CURRENT ELECTRICITY

The electric charge in a current is carried by minute particles called electrons that orbit the nuclei of atoms. Each
electron carries a small electric charge. When a stream of electrons moves from
atom to atom—for example, inside a copper wire—the flow of the charge they carry
is called electric current. Batteries and generators are devices that produce
electric current to power lights and other appliances. Electric currents also
occur in nature—lightning being a dramatic example.

Electric currents flow because atoms and molecules contain
two types of electrical charge, positive and negative, and these opposite
charges attract each other. If there is a difference in the overall charge of
atoms between two points—for example, between two ends of a wire—the negatively
charged electrons will flow toward the positively charged end of the wire,
creating electric current. Direct current (DC) is the flow of electricity in one
direction. Alternating current (AC) intermittently reverses direction because of
the way it is generated.

Electric current flows easily in some substances but not
at all in others. Solids, liquids, and gases that carry electric currents are
called conductors. Many metals are good conductors. More than one conductor may
be needed to build an electric circuit—a path for electric current
to move from one place to another.

The rate at which electric charge flows in a current is
measured in amperes. The unit is named for French physicist Andre Marie
Ampere, who contributed to the study of electrodynamics in the early 19th
century. One ampere of electric current is equal to about 6 billion billion
electrons per second flowing past a point.

The difference in charge between two points creates a
force called the electric potential that drives the current ahead. This force is
measured in volts, and is named after Italian scientist Alessandro Volta. A
typical flashlight battery produces 1.5 volts. Many household appliances run on
110 volts, but some require 220. Voltage in power lines that deliver electricity
around the country is measured in tens of thousands of volts.

Except for the special circumstance in which a substance
becomes a superconductor, all conductors resist the flow of current
to some extent. The measurement of a conductor’s resistance to electric current
is measured in ohms, named after German scientist Georg Simon Ohm.
Ohm’s Law states that the amount of current (I) in a conductor is equal to the
voltage (V) divided by the resistance (R), when these values are measured in
amperes, volts, and ohms.

An electric current’s overall power depends on the amount
of current flowing (measured in amperes) and the electric potential driving it
(measured in volts). Electric power is measured in watts, named after Scottish
scientist James Watt. One watt is equal to one ampere moving at one
volt. Multiplying amperes by volts produces the number of watts. An appliance
that uses 10 amperes and runs on 115 volts consumes 1150 watts of power.
Electricity suppliers charge customers a rate based on the amount of watts the
customers use. Since the number of watts is usually large, it is calculated in a
more convenient unit, the kilowatt, which equals 1000 watts. The standard
measure for large amounts of electricity is the kilowatt-hour, which equals 1000
watts per hour.