Current

Current
Whenever there is movement of charge carriers in a substance, there is an electric
current. Current is measured in terms of the number of electrons or holes passing a
single point in one second.
Usually, a great many charge carriers go past any given point in one second, even if
the current is small. In a household electric circuit, a 100-watt light bulb draws a current
of about six quintillion (6 followed by 18 zeroes) charge carriers per second.
Even the smallest mini-bulb carries quadrillions (numbers followed by 15 zeroes) of
charge carriers every second. It is ridiculous to speak of a current in terms of charge
carriers per second, so usually it is measured in coulombs per second instead. A
coulomb is equal to approximately 6,240,000,000,000,000,000 electrons or holes. A current
of one coulomb per second is called an ampere, and this is the standard unit of
electric current. A 100-watt bulb in your desk lamp draws about one ampere of current.
When a current flows through a resistance—and this is always the case because
even the best conductors have resistance—heat is generated. Sometimes light and
other forms of energy are emitted as well. A light bulb is deliberately designed so that
the resistance causes visible light to be generated. Even the best incandescent lamp is
inefficient, creating more heat than light energy. Fluorescent lamps are better. They
produce more light for a given amount of current. Or, to put it another way, they need
less current to give off a certain amount of light.
Electric current flows very fast through any conductor, resistor, or semiconductor.
In fact, for most practical purposes you can consider the speed of current to be the
same as the speed of light: 186,000 miles per second. Actually, it is a little less.