Physics: Unit one

Infrared radiation

Infrared waves are part of the electromagnetic spectrum. They are the part of the spectrum just beyond visibly red light. We can detect infrared radiation with our skin- it makes us warm.

  • All objects emit (give off) infrared radiation
  • The hotter an object is, the more infrared radiation it emits in a given time.
  • Infrared radiation can travel through a vacuum, as in travelling through space. This is how we get energy from the sun.
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Surfaces and radiation

  • Dark, matt surfaces are good absorbers of infrared radiation. An object painted dull black and left in the sun will become hotter than the same object painted shiny whit
  • Dark, matt surfaces are also good emitters of infrared radiation. So an object that is painted dull black will transfer energy and cool down more quickly than the same object painted shiny white.
  • Light, shiny surfaces are good reflecters of infrared radiation.
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States of matter

The three states of matter are solid, liquid and gas. We can make a substance change between these states by heating or cooling it.

  • In a solid the particles vibrate about fixed positions so the solid has a fixed shape.
  • In a liquid the particles are in contact with each other but can move at random, so the liquid doesn't have a fixed shape and can flow.
  • In a gas the particles are usually far apart and move at random much faster, so a gas doesn't have a fixed shape and can flow. The density of a gas is much less than that of a solid or liquid.
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Conduction

  • Conduction occurs mainly in solids. Liquids and gases are poor conductors.
  • If one end of a solid is heated, the particles at that end gain kinetic energy and vibrate more. This energy is then passed on to the neighboruing particles and in this way the energy is transferred through the solid.
  • This process occurs in metals.
  • In addition, when metals are heated their free electrons gain kinetic energy and move through the metal, transferring energy by colliding with other particles. This makes metals good conductors.
  • Poor conductors are called insulators. Materials such as wool and fibreglass are good insulators because they contain trapped air.
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Convection

  • Convection occurs in fluids. Fluids are liquids and gases.
  • When a fluid is heated it expands. The fluid becomes less dense and rises. The warm fluid is replaced by cooler, denser fluids. The resulting convection current transfers energy throughout the fluid.
  • Convection currents can be on a very snall scale, such as heating water in a beaker, or on a very large scale, such as heating the air above land and sea. Convection currents are responsible for onshore and offshore breezes.
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Evaporation and condensation

Evaporation occurs when a liquid turns into a gas. Evaporations occurs because the most energetic liquid molecules escape from the liquid's surface and enter the air. Therefore, the average kinetic energy of the remaining molecules is less so the temperature of the liquid decreases. This means that evaporation causes cooling.

The rate of evaporation is increased by:

  • Increasing the surface area of the liquid.
  • Increading the temperature of the liquid.
  • Creating a draught of air across the liquid's surface.

Condensation is when a gas turns into a liquid. This often takes place on cold surfaces such as windows and mirrors.

The rate of condesations is increased by:

  • Increasing the surface area.
  • Reducing the surface temperature.
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Energy transfer by design

The greater the temperature difference between an object and its surroundings, the greater the rate at which energy is transferred.

The rate at which energy is transferred also depends on the materials the object is in contact with, the object's shape and the object's surface area.

Sometimes we want to maximise the rate of energy transfer to keep things cool. To do this we use things that are good conductors, are painted dull black and have the air flow around them maximised.

Sometimes we want to minimise the rate of energy transfer to keep things warm. To do this we need to minimise the transfer of energy by conduction, convection and radiation. We may use things that are good insulators, are white and shiny and that prevent convection currents by trapping air in small pockets.

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The vacuum flask

It has a double-walled glass or plastic container, a plastic protective cover and inside there is either a hot or cold liquid. There's a sponge pad for protection and a plastic cao. Inside surfaces are silvered to stop radiation, vacuum prevents conduction and convection and there's a plastic spring for support.

The flask minimises energy transfer by conduction, convection and radiation. This reduces the rate of energy transfer and keeps hot things hot and cold things cold.

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Specific heat capacity

When we heat a substance, we transfer energy to it which will increase its temperature. The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1 kilogram of the substance by 1 degree celsius.

Difference substances have different specific heat capacities. The greater the specific heat capacity, the more energy required for each degree temperature change.

The greater the mass of a substance being heated, the more energy required for each degree temperature change. If we had a 2kg piece of copper, we would need to transfer twice the energy needed to raise the temperature of 1kg of copper by the same amount.

