# Current Electricity - Physics - AQA Additional Science

Information on electric circuits, resistance, series circuits and parallel circuits.

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• Created by: Rukhsar
• Created on: 22-04-10 18:17

## Electric circuits

A cell pushes electrons around a circuit. A battery consists of two or more cells.

A switch enables the current in a circuit to be switched on or off.

A bulb/indicator is designed to emit light as a signal when a current passes through it or as a light source such as a lamp.

A diode allows current through in one direction.

An ammeter is used to measure electric current.

A voltmeter is used to measure potential difference/voltage.

A fuse is designed to melt and therefore break the circuit if the current through it is greater than a certain amount.

A variable resistor allows the current to be varied.

A fixed resistor limits the current in a circuit.

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## Resistance

Current - potential difference graphs are used to show how the current through a component varies with the potential difference across it.

Ammeters are always placed in series with the component. The unit of current is the ampere, A. Voltmeters are always parallel with the component. The unit of potential difference is the volt, V.

Potential difference = current x resistance

If the resistor is kept at a constant temperature the graph shows a straight line passing through the origin.

Resistance is measured in ohms. Resistance kept at a constant temperature means the resistance stays constant.

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## Current - potential difference

The current - potential difference graph for a filament lamp curves. So the current is not directly proportional to the potential difference.

The resistance of the filament increases as the current increases. This is because the resistance increases as the temperature increases.

The current through a diode flows in one direction only. In the reverse direction the diode has a very high resistance so the current is zero.

As the light falling on it gets brighter, the resistance of a light - dependant resistor (LDR) decreases.

As the temperature goes up, the resistance of a thermistor goes down.

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## Series circuits

In a series circuit all of the components are connected one after another. If there is a break anywhere in the circuit the charge stops flowing. There is no other route for the charge as it flows around the circuit so the current through each component is the same.

current = potential difference of supply / total resistance

The p.d of the supply is shared between all the components in the circuit. The p.ds across individual components add up to give the p.d of the supply.

The resistance of the individual components in series add up to give the total resistance of the circuit.

The bigger the resistance of a component, the bigger its share of the supply p.d.

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## Parallel circuits

In a parallel circuit each component is connected across the supply, so if there is a break in one part of the circuit charge can still flow in the other parts.

Each component is connected across the supply p.d, so the p.d across each component is the same.

There are junctions in the circuit so different amounts of charge can flow through different components. The current through each component depends on its resistance. The bigger the resistance of a component, the smaller the current through it.

The total current through the whole circuit is equal to the sum of the currents through the separate components.

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