# Module P5 – Electric Circuits OCR 21st Century

Module P5 – Electric Circuits OCR 21st Century

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• Created by: bhiruntha
• Created on: 28-01-12 19:18

## Static electricity

·         Build up of static electricity is cause by friction.
·         When you rub two insulating materials electrons move from one to the other, so one is positive and one is negative (electrons are negative).
·          Electrically charged objects attract small objects.
·         ONLY ELECTRONS MOVE. If it loses electrons it becomes positive, if it gains them it becomes negative.
·         Positive + Positive repel. Negative + Negative repel. Negative + Positive attract.

·         Dust is easily attracted to insulators (which easily gain a charge) and so things such as TV’s get dusty easily.

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## Electric Current

·         Electric current is the flow of charge around a current.
·         It’s measured in Amps.
·         Electrical charge flows along a conductor (the metal in the wires) because there are lots of free electrons that are free to move around.
·         The battery pushes the charge all the way round the circuit. It’s not used up and it doesn’t disappear.
·         Current depends on voltage and resistance.
·         Voltage is the driving force that pushes current around, its measures in Volts (V).
·         Resistance is anything slows down the flow of current in a circuit, ohms (Ω).

·         Increase in voltage = more currenT

•          Increase in resistance = less current.
•       We normally say electrons flow from positive to negative, but the actually flow from negative to positive. Opposite to ‘conventional current.’
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## Circuits – The basics

· Voltmeters measure the potential difference between two points. You have 1 around the battery and 1 around the component so you can see how much energy the component takes out compared to the amount of energy produced by the battery.

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## Resistance

·         The slope of a voltage current graph shows the resistance.
·         The current through a resistor is proportional to the voltage.
·         The steeper the slope the lower the resistance.
·         Calculating resistance – R = V/I
·         Resistors get hot as the electrons collide with the stationary particles in the resistor, which makes the resistance increase. A filament lamp contains a piece of wire with such a high resistance that it gets so hot and it glows.
·         Light dependant resistor (LDR) – Dark = high resistance
Light = Low resistance
·         LDR useful for automatic lights or burglar alarms.

•        A thermistor – Cool = High resistance     Hot = Low resistance

v=I X R , I = V/R , R= V/I

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## Series Circuits and Parallel Circuits

·         Series circuits are one continuous loop.
·         If one thing disconnects, the circuit is broken, so they aren’t very handy and very few things are connected.
·         Potential difference (Voltage) is shared around the components. V = V₁ + V₂
·         Resistance adds up, so if 4 components are in the circuit 1 with 5 ohs, 1 with 3 ohms ect, the resistance adds up.
·         Cell voltages add up too, so three 12v batteries will mean the circuit has 36v in it.
Parallel Circuits:

·         Each component is separately connected to the + and – side of the battery.
·         If you remove one, it’ll hardly affect the others.
·         In parallel circuits V₁ = V₂ ect
·         Current is shared between branches though, so A = A₁ + A₂
·         The components have the same p.d (voltage) through them all, so the one with most resistance will have the lowest voltage and vice versa.

Resistance controls how much current the voltage is able to push through a component.

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## Mains Electricity

Mains supply is AC, battery supply is DC.
·         UK mains are about 230v.The current is constantly changing direction (alternating current).
·         Battery supply is DC (direct current).
·         AC is used for mains as it’s easy to generate and is distributed more evenly.
·         Moving a magnet in a coil of wire induces current.    This is called electric induction.
·         The direction of the current depends on the way you move the magnet.
·         AC generators just turn the magnet and there’s current.
·         As the magnet turns the magnetic field in the wire changes which induces a voltage, which makes a current flow in the coil.
·         When the magnet is turned a half turn the direction of the current changes (when north goes from left side to right side). This is why you get AC current.

·         Transformers change the voltage, but only AC voltages.
·          current goes to transformer, The more coils on the output of a transformer the higher the volts.  Fewer coils on the output reduce the voltage.
·         They work by electromagnetic induction.
·         Primary coil produces a magnetic field, which stays in the iron core.
·         Because it is AC the magnetic field is constantly changing.
·         The changing magnetic field induces an AC current in the secondary coil.
·         The relative number of coils in the secondary coil determines more or less voltage.
·         If you supplied DC to the primary coil, you’d get a magnetic current but no induction because you need a changing field to induce voltage.

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## Mains Electricity part 2

• Transformers change the voltage, but only AC voltages.
• ·          current goes to transformer, The more coils on the output of a transformer the higher the volts.  Fewer coils on the output reduce the voltage.
• ·         They work by electromagnetic induction.
• ·         Primary coil produces a magnetic field, which stays in the iron core.
• ·         Because it is AC the magnetic field is constantly changing.
• ·         The changing magnetic field induces an AC current in the secondary coil.
• ·         The relative number of coils in the secondary coil determines more or less voltage.
• ·         If you supplied DC to the primary coil, you’d get a magnetic current but no induction because you need a changing field to induce voltage.
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## Electrical Energy

·         Energy is transferred from cells and other sources.
·         Anything that supplies electricity (cells, batteries, generators ect) transfers energy into charge, which then transfers it to components in the circuit.
·         Power is the rate of energy transfer.
·         So, if something needs more power, it takes more energy, so a light might only take 100W ( 100J/s) but a kettle might take 2.5kW (2500J per second).

·         Energy transferred (J) = Power (W) x Time (s) & Kilowatt-hours (kWh) are UNITS of energy.

·         Energy is usually measured in joules, but that’s tiny. A kWh is the amount of electrical energy converted by a 1 kW appliance left on for 1 hour.
·         Longer left on and high power rating means more cost.
·         Cost = Number of kWh x Cost per kWh
·         Power ratings of appliances:
o   High power rating transfers a lot of energy in a short time.
o   Power = Voltage x Current, re-arrange as per question

More efficient machines waste less energy. Efficiency = Useful power out/ Total power in .

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