The equation for specific heat capacity is: E = m x c x θ

E is energy transferred (J)
m is mass (kg)
C is specific heat capacity (
J/kg °C)
θ is temperature change (°C)

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Heating and insulating buildings

Most people want to minimise the rate of energy transfer out of their homes to reduce fuel bills. This can be done by fitting:

  • Fibreglass loft insulation to reduce energy transfer by conduction
  • Cavity wall insulation that traps air in small pockets to reduce energy transfer by convection
  • Double glazing to reduce energy transfer by conduction through windows
  • Draught proofing to reduce energy transfer by convection
  • Aluminium foil behind radiators to reflect infrared radiation back into the room

The U-Value of a material tells us how much energy per second passes through it. Knowing the U-Values of different materials allows us to compare them. The lower the U-Value the better the material is as an insulator.

Solar heating panels contain water that is heated by radiation from the sun. This water may then be used to heat the building or provide domestic hot water. Solar heating panels are cheap to run because they do not use fuel. However they are expensive to buy and install, and the water is not heated at night.

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Forms of energy

Energy exists in different forms such as: light, sound, kinetic (movement), nuclear, electrical, gravitational potential, elastic potential and chemical.

The last three are forms of stored energy.

  • Energy can be transferred from one form to another.
  • Any object above the ground has gravitational potential energy.
  • A falling object transfers gravitational potential energy to kinetic energy.
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Conservation of energy

It is not possible to create or destroy energy, it is only possible to transfer it from one form to another or from one place to another.

This means that the total amount of energy is always the same. This is called the conservation of energy and it applies to all energy transfers.

  • For example, when an object falls, gravitational potential energy is transferred to kinetic energy
  • Stretching an elastic band transfers chemical energy to elastic potential energy.
  • In a solar cell, light energy is transferred to electrical energy.
  • Swinging a pendulum transfers energy from gravitational potential energy to kinetic energy and back again as it swings.
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Useful energy

A machine is something that transfers energy from one place to another or from one form to another.

The energy we get out of a machine consists of: Useful energy which is transferred to the place we want and in the form we want it and wasted energy which is not usefully transferred.

Both the useful and the wasted energy will eventually be transferred to the surroundings, and make them warm up. As the energy spreads out, it becomes more difficult to use for further energy transfers.

Energy is often wasted because of friction between the moving parts of a machine. The energy warms the machine and the surroundings.

Sometimes friction may be usueful, for example in the brakes of a bicycle or a car. Some of the kinetic energy of the vehicle is transferred to energy heating the brakes.

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Energy and efficiency

Energy is measured in joules, this uit is used for all forms of energy. The energy supplied to a machine is often called the input energy. From the conservation of energy we know that: input energy (energy supplied) = useful energy transferred + energy wasted

The less energy that is wasted by a machine, the more efficient the machine is.

We can calculate the efficiency of any appliance that transfers energy by: (useful energy transferred ÷ energy supplied) x 100

The efficiency can be left as a fraction or multiplied by 100 to give a percentage.

No appliance can be 100% efficient, except an electric heater, which usefully transfers all of the electrical energy supplied to it by heating its surroundings.

The energy transfer through an appliance can be represented with a sankey diagram

Efficiency is a ratio and it does not have a unit.

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Electrical appliances

Electrical appliances are extremely useful. They transfer electrical energy into whatever form of energy we need at the flick of a switch.

Common electrical appliances include:

  • Lamps, to produce light
  • Electric mixers, to produce kinetic energy
  • Speakers, to produce sound energy
  • Televisions, to produce light and sound energy.

Many electrical appliances transfer energy by heating. This may be a useful transfer, for example in a kettle, but energy is often wasted. Appliances should be designed to waste as little energy as possible.

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Electrical power

The poer of an appliance is the rate at which it transfers energy.

The unit of power is the watt (W). An appliance of 1 watt transfers 1 joule of electrical energy to other forms of energy every second.

Often a watt is too small a unit to be useful, so power may be given in kilowatts (kW) which is 1000 watts.

Power is given by the equation P = E ÷ t
P is the power in watts
E is the energy in joules
t is the time taken in seconds for the energy to be transferred

Power is the energy per second transferred or supplied, so we can write the efficiency equation in terms of power: efficiency = useful power out ÷ total power in (x 100 %)

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Using electrical energy

Companies that supply mains electricity charge customers for the amount of electrical energy used. Because of the large numbers involved, the joule is not a suitable usit. The amount of energy is measured in kilowatt-hour (kWh). A kilowatt-hour is the amount of energy transferred by a one-killowatt appliance when used for one hour.

