Conceptual Physics

26.12 - Gotchas

The total amount of charge on the plates of a capacitor is designated by q. No, the amount of charge on one plate of the capacitor is q. Each

plate has the same amount of charge: positive on one plate and negative on the other. The total charge is zero.

Capacitors have a net charge. No, the two plates have equal and opposite amounts of charge, so the capacitor is electrically neutral. When you

say “the capacitor has a charge” or the like, it means each plate is charged.

26.13 - Summary

A capacitor is a device with two conductors that are placed in close proximity, and

which store equal but opposite amounts of charge.

Capacitors are characterized by their capacitance, represented by the symbol C,

which equals the amount of charge on one conductor divided by the potential

difference between the conductors. The unit of capacitance is the farad (F).

1 F = 1 C/V.

By storing charge, capacitors also store electric potential energy. A battery or other

device causes charge to accumulate on the conductors of the capacitor. The

opposite charges on the conductors create an electric field between them. Electric

potential energy is stored in the electric field.

Between the conductors of a capacitor there may be a dielectric, an insulator that

can increase the capacitance by decreasing the field strength between the

conductors for a given charge. The factor characterizing the increase in capacitance

(or the decrease in field strength) is called the dielectric constant, ț.

Definition of capacitance

Potential energy in a capacitor

Dielectric constant

ț = Evacuum/Edielectric

Cț = țC

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