P1 - Energy for the Home (OCR Gateway Science B)

Heat is a Measure of Energy:

• When a substance is heated, its particles gain kinetic energy (KE). This energy makes the particles in a gas or liquid move around faster. In a solid, the particles vibrate more rapidly. This is what eventually causes solids to melt and liquids to boil.
• This energy is measured on an absolute scale. (This means it can't go lower than zero, because that's a limit to how slow particles can move.) The unit of heat energy is the joule (J).

Temperature is a Measure of Hotness:

• Temperature is a measure of the average kinetic energy of the particles in a substance. The hotter something is, the higher its temperature, and the higher the average KE of its particles.
• Temperature is usually measured in degrees Celsius, nut there are other temperature scales like degrees Fahrenheit. These are not absolute scales as they can go below zero.

Energy tends to flow from hot objects to cooler ones. eg: warm radiators heat the cold air in your room - they'd be no use if heat didn't flow: If there's a DIFFERENCE IN TEMPERATURE between two places, then ENERGY WILL FLOW between them. The greater the difference, the faster the rate of cooling will be. eg: a hot of coffee will cool down quicker in a cool room than a warm room.

Specific Heat Capacity tells you how much energy stuff can store:

• It takes more heat energy to increase the temperature of some materials than others. eg: you need 4200 J to warm 1 kg of water by 1*c but only 139 J to warm 1 kg of mercury by 1*c.
• Materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. They can 'store' a lot of heat.
• The measure of how much energy a substance can store is called its specific heat capacity.
• Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1*c. Water has a specific heat capacity of 4200 J/kg*c.
• The specific heat capacity of water is high. Once water's heated, it stores a lot of energu, which makes good for central heating systems. Also, water's a liquid so it can easily be pumped around a building.
• You'll have to do calculations involving specific heat capacity. This is the equation to learn: Energy = Mass x Specific Heat Capacity x Temperature Change
  
• example: How much energy is needed to heat 2 kg of water from 10*c to 100*c? Energy needed = 2 x 4200 x 90 = 756000 J

You need to put in energy to break intermolecular bonds:

• When you heat a liquid, the heat energy makes the particles move faster. Eventually, when enough of the particles have enough energ to overcome their attraction to each other, big bubbles of gas form in the liquid - this is boiling.
• It's similar when you heat a solid. Heat energy makes the particles vibrate faster until eventually the forces between them are overcome and the particles start to move around - this is melting.
• When a substancec is melting or boiling, you're still putting in energy, but the energy's used for breaking intermolecular bonds rather than raising the temperature - there are flat spots on the heating graph.
• When a substance is condensing or freezing, bonds are forming between particles, which releases energy. This means the temperature doesn't go down until all the substance has turned into a liquid (condensing)

Specific Latent Heat is the energy needed to change a state:

• The specific latent heat of melting is the amount of energy needed to melt 1 kg of material without changing its temperature (eg: the material's got to be at its melting temp already)

• The specific latent heat of boiling is the energy needed to boil 1kg of material without changing its temperature (eg: the material has to be at its boiling temperature already).
• Specific latent heat is different for different materials, and it's different for boiling and melting.
• The formula for this is: Energy = Mass x Specific Latent Heat
• Example: The specific latent heat of water (for melting) is 334000 J/kg. How much energy is needed to melt an ice cube of mass at 7g at 0*c? Energy = 0.007 x 334000 J = 2338 J/kg

Conduction occurs mainly in solids:

Houses lose a lot of heat through their windows even when they're shut. Heat flows from the warm inside face of the window to the cold outside face mostly by conduction.

• In a solid, the particles are held tightly together. So wen one particle vibrates, it bumps into the other particles nearby and quickly passes the vibrations on.
• Particles which vibrate faster than others pass on their extra kinetic energy to neighbouring particles. The particles then vibrate faster themselves.
• This process continues throughout the solid. This causes a rise in temperature the other side.
• CONDUCTION OF HEAT is the process where vibrating particles pass on extra kinetic energy to neighbouring particles.
• Metals conduct heat really well because some of their electrons are free to move inside the metal. Heating makes the electrons move faster, and collide eith other free electrons, transferring energy. These then pass on their extra energy to other electrons, etc. Because the electrons move more feely, this is a much faster way of transferring energy than slowly passing it between jostling neighbour atoms.
• Most non-metals don't have free electrons, so warm up more slowly, making them good for insulating things - that's why metals are used for saucepans but non-metals are used for saucepan handles.