The amount of energy transferred to a mains appliance can be found using the equation: E = P x t
E is the energy transferred (kWh)
P is the power of the appliance (kW)
t is the time taken (hours) for the energy to be transferred.

The electricity meter in a house records the number of kWh of energy used. If the previous meter reading is subtracted from the current reading, the electricity used between the readings can be calculated.

The cost of the electricity energy supplied is: total cost = number of kWh x cost per kWh. The cost per kWh is given on the electricity bill.

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Cost effectiveness matters

To compare the cost effectiveness of different appliances we must conside a number of different costs, including:

  • The cost of buying the appliance
  • The cost of installing the appliance
  • The running costs
  • The maintenance costs
  • Environmental costs
  • The interest charged on a loan to buy the appliance

Many householders want to reduce their energy bills. To do this they may buy newer, more efficient appliances (such as a new fridge). They could also install materials designed to reduce energy wastage (such as lost insulation).

The payback time is the time it takes for an appliance or installation to pay for itself in terms of energy saving.

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Fuel for electricity

In most power stations, water is heated to produce steam. The stream drives a turbine which is coupled to an electrical generator that produces the electricity. The energy can come from burning a fossil fuel such as coal, oil or gas. Fossil fuels are obtained from long-dead biological material. In some gas-fired power stations, hot gases may drive the turbine directly. A gas-fired turbine may be switched on very quickly.A biofuel is any fuel obtained from living or recently living organisms. Some biofuels can be used in small-scale, gas-fired power stations. Biofuels are renewable sources of energy.

In a nuvlear power station, the fuel used is uranium (or sometimes plutonium). The nucleus of a uranium atom can undergo a process called nuclear fission and this process releases energy. There are lots of uranium nuclei, so lots of fission reactions take place, releasing lots of energy. This energy is used to heat water, turning it into steam.

More energy is released per kilogram of uranium undergoing fission reactions than each kilogram of fuel that we burn. Nuclear power stations don't release any greenhouse gases, unlike fossil fuel power stations. However, they do produce radioactive waste that must be safely stored for a long period of time.

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Energy from wind

Energy from wind (water and tide) is called renewable energy because these sources of energy can never be used up, unlike fossil fuels or nuclear fuels.

We can use energy from wind and water to drive turbines directly.

In a wind turbine, the wind passing over the blades makes them rotate and drive a generator at a top of a narrow tower.

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Energy from water

Electricity can be produced from energy obtained from falling water, waves or tides.

Hydroelectric power: At a hydroelectric power station, water is collected in a resevoir. This water is allowed to flow downhill and turn turbines at the bottom of the hill.

In a pumped storage system, surplus energy is used, at times of low demand, to pump the water back up the hill to the top resevoir. This means that the energy is stored. Then at times of high demand the water can be released to fall through the turbines and transfer the stored energy to electrical energy.

Wave power: We can use the movement of the waves on the sea to generate electricity. The movement drives a floating turbine that turns a generator. Then the electricity is delivered to the grid system on shore by a cable.

Tidal power: The level of the sea around the coastline rises and falls twice each day. These changes in sea level are called tides. Id a barrage is built across a river estuary, the water at each high tide can be trapped behind it. When the water is released to fall down to the lower sea level, it drives turbines.

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Power from the sun

Solar energy from the sun travels through space to the Earth as electromagnetic radiation.

A solar cell can transfer this energy into electrical energy. Each cell only produces a small amount of electricity, so they are useful to power small devices such as watches and calculators.

We can join together large numbers of the cells to form a solar panel.

Water flowing through a solar heating panel is heated directly by energy from the sun.

A solar power tower uses thousands of mirros to reflect sunlight onto a water tank to heat the water and produce steam.

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Power from the Earth

Geothermal energy is produced inside the Earth by radioactive processes and this heats the surrounding rock. In volcanic or other suitable areas, very deep holes are drilled and cold water is pumped down to the hot rocks. There is is heated and comes back to the surface as steam. The steam is used to drive turbines that turn generators and so electricity is produced.

In a few parts of the world, hot water comes up to the surface naturally and can be used to heat buildings nearby.

