# Physics P5

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Static Electricity:

Static electricity is caused by friction. When two insulating materials are rubbed together, electrons are also rubbed off leaving the materials with charges. The like charges repel each-other while opposite charges are attracted to each-other. For example hair particles get charged and because they have the same charge, they repel each-other. Dust particles are also charged so they are attracted to objects made of insulating materials. And clothes get charged too.

Equations:

Power = Voltage x Current.

Resistance = Voltage / Current.

Circuits:

Current is the flow of charge around a circuit. Current will only flow through a component if there’s a voltage across it. More voltage = More current. Less voltage = Less current. It is measured in amps (A).

Power is the rate of energy transfer to an appliance. It is measured in watts (W).

Resistance resists the flow of current. Resistors get hot when current passes through them because the electrons in the current collide with the positive ions in the resistor. This causes the ions to vibrate more thus increasing the temperature. A filament lamp has a high resistance so when current passes through, temperature increases so much that it glows – this is the light seen.

The different circuit symbols are: cell, battery, power supply, switch open, switch closed, filament lamp, fixed resistor, variable resistor (e.g. Thermistor, LDR), ammeter (measures current) and a voltmeter (measures potential difference between 2 points).

In a LDR (light dependant resistor), resistance is high in the dark while lowest in the light.

In a thermistor (temperature dependant resistor), resistance is high at cool temperatures while lowest in hot temperatures.

Series – In a series circuit, potential difference is shared between components, current is the same and resistance adds up to the total.

Parallel

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