Page 35 - Teach Yourself Electricity and Electronics

Static electricity 15

When most of the charge carriers are electrons, the semiconductor is called
N-type, because electrons are negatively charged. When most of the charge carriers are
holes, the semiconducting material is known as P-type because holes have a positive
electric charge. But P-type material does pass some electrons, and N-type material car-
ries some holes. In a semiconductor, the more abundant type of charge carrier is called
the majority carrier. The less abundant kind is known as the minority carrier.
Semiconductors are used in diodes, transistors, and integrated circuits in almost
limitless variety. These substances are what make it possible for you to have a computer
in a briefcase. That notebook computer, if it used vacuum tubes, would occupy a sky-
scraper, because it has billions of electronic components. It would also need its own
power plant, and would cost thousands of dollars in electric bills every day. But the cir-
cuits are etched microscopically onto semiconducting wafers, greatly reducing the size
and power requirements.

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 cur-
rent 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 cur-
rent 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.

Static electricity

Charge carriers, particularly electrons, can build up, or become deficient, on things
without flowing anywhere. You’ve probably experienced this when walking on a car-
peted floor during the winter, or in a place where the humidity was very low. An excess
or shortage of electrons is created on and in your body. You acquire a charge of static