• Liquids and gases conduct heat more slowly than solids - the particles aren't held so tightly together which prevents them bumping into each other so often, so air is a good insulator.

Convection occurs in liquids and gases:

• When you heat up a liquid or gase, the particles move faster, and the fluid (liquid or gas) expands, becoming less dense.
• The warmer, less dense fluid rises above its colder, denser surroundings, like a hot air balloon does.
• As the warm fluid rises, cooler fluid takes its place. As this process continues, you actually end up with a circulation of fluid (convection currents). This is how immersion heaters work.
• CONVECTION occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them.
• Radiators in the home rely on convection to make the warm air circulate the room.
• Convection can't happen in solids befcause the particles can't move - they just vibrate on the spot.
• To reduce convection, you need to stop the fluid moving. Clothes, blankets and cavity wall foam insulation all work by trapping pockets of air. The air can't move so the heat has to conduct very slowly through the pockets of air, as well as the material in between.

Radiation is how we get heat from the sun:

As well as by conduction and convection, heat can be transferred by radiations. Heat is radiated as infared waves - these are electromagnetic waves that travel in straight lines at the speed of light. Radiation is different from conduction and convection in several ways:

• It doesn't need a medium (material) to travel through, so it can occur in a vacuum, like space. This is the only way that heat reaches us from the sun.
• It can only occur through transparent substances like air, glass and water.
• The amount of radiation emitted or absorbed by an object depends to a large extent on its surface colour and texture. This definitely isn't true for conduction and convection.

All objects emit and absorb heat radiation:

• All objects are continually emitting and asorbing heat readiation.
• The hotter an object gets, the more heat radiation it emits.
  
• Cooler objects will absorb the heat radiation emitted by hotter things, so their temperature increases. You can feel heat radiation, for example if you're indoors and the Sun shines on you through a window.

• Matt black surfaces are very good absorbers and emitters of radiation. You should really paint your radiators black to help emit radiation, but leave your fridge a nice shiny white to help reflect it.
• Light-coloured, smooth and shiny objects are very poor absorbers and emitters of radiation. They effectively reflect heat radiation - eg: some people put shiny foil behind their radiators to reflect radiation back into the room rather than heat up the walls.

Heat radiation is important in cooking:

• Grills and toasters heat food by infared (heat) radiation. the heat radiated by a grill is absorbed by the surface particles of the food, increasing their kinetic energy. The heat energy is then conducted or convected to more central parts.
• People often line their grill pan with shiny foil. This reflects the heat radiation back onto the bottom of the food being grilled, so the food is cooked more evenly, as well as stopping the grill pan getting dirty.
• Microwave ovens also use radiation to cook food - microwaves are electromagnetic waves that have a diferent wavelength to infared.
• Microwaves penetrate about 1cm into the outer layer of food where they're absorbed by water or fat molecules, increasing their kinetic energy. The energy is then conducted or convected to other parts 

• You don't cover food with foil in a microwave oven though - the microwaves will be reflecter away so they won't cook the food, AND it can cause dangerous sparks inside the over. It's okay to cover food with glass or plastic though as microwaves can pass right through.

Insulating your house saves energy and money:

• Energy in the home is emitted and transferred (or wasted) in different areas.
• Things that emit enery are called sources, eg: radiators. Things that transfer and waster or lose energy are called sinks, eg: windows and computers.
• To save energy, you can insulate your house so the sinks 'drain' less energy, eg: use curtains to reduce energy loss. You can also make sources and sinks more efficient, so they waste less energy, eg: use an energy-saving light bulbs instead of normal ones.
• It costs money to buy and install insulation, or buy more efficient appliances, but it also saves you money, because your energy bills are lower.
• Eventually, the money you've saved on energy bills will equal the initial cost - the time this takes is called the payback time.
• If you subtract the annual saving from the inital cost repeatedly then eventually the one with the biggest annual saving must always come out as the winner, if you think about it.
• But you might sell the house (or die) before that happens. If you look at it over, say, a five-year period then a cheap oand chearful hot water tank jacket wins over expensive double glazing.

