Two conductors separated by an insulator make a capacitor. Put a voltage across it and it stores charge, and with it energy, in the electric field between the conductors. Capacitance is how much charge it holds for each volt.
How it works
Typical dielectric constants. Bigger plates, closer plates or a better dielectric all raise the capacitance.
Capacitance44.3 pF
Connect a battery and electrons are drawn off one plate and pushed onto the other. The two plates now carry opposite charges that attract each other across the gap, and that attraction is what holds the charge in place. Three things raise the capacitance:
Bigger plates. More room for charge.
A smaller gap. Opposite charges are closer, so they hold each other more strongly.
A better dielectric. The insulator's molecules turn to line up with the field, which cancels part of it. Less field per unit of charge means the same voltage holds more charge. The factor is the dielectric constant, εr.
Dielectric
Typical εr
Air
1
PTFE (Teflon)
about 2
Polyester film
about 3
Mica
about 6
High-k ceramic
hundreds to thousands
No current flows through the insulator. Current flows in the wires only while the voltage is changing, as the plates charge or discharge: i = C × (rate of change of voltage). Once the voltage is steady, current stops. So a capacitor blocks DC but passes AC, because AC never stops changing. How well it passes AC depends on frequency; see Reactance and impedance.
Red arrows: electric field. Blue loops: magnetic field.
The numbers
A 100 µF capacitor across a 13.8 V supply holds Q = 100 µF × 13.8 V = 1.38 mC and stores ½ × 100 µF × 13.8² = 9.5 mJ. Energy grows with the square of the voltage, so doubling the voltage stores four times as much.
Capacitors combine opposite to resistors, and the plates explain why:
Connection
Result
Why
Parallel
C = C1 + C2
plate areas add
Series
1/C = 1/C1 + 1/C2
gaps stack, so the gap grows
Two 100 pF capacitors in series give 50 pF; in parallel, 200 pF. The voltage rating of a series pair is also higher, but only if the voltage divides evenly.
In practice
A capacitor is rated for a maximum voltage as well as its capacitance. Exceeding it breaks down the dielectric.
Real capacitors are not ideal: their leads add a little inductance, and their dielectric leaks. Choosing among ceramic, film, electrolytic and variable types is covered in Capacitors.
Large capacitors store energy long after the power is off. A charged filter capacitor in a power supply can be dangerous; see Electrical safety.