# P5- electric circuits

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P5- Electric Circuits
Static electricity
When you get out of car, you sometimes get a small electric shock when you touch the metal door ­
and you might hear a little "crack" as a spark jumps off your hand and the car door. This is caused by
static electricity. Electricity is part of our everyday world.
When you rub a piece of plastic, it is somehow changed: it can then affect objects nearby. The more it
is rubbed, the stronger the effect. It seems that something is being stored on the plastic. If a lot is
stored, it may escape by jumping to a nearby object, in the form of a spark. We say that the plastic
has been charged.
If you rub two identical plastic rods and then hold them close together, the rods push each other
apart - they repel. The forces exert on each other are very small.
If you try this with two rods of different plastics, however, you can find some pairs that attract each
other. Scientists' explanations for this are that there are two types of electric charge. If two rods
have the same type of charge, they repel each other. But if they have charges of different types,
they attract. The two types of charges are called positive and negative.
Scientists believe that charge is not made but is moved around when two things are rubbed
together. If you rub a plastic rod with a cloth, both the rod and the cloth become charged. Each object
gets a different charge: if the rod has a positive charge, the cloth has a negative one. Rubbing does
not make the charge. It separates charges that were there all along.
Charge is a basic property of matter, which cannot be explained in terms of anything simpler. All
matter is made of atoms, which in turn are made out of protons, neutrons and elections. In most
materials there are equal numbers of positive and negative charges, so the whole thing is neutral.
When you charge something, you move some electrons to it or from it.
Chemists think of the atoms as a tiny nucleus with a positive charge, surrounded by a cloud of
electrons, which have negative charge. As the electrons are on the outside, they can be "rubbed off"
on to another object.
Although they cannot explain charge, scientists have developed useful ideas for predicting its
effects. An example is the idea of an electric field. Around every charge there is an electric field. In
this region of space, the effects of the charge can be felt. Another charge entering the field will
experience a force.
When a van de Graaff generator is running, charge collects on its dome. As the charge builds up, the
electric field around the dome gets stronger. The charge may jump to a nearby object, in the form of
a spark. If you hold a mains-testing screwdriver close to the time and touch its metal end-cap to the
dome, the indicator light inside the screwdriver lights up. So there is an electric current through the
lamp, making it light. Charge on the dome is escaping across the air gap, through the indicator lamp
and through you to the Earth. This suggests that an electric current is a flow of charge.

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Simple circuits
If you break anywhere in the circuit, everything stops.
This suggests that something has to go all around the circuit to make it work. This something is
electric charges. If it was enough for the charges simply to go from the battery to the lamp, the one if
the lamps would be lit, even with the switch open. But this does not happen. There has to be a
complete loop.
Both lamps come on immediately when the circuit is completed.…read more

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At junctions in parallel circuits, the current splits, with some charges flowing through one branch and
the rest flowing through the other branch. Current is the amount of charge passing through a point
every second. So the amounts in the two branches must add up to equal the total amount in the
single wire before or after the branching point.
Controlling the current
The next step is to ask how the size of the current can be controlled.…read more

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LDR can be used to measure the brightness of light or to switch another device on or off when the
brightness of light changes. For example, it could be used to switch an outdoor light on in the evening
and off again in the morning.
A thermistor is another device made from semiconductor material. Its resistance changes rapidly with
temperature. The commonest type has a lower resistance when it is hotter.…read more

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The power is proportional to
the current.
Power (watts) = Current (amps) x voltage (volts)
The unit of power is the watt (w). One watt is equal to one joule per second.…read more

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Its length and thickness is chosen so that it melts if the current goes above the value marked on the
fuse. So it is, `the weakest link' in the circuit. It will melt first if the current for any reason gets bigger
than it should be.
An electricity supply
Generators work on the principle of electromagnetic induction. This phenomenon was discovered

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Distributing electricity
An electric current can be generated by moving a magnet into or out of a coil of wire. The moving
magnet could be replaced by an electromagnet. If a coil is wound round an iron core, it becomes
quite a strong magnet when a current flows through it.
When the current in the electromagnet is switched on, this has the same effect as plunging a bear

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Summary
Electric charge
Electric charge is a fundamental property of manner.
Charge cannot be created or destroyed. But positive and negative charges can be separated, and
moved from one object to another, for example by rubbing.
Electric current
A working electric circuit always consists of a closed loop of conducting material, between the
positive and negative terminals of a battery.
An electric current is a flow of charges, which are already present in the materials of the circuit.…read more

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Keywords
Static electricity
Electric charge
Repel
Attract
Positive, negative
Electron
Electric field
Electric current
Electric circuit
Ammeter
In series
In parallel
Voltage
Resistance
Ohm's law
Voltmeter
Potential difference
Power
Kilowatt-hour
Efficiency
Generator
Electromagnetic induction
Alternating current
Direct current