Page 292 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 24

Electric Current

ELECTRIC CURRENT

A flow of charge from one place to another constitutes an electric current.An electric circuit is a closed path
in which an electric current carries energy from a source (such as a battery or generator) to a load (such as a
motor or a lamp). In such a circuit (see Fig. 24-1), electric current is assumed to go from the positive terminal of
the battery (or generator) through the circuit and back to the negative terminal of the battery. The direction of a
current is conventionally considered to be that in which positive charge would have to move to produce the same
effects as the actual current. Thus a current is always supposed to go from the positive terminal of a battery or
generator to its negative terminal.

Fig. 24-1

Aconductorisasubstancethroughwhichchargecanfloweasily,andaninsulatorisonethroughwhichcharge
can flow only with great difficulty. Metals, many liquids, and plasmas (gases whose molecules are charged) are
conductors; nonmetallic solids, certain liquids, and gases whose molecules are electrically neutral are insulators.
A number of substances, called semiconductors, are intermediate in their ability to conduct charge.
Electric currents in metal wires always consist of flows of electrons; such currents are assumed to occur in
the direction opposite to that in which the electrons move. Since a positive charge going one way is for most
purposes equivalent to a negative charge going the other way, this assumption makes no practical difference.
Both positive and negative charges move when a current is present in a liquid or gaseous conductor.
If an amount of charge q passes a given point in a conductor in the time interval t, the current in the
conductor is
q
I =
t
charge
Electric current =
time interval
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