Loft Insulation:

• Fibreglass 'wool' laid on the loft floor and ceiling rduces energy loss from the house by conduction and convection.
• Initial cost: £200
• Annual saving: £100
• Payback time: 2 years

Hot water tank jacket:

• Reduces conduction.
• Initial cost: £60
• Annual saving: £15
• Payback time: 4 years

Thick curtains:

• Reduce heat loss by convection and conduction through the windows.
• Initial cost: £180
• Annual saving: £20
• Payback time: 9 years

Double Glazing:

• Two layers of glass with an air gap between reduce conduction
• Initial cost: £2400
• Annual saving: £80
• Payback time: 30 years

Draught-proofing:

• Strips of foam and plastic around doors and windows stop hot air going out - reducing convection
• Initial cost: £100
• Annual saving: £15
• Payback time: 7 years

Cavity walls and insulation:

• Two layers of bricks with a gap between them reduce conduction but you still get some energy lost by convection. Squirting insulating foam into the gap traps pockets of air to minimise this convection.
• Initial cost: £150 
• Annual saving: £100 
• Payback time: 18 months
• (Heat is still lost through the walls by radiation, though. Also if there are any spaces where air is not trapped there'll be some convection too.)

Thermograms show where your house is leaking heat:

• A thermogram is a picture taken with a thermal imaging camera.
• Objects at different temperatures emit infrared rays of different wavelengths. The thermogram displays these temperatures as different colours. The hotter parts show up as white, yellow and red, whilst the colder parts are black, dark blue and purple. If a house looks 'hot', it's losing heat to the outside.

Machines always waste some energy:

• Useful machines are only useful because they convert energy from one form to another. Take cars for instance - you put in chemical energy (petrol or disel) and the engine converts it into kinetic (movement) energy.
• The total energy output is always the same as the energy input, but only some of the output energy is useful. So for every joule of chemical energy you put into your car you'll only get a fraction of it converted into useful kinetic energy.
• This is because some of the input energy is always lost or wasted, often as heat. In the car example, the rest of the chemical energy is converted (mostly) into heat and sound energy. This is wasted energy.
• The less energy that is wasted, the more efficient the device is said to be.

More efficient machines waste less energy:

The efficiency of a machine is defined as: 

Efficiency = USEFUL Energy Output/ TOTAL Energy Input (x100%)

• To work out the efficiency of a machine, first find out the total energy INPUT. This is the energy supplied to the machine.
• Then find how much useful energy the machine delivers - the useful energy OUTPUT. The question might tell you this directly, or it might tell you how much energy is wasted as heat/sound.
• Then just divide the smaller number by the bigger one to get a value for efficiency somewhere between 0 and 1. If it's bigger, the division is upside down.
• You can convert the efficiency to a percentage, by multiplying it by 100.
• In the exam you might be told the efficiency and asked to work out the total energy input, the useful energy output or the energy wasted so you would need to rearrange the formula.

The thickness of the arrow represents the amount of energy:

• Sankey diagrams are just energy transformation diagrams - they make it easy to see at a glance how much of the input energy is being usefully employed compared with how much is being wasted.
• The thicker the arrow, the more energy it represents - so you see a big thick arrow going in, then several smaller arrows goig off it to show the different energy transformations taking place.
• You can have either a little sketch or a properly detailed diagram where the width of each arrow is proportional to the number of joules it represents.

Waves have amplitude, wavelength and frequency:

Waves have certain features:

• The amplitude is the displacement from the rest of the position to the crest. (NOT from a trough to a crest)
• The wavelength is the length of a full cycle of the wave, eg: from crest to crest.
• Frequency is the number of complete cycles or oscillations passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is 1 wave per second.

Wave Speed (m/s) = Frequency (Hz) x Wavelength (m)

You may need to convert your units:

• The standard (SI) units involved in wave equations are: metres, seconds, m/s and hertz (Hz). Always convert into SI units (m, s, m/s, Herz) before you work anything out.
• 1kHz (kilohertz) = 1000Hz,                        1MHz (1 megahertz) = 1000000Hz
• Wavelengths can also be given in other units, eg: km for long-wave radio.

All waves can be reflected, refracted and diffracted:

• Waves travel in a straight line through whatever substance they're travelling in.
• When waves arrive at an obstacle (or meet a new material), their direction of travvel can be changed.
• This can happen by reflection or by refraction or diffraction.

