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INTRODUCTION TO CIRCUITS
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CURRENT IN A CIRCUIT
An electric current is a flow of electrical charge going round a circuit. The charge is carried in electrons,
which move round the circuit. Each electron carries a very tiny amount of electrical charge.
An electric circuit usually consists of the following items:
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a power source, such as a battery
a path, such as a wire
a load, such as a bulb
a control, such as a switch
an indicator, such as an ammeter or voltmeter
As current is not 'used up', the total current will be the same all the way round the circuit.
SERIES & PARALLEL CIRCUITS
Circuits can be constructed in series or in parallel.
A series circuit occurs when the electricity
leaves the source (battery), travels along the
pathway (wire), through the bulb (receiver) and
then back along the pathway to the source.
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A parallel circuit involves two or more branches
and the current splits to flow through each
branch. Part of the current passes through each
branch and then joins up again later to complete
the circuit. In a parallel circuit, each bulb
(receiver) has its own pathway to the source so
that it forms its own circuit. In a parallel circuit,
if one bulb fails, the other bulbs will stay on.
SWITCHES
Consider the circuit described earlier which has a bulb as the load or receiver. How would you turn the
bulb off in this construction?
To turn the bulb off you need to break the circuit and stop the current flow. This can be done by placing
a switch in the line. A switch is a pair of contacts that open and close. When the switch is closed,
electrons can flow along the circuit. When the switch is open, there is a break in the circuit and electrons
can no longer flow. This means that the bulb will not light up.
TYPES OF CURRENT
Current carries electrical energy from a power supply, such as a battery, to the components of the
circuit, where it is converted into other forms of energy, such as heat, light, or motion. There are two
types of electrical current — direct current and alternating current.
Direct current is current that always flows in the same direction. It is the type of current that is
produced in a circuit containing a battery.
Alternating current is current that is reversed at regular intervals. Electrical current supplied by the ESB
is alternating current. It changes direction 50 times each second. The advantages of A.C. are (i) it is
easier to generate than D.C. and (ii) less energy is lost in sending high voltage electricity across the
country.
Current produced by a battery is direct current
Current coming from the mains is alternating current
Diodes are electrical one-way valves. A diode allows current to flow in one direction but not the other. It
can be used to convert AC current into DC current. Mobile phone chargers are just one example of an
everyday item that uses a diode.
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EFFECTS OF ELECTRICITY
Electricity has three main effects:
1. Heating effect
Electricity going through a wire causes it to heat up. You will be familiar with this effect as lots of
everyday appliances use it, such as a cooker, an iron and a heater.
2. Chemical effect
When it passes through a liquid, electricity can sometimes cause a chemical change. In the case
of water, it causes the water to split into H2 and O2.
3. Magnetic effect
A current carrying wire has a magnetic field around it. You can 'see' this by bringing a compass
close to an electric wire with current flowing through it. It should deflect the compass needle. If
you want to know what direction the magnetic field is pointing around the wire, you can use the
right-hand rule.
These three effects are used in lots of ways in our lives. The heating effect of electricity is used to iron
your clothes or cook your food. The chemical effect is used in an industrial process called electroplating,
where one metal is covered with a thin layer of another metal to protect it. Silver-plated jewellery or
shiny chrome car bumpers are two examples. The magnetic effect of electricity is used in some door
bells.
HOUSEHOLD ELECTRICITY
The watt is used to specify the rate at which electrical energy is converted in electrical circuits and
systems.
Power is the product of the current and the voltage.
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Each electrical appliance has a power rating. The power rating, in watts, indicates the rate at which the
device converts electrical energy into another form of energy, such as light, heat or motion. A unit of
electricity is equivalent to one kilowatt (1000 watts) of power used for one hour. The units are kilowatthours (kWh).
To calculate the number of units of electricity that an appliance has used, multiply the power rating of
the appliance (in kW) by the time in hours that it has been used for. Typical power ratings for some
domestic appliances are shown below.
Ghettoblaster: 60W
Drill: 480W
Kettle: 2,400W
TV: 120W
Think About This…
If you used a kettle for 2 minutes and a TV for 15 minutes, which one would use more power?
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