Potential energy for a system of electrically charged particles, q_{i}
can be defined as

where = 8.854 x 10
C/N m
is the permittivity of free space, q' is a test charge, qi
is a charged particle, and ri is the distance betwen
the test charge and the charges q_{i}.

Potential difference (or simply 'potential', sometimes referred to as 'voltage') is related to electrical potential energy, U, by:

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