — the dielectric — between them. Apply a voltage and positive charge gathers on one plate while an equal negative charge gathers on the other; remove the voltage and the charge can flow back out through a circuit.
Charging and discharging
Charging happens when the plates are connected across a supply such as a battery. Charge moves onto the plates, the field between them grows, and the voltage rises until it equals the supply — at which point the current stops. Discharging happens when the plates are instead connected across a load: the stored charge flows back through the load, the field shrinks, and the voltage falls back to zero.
Reading the diagram
The two grey bars are the plates, with the dashed box between them marking the dielectric and the blue arrows showing the electric field running from the positive plate to the negative one. Below, a two-way switch selects either the battery (charging) or the load resistor (discharging). The dashed blue lines trace the current: one set runs while charging, and the reverse set runs while discharging.
1. The switch connects the battery
With the switch against the battery terminal, the supply is wired across the two plates. One terminal pushes charge toward a plate while the other draws charge away from the opposite plate, so a current begins to flow in the wires.
2. Charge builds up on the plates
Negative charge — electrons — is pulled onto one plate, leaving the other short of electrons and therefore positive. The two charges are always equal and opposite, so the plates become a matched pair, one positive and one negative.
3. An electric field forms between them
Opposite charges on facing plates pull on each other across the insulator, and the result is an electric field stretched through the dielectric. It is this field, not the metal, that holds the stored energy.
4. Charging stops at the supply voltage
As charge accumulates, the voltage across the capacitor rises. It stops rising when that voltage equals the battery's: from then on the field is as strong as the battery can make it, so no more charge is pushed on and the current falls to zero.
5. Switching to the load discharges it
Move the switch to the resistor and a path opens between the plates. Charge flows back through the load in the opposite direction, the field weakens, and the voltage falls — until the plates are neutral again and the capacitor is empty.
What is a capacitor?
A capacitor is a component that stores energy in an electric field. It is made of two conducting plates separated by an insulator, called the dielectric. Charge collects on the plates when a voltage is applied, and the energy stays there until the circuit lets it flow out again.
Does current flow through a capacitor?
No steady current crosses the insulator between the plates. During charging and discharging, charge flows in the wires up to the plates and off them again, so it looks like current is passing through — but nothing actually crosses the gap.
When does charging stop?
Charging stops when the voltage across the capacitor equals the supply voltage. At that point the field is as strong as the battery can make it, so no more charge is pushed on and the current falls to zero.
Where is the energy stored?
In the electric field between the plates, not in the wires. Separating positive and negative charge stretches a field across the dielectric, and that field holds the energy until the plates are allowed to recombine.
Why do capacitors matter?
Because they can release a lot of energy very quickly, and they can smooth a changing voltage. That makes them useful for camera flashes, power-supply smoothing, timing circuits and filtering noise out of signals.