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P5 physics gcse ocr 21st cetuary science

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  • Created by: callum
  • Created on: 23-05-11 17:02

Electric current

Electric current is a flow of charge. 

Batterys make these free changes move.

An ammeter can be used to measure current in amps A.

Metal conductors have many electrons that are free to move.

Electric current is the movement of these free electrons.

Insulators have no charges that are free to move so do not conduct electricity.

The bigger the voltage the bigger the current.      

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Resistance

For any ciruit, the bigger the resistance, the smaller the current.

Resistance ( ohms ) = voltage ( volts ) / current ( amper )

Resistors gett hotter when a current passes through them. This is becuase the moving electrons bump into stationary atoms in the wire.

two resistors in a series have more resistance than just one on its own. This is because the battery has to push charges though both reisistors now.

Two reisistors in parallel have less resistance tha just one on its own. This is because there are now more paths for charges to flow through.

The resistance of a light dependant resistor ( LDR ) changes with light intensity. The darker it is the more resistance.

A thermistors resistance is dependant on temperature. The colder it is the more resistance.

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Potential difference

The Voltage of a battery shows it push on charges in the circuit. Potential difference means the same as Voltage.

To measure the Potential difference across a component connect a Voltmeter.

More Batterys increase voltage and potential difference.

Parallel batteries provide no extra push because there are now more paths for the charges to be pushed along.

Potential difference is the biggest across the component with the greatest reistance. This is because the component with the greatest difference is doing the most work on the current.

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Electromagnetic induction.

When you move a megnet into a coil of wire a voltage is Induced. This is called Electromagentic induction.

You can induce a voltage in the opposite direction by:

- Moving the magnet out of the coil

- Moving the other pole of the magnet into the coil

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Generating main electricity

Generators make electricity by Electromagnetic induction.

You can increase the induced voltage by:

- spinning the magnet faster

- adding an iron core to the wire

- using a magnet with a stronger magnetic field

- addig more turns to the coil of wire

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Alternating and direct current

The generators magnet turns all the time which means its magnetic field constantly changes direction. So the induced current changes all the time. This is called Alternating current ( A.C ).

A batterys current does not change. It is a Direct current ( D.C ).

Mains electricity is supplied as Alternating current . This is because :

- Alternating current is easier to generate.

- Alternating current is more efficient, with less energy wasted as heat.

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Transformers

A Transformer changes the size of an Alternating voltage.

The voltage in the Primary coil changes all the time. It is a strong magnet and induces a current in the secondary coil where a current now flows.

To work ou the secondary coils voltage:

num. of coils across primary / num. of coils across secondary X primary voltage.

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Electricity at home.

When electic current flows through a device, such as a computer, energy is transferred by the device.

The Power of an appliance is the rate at which energy is transferred to it.

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units needed

energy transferred: Joules J

Power: Watt W

Time: Seconds S

current: Ampere A

voltage: Volts V

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Effieciecy formula

Efficiency = Energy usefully transferred / total energy supplied X 100%

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Comments

RROD

thx will use this. 4/5

Priya Farmah

Thanks :)

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