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Energy and the environment

Coal, oil, gas and uranium are non-renewable energy resources. The rate they are being used up is much faster than the rate they are produced. Oil and gas will probably run out in the next 50 years, although coal will last much longer.

Renewable energy resources will never run out. They can be produced as fast as they are being used.

Scientists are investigating ways to reduce the environmental impact of using fossil fuels. For example, sulfir may be removed from fuel before burning. Instead of allowing carbon dioxide to be released into the atmospher from power stations, it could be captured and stored in old oil and gas fields.

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Advantages and disadvantages of non-renewable reso

Coal
Advantages: Bigger reserves than the other fossil fuels, is reliable.
Disadvantages: Non-renewable, production of CO2 (a green house gas) and S02 (causing acid rain)

Oil
Advantages: Reliable.
Disadvantages: Non-renewable, produces CO2 and SO2

Gas
Advantages: Reliable
Disadvantages: Non- renewable, produces CO2

Nuclear
Advantages: No production of polluting gases, is reliable.
Disadvantages: Non renewable, produces hazardous nuclear waste, which is difficult to dispose of safely. Small risk of a big nuclear accident.

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Advantages and disadvantages of water and wind ene

Wind
Advantages: Renewable, no production of polluting gases, free energy resource
Disadvantages: Requires many large turbines, unreliable- the wind doesn't always blow

Falling water:
Advantages: Renewable, no production of polluting gases, reliable in wet areas, free energy resourse
Disadvantages: Only works in wet and hilly areas. Damming areas causes flooding and affects the local ecology

Waves:
Advantages: Renewable, no production of polluting gases, free energy resource
Disadvantages: Can be hazardous to boats,unreliable

Tides:
Advantages: Renewable, no production of polluting gases, reliable (always tides twice a day), free energy resource
Disadvantage: Only a few river estuaries are suitable, building a barrage affects local ecology.

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Advantages and disadvantages of solar and geotherm

Solar
Advantages: Renewable, no production of polluting gases, reliable in hot countries in the daytime, free energy resource
Disadvantages: Solar cells onto produce a small amount of electricity, unreliable in less sunny countries.

Geothermal
Advantages: Renewable, no production of polluting gases, free energy resource
Disadvantages: Only economically viable in a very few places, drilling through large depth of rock is difficult and expensive.

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The National Grid

In Britain, electricity is distributed through the National Grid. This is a network of pylons and cables that connects power stations to many buildings. Since the whole country is connected to the system, power stations can be switched in or out of the grid according to demand. The cables are carried long distances across the countryside supported by overhead pylons. In towns and close to homes the cables are buried underground.

The National Grid's voltage is 132,000 V or more. Power stations produce electricity at a voltage of 25,000 V.

In power stations, electricity is generated at a particular voltage. This voltage is increase by a step-up transformer before the electricity is transmitted across the National Grid. This is because transmission at high voltage reduces the energy wasted in the cables, making the system more efficient.

It would be dangerous to supply electricity to consumers at these voltages so at local substations, step-down transformers are used to reduce the voltage to 230V for use in homes.

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Big energy issues

A constant amount of electricity is provided by nuclear, coal-fired and oil-fired power stations. This is called the base load demand.

The demand for electricity varies during the day and between the summer and winter.

This variable demand is met using gas-fired power stations, pumped-storage schemes and renewable energy sources.

When demand is low energy is stored by pumping water to the top resevoir of pumped storage schemes.

Different types of power station have different start-up times. Gas-fired power stations have the shortest start-up times and nuclear power stations have the longest.

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The nature of waves

We use waves to transfer energy and information. The direction of travel of the wave is the direction in which the wave transfers energy. There are different wave types:

  • For a transverse wave the oscillation (vibration) of the particles is perpendicular to the direction in which the wave travels.
  • For a longitudinal wave the oscillation of the particles is parallet to the direction of travel of the wave. A longitudinal wave is made up of compressions and rarefactions
  • Electromagnetic waves e.g. light waves and radio waves, can travel through a vacuum. There are no particles moving in an electromagnet wave, as these waves are oscillations in electric and magnetic fields. The oscillations are perpendicular to the direction of travel of the waves. So all electromagnetic waves are transverse waves.
  • Mechanical waves e.g. waves on springs, and sound waves, travel through a medium (substance). Mechanical waves may be transverse or longitudinal.

