EMAT 6690

Electricity Terms

Amp (A)

The unit of electrical current. Properly called the ampere in honour of the French
Scientist André Ampere, it is usually shortened to amp. The maximum current a
fuse will take is written on the side of the fuse in amps, for example 5A, 13A.

Current, electric

An electric current is the flow of electricity through a conductor such as a wire. An
electric current can be understood by comparing it with the way water flows in a
river. Imagine the channel of the river as being like a wire and the flow of water
through it like the current flowing through the wire. The bigger the wire (or the
river channel) the more current (or water) can flow.

The amount of current that flows in a wire also depends on the pressure on the
electric current. The pressure is known as the voltage, or volts (V). More current
flows if the voltage is increased. This is like the way water flows in a river faster
when the bed slopes steeply. So, for example, if you double up the number of cells
in a battery, a bulb connected in the circuit will have more current flowing through
it and it will shine more brightly.

The way current and voltage are related is given by a law called Ohm's Law (named
after the scientist who discovered it). Ohm's Law simply says that the more voltage
(pressure) you put on to a circuit, the more current will flow.

There are tow kinds of current used. The current produced by a battery is a direct
current. It is often shown by DC. Direct current is used in all cars and in all
electronic circuits. Direct current is most useful at low voltages.

The other kind of current is used in mains supplies. It is called alternating current
(shortened to AC). AC is only used for carrying electricity economically across the
country. In electronic circuits there is a special device (called a transformer), that
steps down mains voltage into the low voltage needed for use by electronics and
another device (called a rectifier) that turns AC into DC. Some simple circuits, such
as light bulbs, motors and heaters, can use AC directly. Motors that run on AC are
also much more reliable than those that run on DC because they have a simpler
design.

Electricity, electric energy

Electricity is a form of energy. Electricity and magnetism are inseparable because
electricity produces magnetism. The discovery of electricity has effected our
modern world perhaps more than any other discovery. Inventions that use
electricity include electric light, electric heat, computers, motors, generators,
telephones, radio and television, and the circuits that run motor cars.

The first people to become aware of electricity were the Greeks who found that a
natural material called amber attracted small piece of fur after it has been rubbed by
a cloth. They had discovered static electricity. The word electricity comes from the
Greek word for amber, elektron.

In 1747, both William Watson in Britain and Benjamin Franklin in America
realised that electricity could flow through materials, and that rubbing materials
together simply transferred the electricity from one body to another. In fact we now
know that small particles called electrons are transferred. It is the movement of
electrons in a material that make electricity flow.

It was Franklin who called called an excess of electrical charge positive and a lack
of charge negative. He therefore believed that electricity flowed from positive to
negative.

In the 18th century, an assistant of the Italian scientist Luigi Galvani noticed that a
dissected frog's leg twitched when he touched its nerve with a metal scalpel.
Experimenting further, Galvani found that a frog's muscle would twitch when hung
by a brass hook on an iron railing. Another Italian scientist, Alessandro Volta (for
whom the word voltage is named) showed that when two metals are connected by
some kinds of liquid, a flow of electricity results. By 1800, Volta had invented the
electric cell (and battery).

An electric cell separates electrons by chemical means (rather than by rubbing, as is
the case with static electricity). If the electrons are removed in some way, the
battery separates more electrons, thus changing chemical energy into electrical
energy. This is current electricity.

Electricity grid

A network of cables designed to connect power stations with their customers in
offices, homes, schools and factories. The cables are often carried on tall overhead
pylons across country, but they are buried underground in cities.

Generator

Electric generator, Electric power generator

A machine for producing electricity. It is like a motor in reverse. The spindle is
turned and electricity flows from the surrounding coiled wire.

Many forms of energy can be used to turn the shaft. For example, the shaft may be
connected to the wheel of a bicycle, then, as the bicycle wheel turns, the shaft of the
generator (in this case called a dynamo) also turns, and power is fed to the cycle
bulbs.

The shaft can also be connected to a water wheel, or the blades of a wind generator.

The biggest power stations, however, use steam to turn the shaft.

