# Tricky Key Points- AQA Physics Unit 3

The key points I find particularly difficult to remember... hope it helps.

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• Created by: Amelie
• Created on: 25-05-11 16:35

## Sound

What is sound?

Longitudinal waves caused by vibrations

What is the frequency of sound able to be heard by Humans?

20-20000 Hz

What is the relationship between Frequency and Pitch?

Pitch increases when Frequency increases

• Frequency is the number of cycles per second

What is the relationship between Amplitude and Loudness?

Loudness increases when Amplitude increases

• Amplitude is the size of the waves
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## Ultrasound

What is Ultrasound?

Sound waves above 20000 Hz i.e. cant be heard by humans.

How is Ultrasound useful?

Used in medicine. Ultrasound partially reflects off internal boundaries. Time taken for reflection is the measure of how far away it is.

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## Transformers

How does a Transformer work?

An alternating current in the primary coil produces a changing magnetic field in the iron core, hence introducing an alternating p.d. across the secondary coil.

How is the efficiency of the transformer improved?

The iron core is in layers (laminated), to reduce energy loss.

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## Electromagnetic Induction

What is the Motor Effect?

The Force exerted on a wire carrying current, in a magnetic field.

What use can be made with the Motor effect?

Split Ring Commutator

How can the Motor effect be changed?

Changes direction if:   -current is reversed                                                                                                   -magnetic field is reversed

What is the generator Effect?

Potential Difference induced in a wire as it cuts through magnetic field but must be perpendicular.

Continues on next card...

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## continued...

What use is made of the generator effect?

Slip Ring Commutator

• Changes direction every half turn

Other examples of Electromagnetic Induction?

Dynamo effect

• deflection of ammeter pointer as a wire moves in a magnetic field
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## The Solar System

Why have stars a long time of stability?

Due to Gravity and Radiation pressure.

And because Hydrogen fusion continues until Hydrogen runs out.

What is the life cycle of stars?

Low mass star: Nebula, Protostar, Main stage, Red Giant, White Dwarf, Black Dwarf

High Mass star: Nebula, Protostar, Main Stage, Red Supergiant, White Dwarf, Supernova, Black Hole

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## Mirrors and Lenses

What sort of images do Convex and Concave Mirrors produce?

Convex: Always Virtual (conVex = virtual)

Concave: Both

• Virtual if object is between mirror and principal focus
• Real if object is beyond principal focus

What are Virtual and Real Images?

Real = converge. Rays are brought to a focus.

Virtual = diverge. Image produced is upright and behind the mirror or lens.

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Quite good, but a couple of things:

- Amplitude is the maximum disturbance of the waves (or the height on an oscilloscope), as size is too vague and unspecific.

- Ultrasound is partially reflected off of internal boundaries in tissue or in metals etc, not just tissue and bones.

Other than that, very good job )

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Sorry about some questions not being in the right colour. Its driving me mad and it doesnt seem to change it whenever i try. Hopefully you are not as picky as I am :J

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Worst revision notes ever

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loving it

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