the EMF is doubled, the current is doubled. If the resistance is doubled, the current is cut in half.
This important law of electrical circuit behavior is covered in detail later in this book.
It is possible to have an EMF without having any current. This is the case just before a light-
ning stroke occurs, and before you touch a metal object after walking on a carpet. It is also true be-
tween 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 current, 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 huge EMF does not necessarily 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 many coulombs of static-electric charge that can ac-
cumulate 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. This is why you don’t get a severe shock.
If there are plenty of coulombs available, a small voltage, such as 117 volts (or even less) can
cause a lethal current. This is why it is dangerous to repair an electrical device with the power on.
The power plant will pump an unlimited number of coulombs of charge through your body if you
are not careful.
Nonelectrical Energy
In electricity and electronics, there are phenomena that involve other forms of energy besides elec-
trical 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. You are certainly acquainted with electric heaters, designed for
the purpose of changing electricity into heat energy. This heat is a form of radiant energy called
infrared (IR). 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
(UV), and X rays. This is done by specialized devices such as radio transmitters, sunlamps, and elec-
tron tubes. Fast-moving protons, neutrons, electrons, and atomic nuclei are an important form of
energy. The energy 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.
When a conductor moves in a magnetic field, electric current flows in that conductor. In this
way, mechanical energy is converted into electricity. This is how an electric generator works. Gener-
ators can also work backward. Then you have a motor that changes electricity into useful mechani-
cal energy.
A magnetic field contains energy of a unique kind. The science of magnetism is closely related
to electricity. Magnetic phenomena are of great significance in electronics. The oldest and most uni-
versal source of magnetism is the geomagnetic field surrounding the earth, caused by alignment of
iron atoms in the core of the planet.
A changing magnetic field creates a fluctuating electric field, and a fluctuating electric field pro-
duces a changing magnetic field. This phenomenon, called electromagnetism, makes it possible to
send wireless signals over long distances. The electric and magnetic fields keep producing one an-
other over and over again through space.
Nonelectrical Energy 13