All revsion notes for p5

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P5 ­ Electric Circuits
Static Electricity
Static electricity is all about charges which are not free to move; causing them to build up in one place
and leave to sparks or shocks when they finally do move. This build-up of static is caused by friction.
When two insulating materials are rubbed together, electrons are scraped off one and transferred
to the other.
Electrons are negatively charged, leaving a positive static charge on one (electrons taken off) and a
negative static charge on the other (gained electrons). The way the electrons are transferred
depends on the two materials involved. The main materials used are polythene and acetate rods
being rubbed with a cloth duster.
With the polythene rob, electrons move from the duster to the rod.
With the acetate rod, electrons move from the rod to the duster.
When the electrons are removed from particle the particles are left positively charged ­ these
charged particles are called ions.
Both +ve and ­ve electrostatic charges are only ever produced by the movement of electrons ­ the
negatively charged particles. The positive charges do not move. A positive static charge is always
caused by electrons moving away elsewhere.
Repelling and Attracting
Two things with opposite electric charges are attracted to each other.
Two things with the same electric charge will repel each other.
When you rub two insulating materials together a whole load of electrons get put together on one of
the insulators, which become negatively charged. They try to repel each other, but can't move apart
because their positions are fixed. The patch of charge that results is called static electricity because it
can't move.
Electric Current
Electric current is a flow of charge. In an electrical circuit the metal conductors are full of charges that
are free to move. So electric charge flow in metal conductors because the electrons are free to
move around. Current can't flow in an insulator because there are few charges free to move.
Current, Voltage and Resistance
Current: (I) will only flow through a component if there's a voltage across that component. Its units
are amperes (amps) A.
Voltage: (V) is the driving force that pushes the current round. Its units are volts, V.

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Resistance: (R) is caused by things in the circuit (such as components) that resist the flow of charge
(slows the charge down). Its units are ohms, .
There is a balance as the voltage is trying to push the current round the circuit, and the resistance is
opposing it ­ the relative sizes of the voltage and resistance decide how big the current will be.
Energy Transfer
Anything that supplies electricity is also supplying energy.…read more

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The Standard Test Circuit
This circuit is used for testing components.
The component, the ammeter and the variable
resistor are all in series, meaning that they can
be put in any order in the main circuit.
The voltmeter can only be placed in parallel
around the component under test. If it were to
be put anywhere else it would not work.
Varying the variable resistor alters the current
flowing through the circuit.…read more

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Calculating Resistance
At a constant temperature the resistance of a component is steady and is equal to the inverse of the
gradient of the line, or 1/gradient. In other words, the steeper the graph the lower the resistance.
When electrons move through a resistor, they collide with stationary
positive ions in the resistor these collisions make the ions vibrate
more, causing an increase in temperature.…read more

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The total resistance is just the sum of the individual resistances. The resistance of two (or more)
resistors in series is bigger than the resistance of just one of the resistors on its own because the
battery has to push charge through all of them.
The bigger the resistance of a component the bigger its share of the total potential difference. This
is because more work is done by the charge when moving through a large resistance, than a small
one.…read more

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A circuit with two resistors will have a lower resistance
than a circuit with either of the resistors by themselves ­ which means it will have a higher current.
Cell current
Cells connected in parallel circuits increase the total current in the circuit. However, the current
through each cell is less than in the rest of the circuit because they join together to make the total
Current Through a Component
Each component is separately connected to the battery.…read more

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If you want a bigger peak voltage and current you could do one of these four things:
Transformers are used to change the size of the voltage ­ they use electromagnetic induction to step
up or step down the voltage. They have two coils of wire, the primary and the secondary coils,
wound around an iron core. The alternating current in the primary coil causes changes in the iron
core's magnetic field, which creates a changing voltage in the secondary coil.…read more

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Current Carrying Wires
There is a magnetic field around a straight, current carrying wire. The field is made up of concentric
circles with the wire in the centre.
Because of its magnetic field, a current carrying wire or coil can exert a force on another current
carrying wire or coil, or a permanent magnet. When a current carrying wire is put in a different
magnetic field, the two magnetic fields affect one another. The result is a force on the wire.…read more

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These forces are just the usual forces which act on any current carrying wire in a magnetic field.
Because the coil is on a spindle and the forces act one up and one down, it rotates. The split ring
commentator is a clever way of swapping the contacts every half turn. This reverses the direction of
the current every half turn to keep the coil rotating continuously in the same direction.…read more


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