Reflection of light lets us see things:

• Reflection of light is what allows us to see objects. Light bounces off them into our eyes.
• When a beam of light reflects from an uneven surface such as a piece of paper, the light reflects off at different angles.
• When it reflects from an even surface (smooth and shiny like a plane mirror) then it's all reflected at the same angle and you get a clear reflection.
• The LAW OF REFLECTION applies to every reflected ray:  

Angle of INCIDENCE = Angle of REFLECTION

(Note: these two angles are ALWAYS defined between the ray itself and the NORMAL, dotted below. Don't label them as the angle between the ray and the surface.)

Total internal reflection depends on the critical angle:

• A wave hitting a surface can experience total internal reflection. This can only happen when the light travels through a dense material like glass, water or Perspex towards a less dense substance like air.
• If the angle of incidence is big enough, the ray doesn't come out at all but reflects back into the material.
• Big enought means bigger than the critical angle for that particlar material - every material has its own, different critical angle.

• If the angle of incidence is LESS than the critical angle - Most of the light is refracted into the outer layer, but some of it is internally reflected.
• If the angle of incidence is EQUAL to the critical angle - The ray would go along the surface (with quite a bit of internal reflection).
• If the angle of incidence is GREATER than the critical angle - No light comes out. It's all internally reflected, ie: total internal reflection.

Diffraction - waves spreading out:

• All waves spread out ('diffract') at the edges when they pass through a gap or pass an object.
• The amount of diffraction depends on the size of the gap relative to the wavelength of the wave. The narrower the gap, or the longer the wavelength, the more the wave spreads out.
• A narrower gap is one about the same size as the wavelength of the wave. So whether a gap counts as narrow or not depends on the wave.
• Light has a very small wavelength (about 0.0005mm), so it can be diffracted but it needs a really small gap.
• This means you can hear someone through an open door even if you can't see them, because the size of the gap and the wavelength of sound are roughly equal, causng the sound wave to diffract and fill the room...
• ...but you can't see them unless you're directly facing the door because the gap is about a million times bigger than the wavelength of light, so it won't diffract enough.
• If a gap is about the same size as the wavelength of light, you can still get a diffraction pattern of light and dark frtion around the fringes, as shown here.
• You can get diffraction around the edges of obstacles too. The shadow is where the wave is blocked. The wider the obstacle compared to the wavelength, the less diffraction it causes so the longer the shadow.

Refraction - changing the speed of a wave can change its direction:

• Waves travel at different speeds in substances which have different densities. So when a wave crosses a boundary between two substances (from glass to air for example) it changes speed.
• If a light hights the boundary 'face on', it slows down but carries on in the same direction. It now has a shorter wavelength but the same frequency.
• But if a wave meets a different medium at an angle, part of the wave hits the denser layer first and slows down while another part carries on at the first, faster speed for a while. So the wave changes direction - it's been REFRACTED.
• eg: when light passes from air into the glass of a window pane (a denser medium), it slows down - causing the light to refract towards the normal. When the light reaches the 'glass to air' boundary on the other side of the window, it speeds up and refracts away from the normal.
• Waves are only refracted if they meet a new medium at an angle. If they're travelling along the normal (ie: the angle of incidence is zero) they will change speed, but are NOT refracted - they don't change direction.

There are seven types of electromagnetic (EM) waves:

• Electromagnetic radiation can occur at many different wavelengths.
• In fact, there is a continuous spectrum of different wavelengths, but waves with similar wavelengths tend to have similar properties.
• Electromagnetic radiation is conventionally split into seven types of waves.
• All forms of electromagnetic radiation travel at the same speed through a vaccuum. This means that waves with a shorter wavelength have a higher frequency.
• About half the EM radiation we receive is from the Sun in visable light. Most of the rest is infared (heat) with some UV (ultraviolet) thrown in. UV is what gives us a suntan.

Properties of EM waves depend on frequency and wavelength:

• As the frequency and wavelength of EM radiation changes, its interaction with matter changes - ie: the way a wave is absorbed, reflected or transmitted by any given substance changes.
• As a rule, the EM waves at each end of the spectrum tend to be able to pass through material, whilst those nearer the middle are absorbed.
• Also, the ones with higher frequency (shorter wavelength) like X-rays, tend to be more dangerous to living cells because they have more energy.

• When any EM radiation is absorbed it can cause heating and ionisation (if the frequency is high enough). Ionisation is where an atom or molecule either loses or gains electrons and it can be dangerous.

