Page 38 - Teach Yourself Electricity and Electronics

18 Basic physical concepts

Law. If the EMF is doubled, the current is doubled. If the resistance is doubled, the cur-
rent is cut in half. This important law of electrical circuit behavior is covered in detail a
little later in this book.
It is possible to have an EMF without having any current. This is the case just
before a lightning bolt occurs, and before you touch that radiator after walking on the
carpet. It is also true between the two wires of an electric lamp when the switch is
turned off. It is true of a dry cell when there is nothing connected to it. There is no cur-
rent, but a current is possible given a conductive path between the two points. Voltage,
or EMF, is sometimes called potential or potential difference for this reason.
Even a very large EMF might not drive much current through a conductor or
resistance. A good example is your body after walking around on the carpet. Although
the voltage seems deadly in terms of numbers (thousands), there are not that many
coulombs of charge that can accumulate on an object the size of your body. Therefore
in relative terms, not that many electrons flow through your finger when you touch a
radiator so you don’t get a severe shock.
Conversely, if there are plenty of coulombs available, a small voltage, such as 117
volts (or even less), can result in a lethal flow of current. This is why it is so dangerous
to repair an electrical device with the power on. The power plant will pump an unlim-
ited number of coulombs of charge through your body if you are foolish enough to get
caught in that kind of situation.

Nonelectrical energy

In electricity and electronics, there are many kinds of phenomena that involve
other forms of energy besides electrical energy.
Visible light is an example. A light bulb converts electricity into radiant energy that
you can see. This was one of the major motivations for people like Thomas Edison to
work with electricity. Visible light can also be converted into electric current or voltage.
A photovoltaic cell does this.
Light bulbs always give off some heat, as well as visible light. Incandescent lamps
actually give off more energy as heat than as light. And you are certainly acquainted
with electric heaters, designed for the purpose of changing electricity into heat energy.
This “heat” is actually a form of radiant energy called infrared. It is similar to visible
light, except that the waves are longer and you can’t see them.
Electricity can be converted into other radiant-energy forms, such as radio waves,
ultraviolet, and X rays. This is done by things like radio transmitters, sunlamps, and
X-ray tubes.
Fast-moving protons, neutrons, electrons, and atomic nuclei are an important form
of energy, especially in deep space where they are known as cosmic radiation. The en-
ergy from these particles is sometimes sufficient to split atoms apart. This effect makes
it possible to build an atomic reactor whose energy can be used to generate electricity.
Unfortunately, this form of energy, called nuclear energy, creates dangerous by-
products that are hard to dispose of.
When a conductor is moved in a magnetic field, electric current flows in that
conductor. In this way, mechanical energy is converted into electricity. This is how a