physics revision

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• Created by: sana
• Created on: 09-05-11 20:55

Producing electricity

There are two types of electric current - direct current (DC) and alternating current (AC).

Direct current - If the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5V. The diagram shows an oscilloscope screen displaying the signal from a DC supply.

Alternating current - If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply.

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Producing electricity

An electric current can be produced by moving a magnet inside a coil of wire. The size of this induced current can be increased by moving the magnet faster, by using a stronger magnet, or by increasing the number of turns on the coil and increasing its area. A dynamo is a type of electricity generator.

An electric current is produced when a magnet is moved into a coil of wire in a circuit. We say that the electric current has been induced, and the process is called induction.To increase the induced current:

• move the magnet faster
• use a stronger magnet
• increase the number of turns on the coil
• increase the area of the coil
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Dynamos

It is not practical to generate large amounts of electricity by passing a magnet in and out of a coil of wire. Instead, generators induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire. As this happens, a potential difference - voltage - is produced between the ends of the coil, which causes a current to flow.

One simple example of a generator is the bicycle dynamo. The dynamo has a wheel that touches the back tyre. As the bicycle moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the bicycle's lights.

The faster the bicycle moves, the greater the induced current and the brighter the lights.

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Voltage, current and resistance

Current is measured in amps (A), using an ammeter in series. Voltage is measured in volts (V), using a voltmeter in parallel. There is a relationship between voltage, current and resistance

voltage = current × resistance.

The resistance in a circuit can be increased by adding more components, such as resistors and lamps.

The filament lamp does not follow Ohm's Law.

The resistance of a thermistor changes with temperature, and the resistance of an LDR changes with different light levels. These components are useful for controlling electrical devices.

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Current - A current flows when an electric charge moves around a circuit. No current can flow if the circuit is broken.

Measuring current - You need to know how to measure the current that flows through a component in a circuit and the voltage across it.

current is measured in amperes, amperes is often abbreviated to amps or A current flowing through a component in a circuit is measured using an ammeter, the ammeter must be connected in series with the component - remember, in a series circuit, electrical devices are placed one after the other in a continuous line in the circuit between the positive and negative poles of the battery

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Voltage

A voltage across an electrical component - such as a lamp - is needed to make a current flow through it. Cells or batteries often provide the voltage needed.

Measuring voltage

The voltmeter is in parallel with the lamp

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