Different sorts of signals have different advantages:

As well as cooking our food and keeping us warm, EM waves are used for communication. eg: radio waves are used for radio, microwavse for mobile phone. Before you communication information through, though, it's changed into an electrical signal, which is then sent off on its own (like you get in an ordinary phone line) or carried on EM wave.

The different types of signals habve advantages and disadvantages:

• Using light, radio and electrical signals is great because the signals travel really fast.
• Electrical wires and optical fibres can carry loads of information very quickly.
• Information sent through optical fibres and electrical wires is pretty secure - they're inside a cable and so can't be easily tapped into. Radio signals travel through air, so they can be intercepted more easily. This is an issure for people using wireless internet networks.

• However - cables can be difficult to repair if they get broken, which isn't a problem for wireless methods.
• Wireless communication also has the advantage that is portable (eg: mobile phones, laptop wifi etc.). It does rely on an aerial to pick up a signal though, and signal strength often depends on location.

• However - cables can be difficult to repair if they get broken, which isn't a problem for wireless methods.
• Wireless communication also has the advantage that is portable (eg: mobile phones, laptop wifi etc.). It does rely on an aerial to pick up a signal though, and signal strength often depends on location.

Communicating with light can require a code:

• Historically, light was used to speed up communication over long distances.
• By creatinga code of 'on-off' signals, a message could be relayed between stations far away by flashing a light on and off in a way that could be decoded.
• This is the principle behind the Morse code.
• Each letter of the alphabet (and each number 0-9) is represented by a sequence of 'dots' and 'dashes' - which are pulses of light (or sound) that last for a certain length of time.
• The Morse code is a type of digital signal because the light pulse is either 'on' or 'off'.

Light signals can travel throgh optical fibres:

• A more modern use of light for communication is the use of optical fibres, which can carry data over long distances as pulses of light or infared radiation.
• They work by bouncing waves off the sides of a very narrow core which is protected by outer layers.
• The ray of light enters the fibre so that it hits the boundary between the core and the outer cladding at an angle greater than the critical angle for the material. This causes total internal reflection of the ray within the core.

• The pulse of light enters at one end and is reflected again and again until it emerges at the other end.
• Optical fibres are increasingly being used for telephone and broadband internet cables, replacing the old electrical ones. They're also used for medical purposes - to 'see inside' the body without having to operate. 

Using light has a lot of advantages:

• Using light is a very quick way to communicate. In a vacuum, light travels at 300000000m/s - it can't travel that fast through optical fibres (it's slowed down by about 30%) but it's still pretty quick.
• Multiplexing means that lots of different signals can be transmitted down a single optical fibre at the same time, so you don't need as many cables.
• As it's a 'digitla' signal, there's little interference.

Long wavelengths travel well through Earth's atmosphere:

• Radio waves and microwaves are good at transferring information over long distances.
• This is because they don't get absorbed by the Earth's atmosphere as much as waves in the middle of the EM spectrum (like heat for example), or those at the high-frequency and the end of the spectrum (eg: gamma rays or X-rays).

Radio waves are used mainly for communications:

• Radio waves are EM radiation with wavelengths longer than about 10cm.
• Different wavelengths of radio waves refract and diffract in different ways.
• Long-wave radio (wavelengths of 1-10km) can be transmitted from one place and received halfway round the world because they diffract (bend) around the curved surface of the Earth. 
• Radio waves are used for TV and FM radio transmissions have very short wavelengths (10cm - 10m). To get reception, you must be in direct sight of transmitter - the signal doesn't bend around hills or travel far through buildings.
• Short-wave radio signals with wavelengths ofabout 10m - 100m can be received at long distances from the transmitter because of reflection in the ionosphere. Medium-wave signals (the shorter ones) can also reflect from the ionsphere, depending on atmospheric conditions and time of day.

Diffraction makes a difference to signal strength:

• Diffraction is when waves spread out at the edges when they pass through a gap or past an object.
• The amount of diffraction depends on the wavellength or the wave, relative to the size of the gap or obstacle.
• Longer wavelengths can encounter a lot of diffraction because they are large compared with the gap or obstacle
• This means that they are able to bend around corners and any obstacles - such as hills, tall buildings etc.
• So longer wavelength radio waves can travel long distances between the transmitter and reciever without them having to be in the line of sight of each other. Shorter wavelength radio waves and microwaves don't diffract very much, so the transmitters need to be located high up to avoid obstances (and even then they can only cover short distances).
• Some areas have trouble receiving shorter wavelength radio (and microwave) signals - eg: if you live at the foot of a moutain you will probably have short signal strength.
• Diffraction can also occur at the edges of the dishes used to transmit signals. This results in signal loss - the wave is more spread out so the signal is weaker.

