# Circuits

P.51 - 56 AQA GSCE Physics Revision Guide

I've only listed the key points in my notes so they may not be as detailed as my previous ones.

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• Created by: jenny100
• Created on: 25-05-12 10:55

## Circuits - The Basics

Current: flow of electrons round a circuit. It will only flow through a component if a voltage is across that component

Voltage: driving force that pushes the current round

Resistance: anything in the circuit that slows the flow down

• If you increase the voltage - then more current will flow
• If you increase the resistance - then less current will flow

Ammeters must be placed in series (measures current)

Voltmeters must be place in parallel (around the component under test)

When components are in series it means they can be placed in any order round the circuit.

Some components (such as the voltmeter) can only be placed in parallel around the component under test.

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## Resistance

Different Resistors: the current through a resistor is proportional to voltage

Filament Lamp: as the temperature of the filament increases the resistance increases (curve on a graph)

Diode: current will only flow through a diode in one direction

If the graph curves it means the resistance is changing, to work it out use this formula:

Resistance (R)= Potential Difference (V) / Current (I)

Diode = a special device made from semiconductor material such as silicon. It lets current flow freely in one direction but not the other, i.e. there's a very high resistance in the reverse direction

Thermistor (Temperature-dependant Resistor) = in hot conditions the resistance drops, in cool conditions the resistance goes up. Make useful temperature detectors

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## Series and Parallel Circuits

Series Circuit:

• Potential Difference is shared - the voltage always adds up to the source voltage (light bulbs get dimmer)
• Current is the same everywhere

Parallel Circuit:

• Potential Difference is the same across all components - same voltage meaning same brightness of light bulb
• Current is shared between branches - current going into a junction has to equal the total current leaving. The same current will flow through identical components
• Resistance is difficult to work out - current through each component depends on resistance so it's difficult to work out
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