# Physics 3 - QWC's

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• Created by: Katy **
• Created on: 06-04-14 13:29

## Explain the Oersted's Experiment

The Oersted's Experiment involves a single wire and a compass needle.

It says that when there is no current flow, the needle will point from North to South. However when the current is on the needle will start to move. If the current is reversed, the needle will move in the opposite direction.

When the wire is above the compass the needle moves in the oppostie direction to when the wire was below the compass.

The Oersted's Experiment concludes that if iron filings were placed by a wire, these filings will move in a circular motion around the current. This shows the magnetic field. If the current is increased there is a stronger magnetic field, but there is no change in the shape of field or direction that it is going.

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## Factors effecting the force/movement of the wire

By increasing the current the wire will move faster.

By having a stronger magnet the wire will also move faster.

If the current direction is reversed the wire will move in the opposite direction.

If the magnet field is reversed the wire will also move in the opposite direction.

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## The effects of moving a single wire

Moving a wire in a magnetic field will produce an induced current in the wire.

If the wire is stationary there will be no current induced.

By moving the wire faster there will be a greater current, also by having a stronger magnet there will be a greater current in the wire.

By reversing the direction of movement the current direction is reversed, likewise by reversing the direction of the magnetic field it will reverse the direction of the current.

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## What are the factors affecting the induced current

To produce an induced voltage for a conductor (wire) the voltage is induced when a conductor cuts the magnetic field lines. However to produce an induced voltage for a coil (solenoid) the voltage is induced when the magnetic field throught the coil changes.

The faster the rate of change of the magnetic field the larger the induced current is produced. The voltage will drive the current in a complete circiut.

To increase the induced current; the wire has to move faster, a stronger magnet has to be used to make a greater magnetic field, increase the length of a wire in the field & increase the area of a coil.

To change the direction of a current; reverse the magnetic field & move the wire in the opposite direction.

When the wire is moving parallel to the magnetc field, no lines of field are being cut and there is no current produced.

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## Changing the magnitude of induced current

As the coil turns, the rate of change of magnetic field changes.

The rate of change is at its maximum at the horizontal poition & is at zero at the verticle position.

This will change the magnitude (size) of the current from the maximum to zero.

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## Changing the direction of induced current

As the coil turns, the conductor changes the direction in which it moves throught the magnetic field.

There is a change in the dirction, in which the magnetic field through the coil is changing.

This will change the direction in which the current flows thought the coil.

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## Varying current in the primary coil

For there to be a varying current though the primary coil there must be a varying current (voltage).

To produce this induced current in the secondary coil there needs to be a varying magnetic field through the secondary coil.

This means that there must be a varying magnetic field coming from the primary coil.

Therefore, we need a varying current in the primary coils' wires.

AC in = AC out

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## Describe the journey of this distance time graph

1. Stationary

2. Accelerates at a steady speed

3. Steady speed for some time

4. Deccelerates

5. Stops

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## Describe the journey of this speed time graph

1. Stationary

2. Accelerates to a steady speed (uniform)

3. Steady speed for some time

4. Deccelerates (uniform)

5. Stops

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## Newton's Third Law

The rule of Newton's Third Law is that for every action there is an equal & opposite reaction.

For example, firing a gun:

Before firing a gun, the momentum of the gun & bullet is zero (this means that the velocity is also zero).

The conservation of momentum shows us that the total momentum before is equal to the total momentum after:

0 = Pgun + Pbullet

0 = -Mvgun + Mvbullet

Therefore the two velocities must be in two opposite directions.

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## Explain what is 'Absolute Zero'

Temperature indicates or measures the energy of particles, or the mean energy of a number of particles.

Absolute Zero is when the particle have no energy. Particle show their energy in heat, movement, volume (movement) & pressure (momentum).

Zero energy would be when there is no volume or pressure.

At a temperature with zero at absolute & with increments in 'c is called the Kelvin Scale.

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## Increasing temperature on pressure

An increasing temperature will increase the velocity.

This increase will also increase the the change of momentum & reduce the impact time.

The rate of change will then increase.

Both of these factors will increase pressure.

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## Explain Conduction

Conduction is the transfer of heat by particles in contact with eachother & passing on their vibrations.

When a solid is heated their particles will vibrate more. The heat will cause vibrations to transfer into kinetic energy.

The particles are connected to neighbors & cause them to vibrate. These will then pass on the vibrations to their neighbors all the way to the end of the metal.

The metals vibrating electrons are more important for pass on energy as they are more mobile.

Particles are closer together in solids than in liquids, so therefore are better conductors. Likewise with liquids there are more particles closer together than in gasses.

Solid → Liquid → Gases

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## Explain convection in fluids

Convection is the transfer of heat by particles moving. As these particles get hot they rise & take the heat energy with them. This will only occur in liquids & gases.

Firstly the particles will gain heat energy & move. These hot particles take up more space & expand making them less dense, they begin to rise.

As the liquid rises, the particles will take the heat with it.

The colder, more dense particles will fall & replace the particles that have risen.

This is called a convection current.

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## What are the Laws of Refraction

For a wave travelling bewtween two media (air → glass) & at right angles to the boundary the speed of wave will change, but the direction will not change.

From a less dense media to a more dense media the speed of the wave will decrease & the direction of the wave is refracted towards the normal.

From a more dense media to a less dense media the speed of wave will increase & the direction of the wave refracted away from the normal.

This is because the wavefront is meeting the boundary at an angle, so only part of the wavefront is changing speed. The consequence is that the entire wavefront is forced change direction.

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## Rays in an optical fibre

Firstly the ray will travel from a less dense medium to a more dense medium, so it is refracted towards the normal.

The ray will then meet the boundary at a greater angle than the critical angle, so there is a total internal refraction.

Finally the ray will travel fom a more dense medium to a less dense medium, refracting away from the normal.

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## How is a star born

A star begins as a nebula (cloud) of elementary particles.

The star will then contract due to gravity.

This will cause an increase in temperature & perssure at the star's core.

A big enough cloud will cause the temperature & pressure to create fusion.

If the mass is great enough then a sustained continuous fuision occurs, this will make the beginnings of a protostar.

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## What is the life cycle of a star

The mass of the star will determine the sequance & the rate of change in the star.

The smaller stars are slower & more dramatic.

The usage of fuel for fusion will decrease the rate of fusion, this will decrease the outwards radiation pressure & the star will contract & collapse.

The collapse of the star will cause the core temperature & pressure to increase. This will result in a new higher fusion reaction, the star will expand & grow colder. This will decrease the star's pressure.

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