Sound waves are longitudinal waves.

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Wave diagram

(http://www.revisionworld.com/sites/revisionworld.com/files/imce/transverse-wave.gif)

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Measuring waves

  • The amplitude of a wave is the height of the wave crest or the depth of the wave trough from the position of rest. The greater the amplitude of a wave, the more energy it carries.
  • The wavelength of a wave is the distance from one crest to the next crest, or from one trough to the next trough
  • The frequency of a wave is the number of wave crests passing a point in one second. The unit of frequency is the hertz (Hz).

The speed of wave is given by the equation: v = f x λ
v is the wave speed in m/s
f is the frequency in Hz
λ is the wavelength in m

The wavelength of a longitudinal wave is the distance from the middle of one compression to the middle of the next compression. This is the same as the middle of one rarefaction to the middle of the next rarefaction. The frequency of a longitudinal wave is the number of compressions passing a point in one second.

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Wave properties: Reflection

(http://4.bp.blogspot.com/-HvCtfsEAEU8/TlOxNqRM1KI/AAAAAAAAAHo/f6wfjFGLhOk/s1600/reflection.jpg)

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Reflection

The image seen in a mirror is due to the reflection of light. The diagram on the previous card shows how an image is formed by a plane (flat) mirror. The incident ray is the ray that goes towards the mirror. The reflected ray is the one coming back.

We draw a line, called the normal, perpendicular to the mirror at the point where the incident ray hits the mirror. The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the reflected ray and the normal. For any reflected ray the angle of incidence equals to the angle of reflection.

The image in a plan mirror is: The same size as the object, upright, the same distance behind the mirror as the object is in front and virtual.

A real image: Can be formed on a screen because the rays of light that produce the image pass through it.

A virtual image: Cannot be formed on a screen because the rays of light that produce the image only appear to pass through it.

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Wave properties: Refraction

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Refraction

Waves change speed when they cross a boundary between different media. The wavelength of the waves also changes but the frequency stays the same.

Refraction is a property of all waves, including light and sound. For example, a light ray refracts when it crosses a boundary between two media such as air and glass, or air and water. The change in speed of the waves causes a change in direction. When light enters a more dense substance, such as going from air to glass, it slows down and the ray changes direction towards the normal. When light enters a less dense substance, such as going from glass to air, it speeds up and the ray changes direction away from the normal.

However, if the wave is travelling along a normal, then it will not change direction.

Different colours of light have different wavelengths, and are refracted by slightly different amounts. When a ray of white light is shone onto a triangular glass prism we can see this because a spectrum is produced. This is called dispersion.

Violet light is refracted the most, red light is refracted the least.

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Diffraction

Diffraction is a property of all waves, including light and sound. It is the spreading of waves when they pass through a gap or round an obstacle.

The effect is most noticeable if the wavelength of the waves is about the same size as the gap or the obstacle.

TV signals are carried by radio waves. People living in hilly areas may not be able to receive a signal because it is blocked by a hill. Radio waves passing the hill will be diffracted round the hill. If they do not diffract enough, the radio and TV signal will be poor.

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Sound

Sound is cause by mechanical vibrations in a substance, and travels as a wave. It can travel through liquids, solids and gases. Sound waves generally travel fastest in solids and slowest in gases.

They cannot travel through a vacuum. This can be tested by listening to a ringing bell in a 'bell jar'. As the air is pumped out of the jar, the ringing sound fades away.

Are longitudinal waves. The direction of the vibrations is the same as the direction in which the wave travels. The range of frequencies that can be heard by the human ear are 20-20,000 Hz. The ability to hear the higher frequencies declines with age.

Sound waves can be reflected to produce echoes: only hard, flat surfaces such as flat walls and floors reflect sound, soft things like carpets, curtains and furniture absorb sounds and an empty room will sound different once carpets, curtains and furniture are put into it.

Sound waves can be refracted. Refraction takes place at the boundaries between layers of air at different temperatures. They can also be diffracted.

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Musical sounds

The pitch of a note depends on the frequency of the sound waves. The higher the frequency on the wave, the higher the pitch will sound.

The loudness of a sound depends on the amplitude of the sound waves. The greater the amplitude the more energy the wave carries and the louder the sound.

Differences in waveform can be shown on an oscilloscope. Tuning forms and signal generators produce 'pure' waveforms. The quality of a note depends on the waveform. Different instruments instruments produce different waveforms, which is why they sound so different from each other.

