# Physics

## Kinetic Theory

Liquids cannot be compressed because their particles are close together!

Solids = Low energy

Liquids = Medium energy

Gas = High energy

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## Energy transfers and efficiency

Efficiency = A measure of how good a device is at changing energy from one form to another.

Efficiency = useful energy out

total energy in

Energy = Power X Time

When a ball falls in the air, it loses gravitational potential energy and gains kinetic energy.

When an electric heater is switched on, it changes electrical energy into thermal energy.

Wasted energy becomes increasingly spread out.

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## Energy transfer by heating

Conduction: Heat travels through the particles in a solid, energising them. All the particles are eventually heated.

Convection: When a liquid is heated, convection currents push hot particles upwards to heat cool particles before sinking back down again. This cycle continues until all the particles are heated. Some particles that have gained energy at the top evaporate off as a gas with high energy, others lose energy and cool back down as a liquid.

The closer together the particles, the better conductor (e.g. solid) Free electrons in a metal element carry the heat energy through the particles, heating it up.

Convection and conduction are stopped by a vacuum.

The surroundings affect the rate at which a hot object transfers energy.

Animals that live in hot countries have big ears because the surface area to volume ratio is larger which means they can lose heat more easily.

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Infrared radiation is thermal radiation - all hot objects emitt it and absorb it. Infrared radiation signals travel through optical fibres. Optical fibres are thin strands of solid glass. The light is reflected internally and detected at the other end. We use optical fibres because:

1. They are cheaper

2. They are thinner and lighter

3. Signals travel further

The shape and colour of an object affects the rate at which it radiates and absorbs infrared radiation:

Black, dull surfaces absorb heat the best

Light, shiny surfaces are the worst emitters of heat energy

Emitting = Giving out

Absorbing = Taking in

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## Heating and Insulating buildings

Specific heat capacity = Amount of energy needed to raise the temperature of substance by a given amount.

Energy = Mass X SHC X Change in temperature

SHC of water = 4181J

U-Value = Measure of heat loss in a building element such as a wall, floor or roof. The higher the U-Value, the worse the thermal performance of the building element. Low U-Value = High insulation

Payback time = A measure of how cost effective it is.

Payback time (years) =  Cost of installation                 (£)

Savings per year in fuel costs (£)

Reducing energy consumption: Energy efficient appliences, Insulate your home - very efficient and cost-effective

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## Transferring electrical energy

Electrical energy is transferred into heat energy to boil water in a kettle. Energy is wasted kinetically when the kettle vibrates. Sound energy is also wasted. The heat energy is in the steam, as well as the water and the kettle.

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## Generating electricity

Uranium is used as a nuclear fuel because its radioactive.

1. Water is heated to a reactor

2. The steam produced turns a turbine which links to the generator, producing electricity

3. The steam cools in the condenser, transforming back into water which then gets heated once again

Fossil fuels are used to heat the water, e.g. coal, oil, gas.

A gas fired power station takes the least time to start up.

At pumped storage hydroelectric stations water is pumped back into the reservoir when there are periods of low power demand. This is often when there is excess energy being produced by other power stations. During higher demand periods, the water can be allowed to flow back down to produce electricity just like a normal power station. Such a system saves energy and also can be turned on quickly at times of peak demand.

Water power: Hydroelectric, wave, tidal.

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## Generating Electricity 2

Solar cells convert sunlight directly into electricity but they are unreliable.

Wind power is renewable, cheap and has the possibility of tourism but wind farms are generally unsightly, noisy and expensive to set up.

Water power is renewable, cheap but the area has to be suitable.

Fossil fuels are cheap and easily accessible however, they produce greenhouse gases and they are non-renewable.

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## The National grid

Power station -> Step-up transformer -> Overhead power lines -> Step-down transformer -> Houses

Step-up transformer - Voltage increases to reduce amount of energy lost (more efficient)

Step-down transformer- Voltage decreases so it is at a suitable level for safely using electrical devices in our homes.

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## Reflection

The angle of incidence is always the same as the angle of reflection.

A virtual image is formed behind a plane mirror which is the same size and distance as the object.

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## General Properties of Waves

Oscillation = Vibration

In transverse waves, the oscillations are at right angles to the direction of travel and energy transfer.

In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer.

Compression = Frequency increases because of a pushing force.

Rarefaction = Frequency decreases because of a pulling force.

Compression happens when particles are forced/pressed together.Rarefaction occurs when particles are given extra space&allowed to expand.
Frequency = wave speed

wavelength

Amplitude = Height from mid-point to top

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## General Properties of Waves 2

Longest to shortest wavelengths/least to most energy/smallest to biggest frequency:

Microwaves

Visible light

Ultra-violet light

X-Rays

Gamma rays

Reflected - A table

Refracted - Air to water

Diffracted - Blinds

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## General properties of waves 3

Waves are NOT refracted if travelling along the normal.

Velocity = Frequency X Wavelength

If velocity stays the same and frequency increases, the wavelength has to decrease. If the frequency increases, so does the energy.

Microwaves are used for satallite communications for mobile phones etc.

Infrared radiation is used for internet communication and communication via optical fibres.

Visible light waves are used for visible communication for animals and humans.

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## Sound

Sound waves are produced by oscillation pressure. When an object vibrates, it sends out waves which can be heard as sound.

The faster an object vibrates, the higher pitched noise it makes. As sound waves get closer, their frequency increases, so their pitch and volume increases. When getting further away, the pitch and frequency decreases so the sound gets quieter (the Doppler effect).

Sound that bounces back is called an echo. In an empty place, there is nothing to stop/absorb/cut off the sound waves so they bounce off the walls whilst losing energy therefore bounce back to our ears.

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## Red-Shift

Red-Shift is when light is shifted towards the red end of the visible spectrum.

The further away a galaxy is, the more it is red-shifted. The red end of the spectrum has a lower frequency so its links to the Doppler effect because the light/sound is moving away from us in a lower frequency. This means the distant galaxies must be moving away from the earth.

The further away a galaxy is, the greater amount of red-shift which also means faster movement. This is evidence of the big bang and an expanding universe because galaxies are moving away from one another. The big bang indicates that the universe was created by accident by an explosion which left behind background radiation (CMBR).

CMBR = Cosmic Microwave Background Radiation. CMBR has cooled as the universe has expanded and is now slightly less than 3 degrees above absolute zero.

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