An electrical circuit provides pathways along which current
can flow to do work. Current is driven by a power source, such as a BATTERY. This produces an electrical pressure, known as voltage, which
pushes electrons along the wires. Engineers classify circuits into two types.
In a series circuit, the same current flows through all the components (such as
light bulbs) in the circuit. In a parallel circuit, the same voltage is applied
to all the components.
Modern homes depend on electrical circuits. They carry power to the
electric motors in toasters, refrigerators, DVD players, and many other
machines. Electricity also supplies heat and light. Forcing current through
something with RESISTANCE turns electrical energy into heat. The
result may be a red glow that makes toast, or the brilliant white of a light
bulb.
Fuses protect wires from too much current, which could make them hot
enough to start a fire. A fuse is a thin wire in a fireproof casing. Too much
current makes it melt, breaking the circuit safely. An earth wire protects
people from electric shock if the metal casing of an electrical machine
accidentally gets connected to the electricity supply. Instead of current
flowing to earth through a person when they touch the machine (and possibly
killing them), it flows harmlessly through the earth wire.
A battery turns chemical energy into electrical energy.
It consists of one or more cells. Each cell contains two electrodes (pieces of
metal or another substance), and a chemical called the electrolyte that
transports electrons between them. The electrodes are made of different
materials, so one gets more electrons than the other. The excess electrons can
flow around a circuit connected to a battery as an electric current. Different
kinds of battery are used for different purposes. Some, such as torch
batteries, can be used only once. Others, including nickel-cadmium (NiCad)
batteries and car batteries, can be recharged and used again.
A car battery can be used and recharged by the car’s
alternator for years. It can also deliver the huge current needed to start the
car. Its electrodes of lead and lead oxide are immersed in dilute sulphuric
acid.
Modern batteries have a steel case around a layer of manganese
dioxide and a core of zinc powder. Both are coated in a strong alkali (the
opposite of an acid) electrolyte. The manganese dioxide gives up electrons to
the zinc. The electrons travel to the battery’s negative end through a
collector. The current stops when the chemicals are used up.
BIOGRAPHY: ALESSANDRO VOLTA Italian, 1745-1827
In 1800, the scientist Volta made the first battery, a pile of
silver and zinc discs separated by salt-soaked card. His friend Luigi Galvani
had noticed that a frog’s leg twitched when in contact with two
different metals. Galvani thought it was the frog that produced this electrical
effect. Volta showed it was the metals.
Electrons moving along a wire bump into lots of atoms,
which slow the electrons down and make them lose energy. This effect is called
resistance. It limits the current that can flow when a particular voltage is
applied. The energy lost by the electrons makes the wire hotter – hot
enough, perhaps, to light a room.
CIRCUIT TERMS AND SYMBOLS
The electrical pressure that drives current through a circuit.
It is measured between two points, one of which is often the surface of the
Earth. Unit: volt. Symbol: V. A single alkaline battery gives a voltage of
1.4 V.
The flow of electrical charge through a circuit. It is measured
as the charge per second passing one point. Unit: ampère. Symbol: A.
Starting a car can draw a current of 200 A.
The property of a circuit that opposes the flow of current. It
is measured as voltage divided by current. Unit: ohm. Symbol: Ω. An
ordinary torch bulb has a resistance of about 8Ω.
The rate at which energy is consumed or released by a circuit.
It is measured as voltage times current. Unit: watt. Symbol: W. An electric
train uses about 3,000,000 W or 3 MW (megawatts).
(First stated by German physicist Georg Ohm.) The current
through a circuit is given by the voltage across it divided by its resistance.
A triangle helps to show the three relationships: V = I x R, I = V
÷ R, and R = V ÷ I, where V = voltage, I = current, and R =
resistance.
Inside a light bulb is a filament – a length of very thin
wire coiled up twice so it looks shorter and thicker than it really is. The
filament is made from tungsten, a metal that withstands high temperatures. A
60W bulb filament has a resistance of 88Ω, allowing a current of 0.26A to flow
when connected to a 230V supply.
A current is only forced to do some work when it encounters
resistance. In the end, it just generates heat, but on the way it may do
something more interesting, like produce music. The 33Ω resistor here
would draw a current similar to that of a personal radio.
Modern batteries have a steel case around a layer of manganese
dioxide and a core of zinc powder. Both are coated in a strong alkali (the
opposite of an acid) electrolyte. The manganese dioxide gives up electrons to
the zinc. The electrons travel to the battery’s negative end through a
collector. The current stops when the chemicals are used up.