Vibrations created in an instrument when it is played produce sound waves.

In some instruments (e.g. a saxophone) a column of air vibrates. In others (e.g. a violin) a string vibrates. Some instruments vibrate when they are stuck (e.g. a xylophone)

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The electromagnetic spectrum

(http://butane.chem.uiuc.edu/pshapley/GenChem2/A3/electromagnetic-spectrum.jpg)

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Electromagnetic spectrum

Electromagnetic radiations are electric and magnetic disturbances. They travel as waves and ove energy from place to place. They can travel through a vacuum at the same speed but have different wave lengths and frequencies. Together they are called the electromagnetic spectrum, groupded according to their wavelength and frequency:
-Gamma rays have the shortest wavelength and highest frequency.
-Radio waves have the longest wavelength and lowest frequency

The spectrum is continuous. The frequencies and wavelengths at the boundaries are approximate as the different paths of the spectrum are not precisely defined. Different wavelengths of electromagnetic radiation are reflected, absorbed or transmitted differently by different substances and types of surface.The higher the frequency of an electromagnetic wave, the more energy it transfers.

All electromagnetic waves travel through space at a wave speed of 300 million m/s. We can link the speed of the waves to their wavelength and frequency using:
v = f x λ

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Light, infrared, microwaves and radio waves

Visible light is the part of the electromagnetic spectrum that is detected by our eyes. We see the different wavelengths within it as different colours. The wavelength increases across the spectrum from violet to red. We see a mixture of all the colours as white light Visible light can be used for photography.

Infrared radiation is given out by all objects. The hotter the object, the more it emits. Remote controls for devices such as TVs and CD players use IR.

Microwaves are used in communications. Microwave transmitters produce wavengths that are able to pass through the atmosphere. They are used to send signals to and from satellites and within mobile phone networks.

Radio waves transmit radio and TV programmes and carry mobile phone signals.

Microwave radiation and radio waves penetrate your skin and are absorbed by body tissue. This can heat internal organs and may damage them.

Infrared radiation is absorbed by skin, too much will burn your skin.

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Communications

An alternating voltage applied to an aerial emits radio waves with the same frequency as the alternating voltage. When the waves are received they produce an alternating current in the aerial with the same frequency as the radiation received.

The radio and microwave spectrum is divided into different bands.

The different bands are used for different communications purposes.

The shorter the wavelength of the waves the more information they carry, the shorter their range, the less they spread out.

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Mobile phones and optical fibres

Mobile phones commincate with a local mobile phone mast using wavelengths just on the border between radio waves and microwaves.They are usually referred to as microwaves.

Some scientists believe that the radiation from mobile phones may affect the brain, especially in children.

Optical fibres are very think glass fibres. We csn use them to transmit signals carried by visible light or infrared radiation. The signals travel down the fibre by repeated total internal refraction.

Optical fibres carrying visible light or infrared are useful in communications because they carry much more information and are more secure than radio wave and microwave transmissions.

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The Doppler effect

(http://images.tutorvista.com/cms/images/83/doppler-effect-image.PNG)

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The expanding universe

Galaxies are large collections of stars. Light observed from distant galaxies has been 'shifted' towards the red end of the spectrum. This is knows as red-shift and means that the frequency has decreased and the wavelength has increased (like in the doppler effect when the source moves away from the observer)

A blue-shift would indicate that a galazy is moving towards us. We are able to see these effects by observing dark lines in the spectra from galaxies.

Red-shift: The further away the galaxy, the bigger the red-shift. This suggests that distant galaxies are moving away from us, and the most distant galazies are moving the fastest. This is true of galaxies no matter which direction you look.

All the distant galaxies are moving away from each other, so the whole universe is expanding.

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The Big Bang

Red-shift gives us evidence that the universe is expanding outwards in all directions.

We can imagine going back in time to see where the universe came from. If it is now expanding outwards, this suggests that it started with a massive explosion at a very small initial point. This is known as the Big Bang theory.

If the universe began with a Big Bang, then high energy gamma radiation would have been produced. As the universe expanded, this would have become lower-energy radiation.

Scientist discovered microwaves coming from every direction in space. This is cosmic microwave background radiation (CMBR), the radiation produced by the Big Bang.

The Big Bang theory is so far the only way to explain the existence of CMBR.

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