# Physics 3 AQA Gcse

Explainations for answers to questions which are often worth the most marks.

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• Created by: Sarah
• Created on: 26-05-11 14:16

## How can you find the centre of mass of a thin shee

• Suspend the sheet from a pin held in a clamp stand so that it is freely suspended and able to turn.
• When it comes to rest hang a plumbline from the same pin.
• Mark the position of the plumbline on the thin sheet.
• Hang the sheet, using the pin, from a different point and repeat the procedure.
• The centre of mass is found where the lines, marked by the position of the plumbline, intersect/meet.
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## How are ultrasound waves used to clean delicate me

• The object that is being cleaned is placed in a tank of fluid (sometimes water).
• Ultrasonic/ultrasound waves are passed through the tank of fluid
• The waves cause the particles in the fluid to vibrate because to travel the waves cause mechanical vibrations.
• The vibrations of these particles dislodge the dirt particles in the object.
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## How are ultrasound waves used to find flaws in met

• A transmitter sends ultrasound waves into the metal object.
• These waves are partially reflected at boundaries between different media, i.e. where the flaws are.
• A detector detects these reflected rays.
• The time taken for these reflected waves to be detected can be used to work out the position of the flaw in the metal object.
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## How does the motor effect work?

• A current is passed through the coil.
• A force acts on each side of the coil which causes the coil to spin.
• The force on each side of the coil is in the opposite direction to the other side of the coil.
• The split ring commutator reverses the direction of the current passing through the coil with every half turn of the coil. Because the sides of the current swap every half turn the coil is pushed in the same direction so spins round rather than back and forth.

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## How does the generator effect work?

• Force is applied to the coil by either the coil being turned or the magnet being moved in relation to the coil
• This movement cuts the magnetic field lines which induces potential difference along the ends of the coil.
• If the coil is connected to a circuit, current is produced and transfered into the circuit.
• The commutator rings and brushes are used in this effect to complete the circuit so the current can be transfered from the ends of the coil into the circuit when the coil is moving and prevents the wires from the coil becomming tangled.
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## How do transformers work?

• Alternating current is passed into the primary coil.
• This a.c. current changes the magnetic field accross the primary coil.
• The alternating magnetic field is transfered into the secondary coil because of the iron core which is magnetic.
• This induces potential difference across the secondary coil.
• The induced potential difference produces alternating current accross the secondary coil.

NB:

The coils are insulated so current isn't short circuited.

The iron core is laminated so that current doesn't pass through this core which cause over heating and ineffiency.

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## How are elements formed?

How are light ellements formed?

In stars the same size as our sun, fusion between the nuclei of hydrogen produces light ellements to be formed during the main star sequence. This nucleur fusion process gives out large amounts of energy in the form of heat and light.

How are heavy ellements (heavier than iron) formed?

When large stars die the core of the star is compressed into a neutron star. The energy from this is used to fuse light ellements into heavy ellements. These ellements are scattered accross the universe when the large star explodes in a supernova.

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## Comments

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awesome, thanks =)

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