Electrical energy
Electrical energy is a form of potential energy. It comes about
from the attraction and repulsion of electrically charged particles.
When two charged particles come near each other, their action on each
other will either make them move apart or move together. In each case,
electrical potential energy is transformed to kinetic energy of the
moving particles.
The
word electricity comes from a Greek word elektron. Electrical energy
is all to do with electrons and protons, parts of all atoms, the basic
building blocks of our Universe. Atoms are made up of a dense positively
charged nucleus in the centre, surrounded by clouds of negatively charged
particles called electrons. These electrons are arranged in clouds and
it is the outermost electrons, or valence electrons, that are involved
in electrical effects.
These electrons, moving around the outermost parts of the atom are
sometimes easily removed - especially from metal atoms. In metals, these
electrons are shared by all the atoms in the structure and it is the
movement of these electrons through the metal that produces an electric
current.
Materials that allow easy movement of electrons are called electrical
conductors, while those that don't allow electron flow are called electrical
insulators. Good conductors are metals and some non-metals, such as
carbon or graphite. Insulators tend to be plastics, ceramics, glass
and a few non-metal solid elements.
Static electricity
You might have experienced the effects of static electricity when
you walked across a synthetic carpet, then touched a metal object, or
when you removed one piece of clothing from another made from a different
material and experienced an electric shock. Static electricity is caused
by charge separation or transfer. When you walk across a carpet, electrons
may transfer from the soles of your shoes to the carpet or vice versa.
This leaves your shoes, and yourself, with a net charge and the carpet
with the opposite charge. When you then touch a conductor, like a door
knob, the excess charge on your body discharges through the doorknob;
this is the electric shock. It's called static electricity because,
until the charge is transferred, the charge builds up but does not move
in a directional current.
Current electricity
Current electricity is the most common form, and is more useful to us.
In this case electrons flow around a circuit and carry electrical energy
that is transformed into other forms of energy in the circuit. Your
computer uses current electricity, usually coming from the mains supply,
but sometimes from a stored source, like batteries. The focus of the
rest of this topic is on current electricity.
Voltage and current - what's the
difference?
Current is a flow of charge through a conductor. The carriers of electrical
energy are the electrons, but electricians and physicists talk about
current as the movement of positive charge. In atoms, the positive charge
is on protons and they cannot move through a wire because they are firmly
held in place in the nucleus of the atoms. Hence, positive charge cannot
move, only electrons move. This is a historical hangover from the first
definition of current. Confused? Don't be - electrons carry the electrical
energy in a current.
Voltage can be thought of as the ability to cause current to flow in
a conductor. It is a measure of electrical potential energy change.
The greater the voltage, (or potential difference as it is often called)
the greater the current in a circuit. For example, a 12 volt battery
will cause twice as great a current to flow around a circuit as a 6
volt battery.
An everyday example of this concept is turning on a tap above a sink.
Due to its height above the sink, water can flow, but does not actually
flow until the tap is opened. Voltage is like the tap water pressure,
current is like the rate of flow of water, and the tap is like an electrical
switch.
Electrical flow
As electrons flow through a circuit their electrical potential energy
is transformed into other forms of energy. In an electric light globe
the electrical potential energy is transformed into light and thermal
energy; an electric motor transforms electrical potential energy into
kinetic energy; a radiant heater produces thermal energy and your radio
converts electrical potential energy into sound energy. Electricity
is an important power source in our modern society - how does it work?
Think of an ordinary battery (a single unit is really called a
cell but we'll use the common word). One end has a bit sticking out
and this is called the positive terminal, the other end is the negative
terminal.
A battery is a chemical factory that stores chemical potential energy.
When a current flows in a circuit, due to the presence of the battery
this energy is transferred by electrons as electrical potential energy.
It then flows through the circuit, being transformed to other forms
of energy such as thermal, kinetic, sound and light energy.
The supply of chemical potential energy is limited so its ability
to cause a current to flow is also limited. Eventually the battery will
exhaust its usable chemical energy supply and the battery is said to
"lose charge" or "go flat". An important point to
realise here is that batteries do not "create electrons",
they simply transform chemical potential energy into electrical potential
energy. Rechargeable batteries are able to store chemical potential
energy from electrical potential energy in the mains supply - this type
of battery is convenient because it is portable and replaceable.
Starting and stopping the flow
To allow electrons to flow a complete circuit is needed. Usually
wires covered with plastic connect one terminal of the battery to the
other through various components and back to the other terminal of the
battery.
A simple device to produce light is a torch where one or more batteries
are connected through a metal circuit and a globe. The globe has a high
resistance filament in which energy is transformed to thermal energy
and light energy.
To stop the electrical flow, the circuit needs to be broken. A
switch is used to do this, breaking the circuit by making a high insulation
air gap. The switch on any electrical item does the same job - making
and breaking the circuit.
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