Refraction can help radio waves travel further:

When a wave comes up against something that has a different density, it changes speed. If the wave hits the new substance at an angle, it changes direction. This is refraction. When it hapens high up in the atmosphere, it can help waves travel further for long distance communication.

• UV radiation from the Sun creates layers of ionised atoms (atoms that have either gained or lost electrons) in the Earth's atmosphere. These electrically charged layers are called the ionosphere.
• Radio waves travel faster through ionised parts of the atmosphere than non-ionised parts. This causes refraction.
• Short-wave (with wavelengths of about 10m - 100) and medium-wave (about 300m) radio signals are refracted most in the ionosphere - they are effectively bounced back or reflected back to Earth. This means that short and medoum wave radio signals can be received a long way from the transmitter.
• The amount a wave is refracted in the ionosphere depends on its frequency and angle of elevation.  High frequency / short wavelength signals such as short-wave radio don't refract as much as medium-wave.

• Radio waves 'bounce' off the ionosphere in a similar way to how light waves totally internally reflect inside optical fibres.
• Refraction's not always good though. It can disrupt a signal by bending it away from the receiver dish.

Digital radio helps reduce interference:

• There's a limited number of radio wave requencies that can be used to transmit a good analogue signal - so radio stations often broadcast using waves of very similar frequencies.
• These analogue signals often suffer from interference because of this - similar waves covering a smiliar area can combine, which causes 'noise'. This is why radio stations near to each other use different frequencies - so they don't interfere as much.
• Digital Audio Broadcasting (DAB) works in a different way to traditional radio broadcasts - it's digital to start with.
• With DAB, many different signals are compressed, then transmitted as a single wave - this is known as multiplexing.
• They are transmitted across a relatively small frequency bandwidth and seperated out by the receivers at the other end. You need a DAB radio set to pick u and decode the signals.

• DAB suffers less interference than traditional radio broadcasts, and since the signals from many stations can be broadcast at the same frequency (multiplexing), it means an increase in the potential number of radio stations available.
• At the moment there are a limited number of DAB transmitters in the UK (and the world) so some areas can't receive digital radio signals at all.
• Even if you can receive DAB, the sound quality is often not as good as a traditional FM radio broadcast, due to the compression of the signal.

Microwaves are used for satellite communication...:

• Communication to and from satellites (including satellite TV signals and satallite phones) uses microwaves. But you need to use wavelengths which can pass easily through the Earth's watery atmosphere without too much absorption.
• For satellite TV and phones, the signal from a transmitter is transmitted into space, where it's picked up by the satellite's receiver dish orbitting thousands of kilometres above the Earth. The satellite rtransmits the signal back to Earth in a different direction, where it's received by a satellite dish on the ground.
• Microwaves are also used by remote-sensing satellites - to see through the clouds and monitor oil spills, track the movement of icebergs, see how much rainforest has been chopped down etc.

...as well as mobile phones:

• Mobile phone calls travel as microwaves from your phone to the nearest transmitter (or mast). The transmitters pass signals between each other, then back to your mobile phone.
• Microwaves have a shorter wavelength than radio waves, so they don't diffract much. This means they're affected by the curvature of the Earth because they don't bend round it like long-wave radio waves. It also means they're blocked more by large obstacles like hills because they can't bend around them.

• This means that microwave transmitters need to be positioned in line of sight - they're usually high up on hilltops so they can 'see eachother' and they're positioned fairly close together. If ther's a hill or a man-made obstacle between your phone and the transmitter you'll probably get no signal or poor signal.
• The microwave frequencies used are partially absorbed by water, even though they can pass through the atmosphere. So in adverse weather (or if theres a lake nearby) there can be some signal loss due to absorption or scattering.
• Sometimes there's interference between signals, which can also affect signal strength.

Mobile masts may be dangerous - but there's conflicting evidence:

• Microwaves used for communications need to pass through the Earth's wavery atmosphere, but the microwaves used in microwave ovens have a different wavelength - they're actually absorbed by the water molecules in the food.
• t's the absorption that's harmful - if microwaves are absorbed by water molecules in living tissue, cells may be burned or killed.
• Some people think that the microwaves emitted into your body from using a mobile phone or living near a mobile phone mast could damage your health.

