P5 revision

Static electricity

Neutron has a charge of 0.                                                                                                      Proton has a charge of 1.                                                                                                      An electron has a charge of -1.

Static electricity can build up on an object by rubbing two objects together; friction causes electrons to be transferred between the two objects. If the objects are insulators, the charges (electrons) remain (static) on the objects. The object that loses its electrons will have a positive charge. the object that gains electrons will have a negative charge. 

+ + will repel

+ - will attract

- - will repel

Attraction and repulsion are the forces that act between these combinations of particles.

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Hair and Van der Graff generator

Friction inside the Van der Graff generator causes a negative charge to build up on the sphere.

The person touching the sphere also builds up a negative charge. 

Each hair has a negative charge so they repel the hair next to it.

Hair is light and so the small force of repulsion is enough to make the hair stand up.

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Electrical circuits

Electric current- flow of current through a wire in a circuit.

Potential difference (voltage) - the difference in charge that 'pushes' the electrons around the circuit. 

Resistance- the opposition to the flow of current in a component.

Series circuit- only one path in it. Current stays the same throughout the circuit. Voltage divides between components.

Parallel circuit- more than one path around it. Current divides between the branches of the circuit. Voltage stays the same and is the same as the voltage produced by the power source. 

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Ohm's law and resistors

Current is directly proportional to voltage (if the temperature is constant).

Voltage = current * restistance 

To calculate total resistance of 2 resistors in a series circuit add them both together. 

To calculate the total resistance of 2 resistors in a parallel circuit:

Total resistance = (R1 * R2) + (R1 + R2).

LDR - light dependent resistor. As light increases, resistance decreases.

Thermistor- temperature dependent resistor. As temperature increases, resistance decreases. 

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Power

Unit for power = Watts (W)

kwh = kilowatt hour. Measures electrical energy use. 

Energy(kwh) = power(kw) * time(h)

Example: If one unit (kwh) of electricity costs 12p, how much would it cost to watch a 300w TV for 5 hours?

Energy = 0.3kw (remember to convert) * 5

Energy = 1.5kwh

1.5khw * 12p = 18p 

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Generators and AC DC

Electromagnetic induction- Production of electricity by moving a magnet in a coil of wire.

A generator produces energy by rotating a magnet inside a coil of wire which induces an electrical current in the coil.

Ways to increase electricity generated:

- bigger/stronger magnet 

- more coils of wire

AC (alternating current) changes direction repeatedly. Produced generators.

DC (direct current) flows in one direction only. Produces by batteries. 

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