The first people to discover how to generate electricity were Michael Faraday in
Britain and Joseph Henry in the United States. They showed that by moving a
magnet through a coil of wire, electricity could be produced, or generated, in the
wire.

Modern generators spin an electromagnet on a shaft inside huge coils of wire.

The current and the voltage produced depend on the speed the shaft turns and on
the strength of the magnetic field.

Generators are used in power stations. They are very large and some can generate
hundreds of thousands of watts per second.

Hydroelectric power

Power produced by using the power of fast flowing water to turn the shaft of a
generator. Hydroelectric power stations are located near dams or on large rivers.

Mains electricity

Mains electricity, or household supply of electricity, is the way that electricity is
supplied to homes. This supply is alternating current (AC). Mains electricity is
used to run lights, heaters and many other electrical appliances.

The key to understanding the mains supply is to realise that we demand a large
amount of power to our homes. With kettles, heaters, showers and all the other
kinds of domestic appliances going at the same time, we can demand 20kW or
even more.

This is a very substantial amount of power. Power is a combination of current and
voltage. To get more current through a cable while leaving the voltage the same, the
cable has to be thicker. So, to get power into a home you could use fat cables and a
low voltage. This would be as safe to touch as a dry battery.

But fat cables are expensive and difficult to use. The power cable coming into your
home might be the size of a hosepipe! Imagine trying to bend metal this width. It
would be impossible. There is also a much greater waste of power from a low
voltage supply.

But if you increase the voltage, the current and the cable thickness can stay the
same. High voltage cables also waste far less power. This is the reason higher
voltages are used rather more current.

The sensible alternative of using higher voltages simply requires that safety
measures are put in place, for example, that safety plugs are used to connect up
appliances and that good insulation is used around the cables. Nevertheless, it is
still important to take care with mains electricity supplies.

The household circuit is connected up as a parallel circuit. This allows many
appliances to be used at the same time.

Ohm

The unit of resistance. It is named after Georg Ohm.

Power

Power is a way of talking about the amount of electrical energy used.

All electrical equipment uses power. The amount of power used is measured in
Watts (W). Devices that use a large amount of power are always run off the mains

A small mains electric light uses about 25W; a large electric light uses 100W. A
small flashlight bulb used in a torch may use 1.5W.

Appliances such as cookers and heaters use much more power. For example, the
heating ring of an electric cooker may be about 1000W (1kW ? kilowatts). An
electric heater may consume 2000W (2kW ) or more. A shower has to heat water
very fast and may consume 7kW.

The more power taken by an electric circuit, the thicker the supply wires have to be.
That is why the cable use to connect an electric heater to the mains supply is
thicker than the cable used to connect a table lamp to the mains.

In a battery-run circuit, the more power that is used, the faster a battery is run
down.

Power is amps multiplied by volts. So, for example, if a motor is connected to a 9V
battery mains and uses 3 watts of power, we can easily work out how much current
is flowing in the wires.

Power W = Voltage (V) x current (A)

3 = 9 x A

A = 1/3

The motor uses 1/3A of current.

Calculations like these allow electricians to work out the cable to be used to
connect mains appliances and also the size the fuse that should be used.

Resistance

The term for how easy it is for an electrical current to flow through a conductor. A
piece of copper, for example, will allow electricity to flow through it more easily
than a piece of carbon. The copper has a lower resistance than the carbon. As a
result copper is used for wires and carbon is used to make resistors.

Switch

A device for breaking the flow of electricity in a circuit

Transformer

A device for changing the voltage of an electricity supply.

It is mainly used to step down the very high voltage carried by transmission lines
to the lower (240V) voltage needed for home use.

Transmission lines

(US)/Power lines/Power cables

Large cables designed to carry very high voltage electricity from the power station
to where it is needed.

Transmission lines are cables. They are supported on metal frames called pylons.

Voltage

The electrical 'pressure' that a battery or other source of electricity can provide. A
single dry battery normally provides 1.5 volts; a main supply provides 240 volts.

Volts

The unit of measuring voltage. It is named in honour of Alessandro Volta, a
pioneer of electricity.

Watt