• There's no conclusive proof either way yet though. Lots of studies have been published, which has allowed the results to be checked, but so far they have given conflicting evidence.
• Any potential dangers would be increased by prolonged exposure though - eg: living close to a mast or using your phone all the time.
• This means we have to carefully balance the potential risks and the benefits of this technology until we know more - in terms of where we locate masts and how much we choose to use our mobile phones.

The size of receiver depends on the size of wave:

• We use different receivers (sensors) to pick up the different types of EM waves used for communication - eg: television, satellite dishes, microscopes etc.
• The minimum size of receiver needed is linked to the size of the wavelength of the wave - the longer the wavelength, the larger the receiver should be.
• So radio waves need the biggest receivers, the microwaves then infared, then light waves.
• This is because of diffraction - when a wave enters a receiver it passes through a gap. If the wave is diffracted, it spreads out and you lose detail.
• As you've already seen, the amount of diffraction is affected by the size of the gap compared to the wavelength - gaps about the same size as the wagvelength cause lots of diffraction, but as the gap size increases there is less diffraction.
• So the bigger the receiver compared with the wavelength of wave being received, the less diffraction it causes, so the clearer the information received is.

Telescopes detect different types of EM waves:

• Telescopes help you to see distant objects clearer - eg: astronomers look for very distant stars and galaxies using them.

• Different telescopes are used to collect different EM waves - eg: optical telescopes receive visable light, radio telescopes collect radio waves etc.
• Bigger telescopes give us better resolution because they cause little diffraction.
• Telescopes with small gaps compared to the wavelength they're looking for have limited resolving power - they're said to be diffraction-limited.
• Light waves have a relatively small wavelength compared to radio waves.
• Since radio waves can be more than 10000000000 times bigger than light waves, this would mean having a ridiculoudly big receiver than light waves, this would mean having a ridiculously big receiver (about the size of UK for a decent one). So we just make do with a lower resolution (although the dishes are huge).
• To get round this, radio telescopes are often linked together and their signals combined to get more detailed information - acting like a single giant receiver.
• A bigger receiver can also collect more EM waves, giving a more intense image - so a bigger telescope can observe fainter objects.

Optiical microscopes are diffraction limited:

• Optical microscopes have to be small, because you usually use them to look at small samples of tiny things in the lab - you want to collect light coming from small areas only.
• Their small size makes it hard to get a good resolution - the gap needs to be really small, so you still get some diffraction even though light has a small wavelength.

Information is converted into signals:

• To communicate any kind of information (eg: sounds, pictures), it needs to be converted into electric signals before it's transmitted.
• These signals can then be sent long distances down telephone wires or carried on EM waves
• The signals can either be analogue or digital.

Analogue signals vary but digital's just on or off:

• An analogue signal can take any value within a certain range. The amplitude and frequency of an analogue wave vary continuously.
• A digital signal can only take two values. These values tend to be on/off or 1/0. For example, you can send data along optical fibres as short pulses of light.

Digital signals have advantages over analogue:

• Digital and analogue signals weaken as they travel, so they might need to be amplified along their route.
• They also pick up interference or noise from electrical disturbances or other signals.

• When you amplify an analogue signal, the noise is amplified too - so every time it's amplified, the sign loses quality. The noise is easier to remove or ignore with digital, so the signal remains high quality.
• Another advantage of digital technology is that you can transmit several signals at the same time using just one cable or EM wave - this is called multiplexing.
• Multiplexing happens in phone wires. When you're on the phone, your voice is converted into a digital signal and transmitted regularly at very small time intervals. In betweenn your voice signals being transmitted, thousands of other people's voice signals can be slotted in or 'multiplexed'. The samples are seperated out again at the other end so the person you called can here you and only you. This happens so quickly you don't notice it.
• The advantages of digital signals over analogue have played a big part in the 'switching over' from analogue to digital TV and radio broadcasts.

