# Physics P3

Some informtion on a few of the topics for AQA Physics Unit 3

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• Created by: Lil
• Created on: 29-05-12 18:52

## Centripetal Force

When an object in moving in a circular path it's pulled towards the centre of the circle. If it's moving in a circle it also means that the object is constantly accelerating as it's changing direction (but the speed stays the same.) The force which keeps something moving in a circular motion is called a centripetal force.

A centripetal force can be provided by friction, tension or gravity.

The size of the centripetal force can be affected by size, mass and speed. To increase the centripetal force you can : increase the mass of the object, increase the speed of the object and lastly, decrease the radius of the circle.

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## Ultrasound

Sound waves which have a frequency above 20 000 Hz (beyond the frequency human's can hear) are known as ultrasound. Ultrasound are partially reflected at the boundary between media. The time it takes for the waves to get back means that you can calculate the distance that it has travelled.

Sound waves can be used to clean delicate machinery as they are efficient at removing dirt and debris. They can also be used by dentists to clean teeth.

Ultrasound can also be used to detect cracks in metal and for pre-natal scanning.

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## Transformers

Transformers involve the use of electromagnetic induction so they will only work on AC. There are 'step-up transformers' and 'step-down transformers'. Both types of transformer have primary and secondary coils joined on an iron core.

No electricity flows round the iron core it's because it's magnetic. That's why insulated wire is used; otherwise it would short circuit.

Transformers work as the primary coil produces an alternating magnetic field by the alternating current, this then induces an AC voltage in the secondary coil.

The transformer equation

Primary Voltage       = Number of turns on primary

Secondary Voltage    Number of turns on secondary

Transformers only work with AC!!

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## Generators

Generators basically generate electricity. They work by electromagnetic induction. As they coil spins an AC voltage is induced as the coil cuts through the magnetic field lines caused by the permanent magnets.

This can then be used to power things as the slip rings and brushed means that the coil in constantly connected and the circuit is never broken.

The AC voltage can then be used to power something such as a light bulb.

The potential difference can be increased by: increasing the strength of the magnet, increasing the number of turns in the coil and increasing the speed at which the coil moves.

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