Electricity · Magnetism
In electromagnetism and electronics, capacitance is the
ability of a body to hold an electrical charge.
Capacitance is also a measure of the amount of elec-
tric charge stored (or separated) for a given electric po-
tential. A common form of charge storage device is a
two-plate capacitor. If the charges on the plates are +Q
and −Q, and V gives the voltage between the plates, then
the capacitance is given by
The SI unit of capacitance is the farad; 1 farad = 1 cou-
lomb per volt.
The energy (measured in joules) stored in a capacitor
is equal to the work done to charge it. Consider a capacit-
ance C, holding a charge +q on one plate and -q on the
other. Moving a small element of charge dq from one
plate to the other against the potential difference V = q/C
requires the work dW:
where W is the work measured in joules, q is the charge
measured in coulombs and C is the capacitance, meas-
ured in farads.
We can find the energy stored in a capacitance by in-
tegrating this equation. Starting with an uncharged ca-
pacitance (q=0) and moving charge from one plate to the
other until the plates have charge +Q and -Q requires the
• Motion of an electric charge through a
potential difference simulates charging a two-
plate capacitor. Charging an actual capacitor
with no medium between the plates (in free
space) transports no charge through the
vacuum between its plates.
• Current that enters a capacitor’s plates does
not pass through a potential difference within
the capacitor. The conductors that connect to
the capacitor’s plates have the same potential
as their respective plates.
• Within the simulation, q moves from one plate
to the other while q is isolated from force with
any charge that is not creating the potential
difference. Selective charge isolation is
impossible. No point charge is exempt from
Coulomb’s law. Charge isolation from a source
atom requires ionization energy.
• The only portions of a capacitor’s plates used
for the calculation of capaci