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Conservation of Charge Concept

As our first major concept we begin with the notation that there is a thing called charge

which is conserved. It can be stationary or in motion. It can be used to probe an electromagnetic field; it is discrete but in such smali amounts as not to concern us at the

macroscopic level. The charge and its motion are characterised by parameters

DESCRIPTOR SYMBOL UNITS

Charge density per unit volume p Cm-3

Volume current density J Am-2

Surface current density K Am-i

Table 4.1: Important charge and current density descriptors.

such that each of the expressions in Figurę 4.1 gives the amount of charge Crossing

the

indicated boundary per unit time, i.e. the current.