Ultraviolet radiation causes skin cancer:

• If you spend a lot of time in the sun, you can expect to get a tan and maybe sunburn.
• But the more time you spend in the sun, the more chance you also have of getting skin cancer. This is because the Sun's rays include ultaviolet radiation (UV) which damages the DNA in your cells.
• UV radiation can also cause you eye problems, such as cataracts, as well as premature skin aging. 
• Darker skin gives some protection against UV rays - it absorbs more UV radiation. This prevents some of the damaging radiation from reaching the more vulnerable tissues deeper in the body.
• Everyone should protect themselves from the Sun, but if you're pale skinned, you need to take extra care, and use a sunscreen with a higher Sun Protection Factor (SPF).
• A SPF of 15 means you can spend 15 times as long as you otherwise could in the sun without burning (as long as you keep reapplying the sunscreen)
• We're kept informed of the risks of exposure to UV - research into its damaging effects is made public through the media and advertising campaigns, and the government tells people how to keep safe to improve public health.

The ozone layer protects us from UV radiation:

• Ozone is a molecule made of three oxygen atoms (O3). There's a layer of ozone high up in the Earth's atmosphere.
• The ozone layer absorbs some of the UV rays from the Sun - so it reduces the amount of UV radiation reaching the Earth's surface.
• Recently, the ozone layer has got thinner because of pollution from CFCs - these are gases which react with ozone molecules and break them up. This depletion of the ozone layer allows more UV rays to reach us at the surface of the Earth.

There's a hole in the ozone layer over Antarctica:

• In winter, special weather effects cause the concentration of ozone over Antarctica to drop dramatically. It increases again in spring, but the winter concentration has been dropping. The low concentration looks like a 'hole' on satellite images.
• Scientists now monitor the ozone concentration very closely to get a better understanding of why it's decreasing, and how to prevent further depletion.
• Many different studies have been carried out internationally, using different equipment, to get accurate results - this helps scientists to be confident that their hypotheses and predictions are correct.

• Studies led scientists to confirm that CFCs were causing the depletion of the ozone layer, so the internation community banned them. We used to use CFCs all the time - eg: in hairsprays and in the coolant for fridges - but now international bans and restrictions on CFC use have been put in place because of their enviromental impact.

Earthquakes cause different types of seismic waves:

• When there's an earthquake somewhere, it produces shock waves which travel out through the Earth. We record these seismic waves all over the surface of the planet using seismographs.
• Seismologist measure the time it takes for the shock waves to reach each seismograph.
• They also note which parts of the Earth don't receive the shock waves at all.
• There are two different types of seismic waves that travel through the earth - P-waves and S-waves.

P-waves are longitudinal:

• The vibrations are along the direction that the wave travels.
• Vibrations <->, wave travelling this way ->
• P-waves travel through solids and liquids. They travel faster than S-waves.
• P-waves refract as density changes.

S-waves are transverse:

• The virbrations are at right angles to the direction that the wave travels.
• Vibrations this way |, wave travelling this way ->
• S-waves only travel through solids and they are slower than P-waves.
• S-waves can't pass through the liquid outercore.

The seismograph results tell us what's down there:

• About halfway through the Earth, p-waves change direction abruptly. This indicates that there's a sudden change in properties - as you go from the mantle to the core.
• The fact that S-waves are not detected in the core's shadow tells us that the outer core is liquid - s-waves only pass through solids.
• P-waves seem to travel slightly faster through the middle of the core, which strongly suggests that there's a solid inner core.
• (S-wave do travel through the mantle which shows that it's sloid. It only melts to form magma is small 'hot stops'

S-waves are transverse:

• The virbrations are at right angles to the direction that the wave travels.
• Vibrations this way |, wave travelling this way ->
• S-waves only travel through solids and they are slower than P-waves.
• S-waves can't pass through the liquid outercore.

The seismograph results tell us what's down there:

• About halfway through the Earth, p-waves change direction abruptly. This indicates that there's a sudden change in properties - as you go from the mantle to the core.
• The fact that S-waves are not detected in the core's shadow tells us that the outer core is liquid - s-waves only pass through solids.
• P-waves seem to travel slightly faster through the middle of the core, which strongly suggests that there's a solid inner core.
• (S-wave do travel through the mantle which shows that it's sloid. It only melts to form magma is small 'hot stops'

The waves curve with increasing depth:

• The waveds change speed as the properties of the mantle and core change.
• This change in speed causes the waves to change direction - which is the refraction.
• Most of the time the waves change speed gradually, resulting in a curved path.
• Bur when the properties change suddenly, the wave speed changed abruptly, and the path has a kink.