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Click
on the switches to close the circuits.
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This is a
very simple control system.
Without the
switch we couldn't take control. Imagine how
inconvenient it would be at home to have no switches on
your wall to control the room lighting, or on your TV to
switch on the power.
How the circuit
works
Voltage
The power for
the circuits above comes from the battery.
Batteries can be low voltage types (1.5 volts) to higher
voltage types (9 volts) and cars even have 12 volt batteries.
The switch,
when closed, will conduct and let the battery
connect to the circuit. In this position the switch makes the
circuit.
The switch,
when opened, will insulate and disconnect the
power source from the rest of the circuit. In this position the
switch has made an "open circuit"
In our circuit
it is also important to match the battery voltage
with the correct lamp to prevent it overloading and getting
damaged. This is the same for many other components in
electronic circuits.
Current
The battery
gets its power to the lamp through the wires. This
is because the battery in a closed circuit creates current
in
the wires which flows around the circuit. When current flows
through the
lamp it lights up.
Current is measured
in Amperes (Amps) - smaller currents in
electronic circuits are usually measured in milliamps (mA). A
milliamp is equal to one thousandth of an Amp.
1mA = 1/1000
Amp
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Close
the switch
to
see the current flow from the positive (+) side of the battery
to the negative (-) side.
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If you had
no lamp and just connected the battery ends
together with wire you would allow a very high current to flow.
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Click
on the switch to see what would
happen to the wire if there was no lamp and a car battery was
being used.
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Resistance
In the second
circuit by connecting the two ends of the
battery together with wire the battery has been short-
circuited. Because of this, there is very little or no
resistance to the current that flows from the battery. The
wires connected to the battery are glowing redhot and would
soon melt.
The
heat produced in this way can
seriously burn you or create a fire. This can happen because
a car battery is very powerful and can produce an extremely
high current when its terminals are connected together
(shorted).
In the first
example, when the lamp was in the circuit, the
current was much less as the lamp resisted its flow. To
prevent damage to the battery and stop overheating, you
should avoid short-circuits in your practical work.
Q9,
Is this statement true or false ? (you may need to
look back at the Resistance section above).
A "short circuit" across the contacts of a battery
has a very
low resistance - So lots of current can flow through it.
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In a circuit
Ohm's
Law
says:
Power
Different electrical
circuits use different amounts of power. A
torch can use quite a lot of battery power and you have
probably found that its batteries need to be replaced more
often than the batteries in a wrist watch. Power is dependent
on the voltage and the current in the circuit. It is measured
in
Watts.
Power (Watts)
= Voltage (V) x Current (I)