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  • Created by: allie_99
  • Created on: 22-04-15 16:17

Moving and Storing Heat

  • Heat is a measure of energy
    • when a substance is heated, the particles gain kinetic energy making them move faster.
    • measured on an absolute scale (cannot go lower then 0) and in joules (J)
  • temperature is a measure of hotness
    • average kinetic energy of the substances particles
    • measured in ºC but there are others (ºF). these are not absolute
      • a thermogram can be used to show how hot an object is. different temperatures are shown by different colours.
      • hottest colour-  white, red,yellow, green, light blue, dark blue - Coldest colou
  • energy flows from hot to cool objects - if there is a DIFFERENCE IN TEMPERATURE between the two places, then ENERGY WILL FLOW between them
  • specific heat capcity tells you how much something can store
    • energy = mass x specific heat capacity x temperature change
    • water has a specific heat capacity of 4200 J/Kg?ºC which is high
      • needs a lot of energy to raise its temperature as of high SHC
      • so it can store large amounts of thermal energy.
      • so does not need to be pumped quickly around the central heating circuit
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Melting and Boiling

  • when particles have enough energy they overcome the attraction to each other
  • when a substance is melting or boiling you still put in energy but the energy is used for breaking the intermolecular bonds rather then raising the temperature.
    • when condensing or freezing, bonds form which releases energy
  • specific latent heat is the energy needed to change state
    • energy = specific latent heat x mass
    • J = J/Kg x Kg
  • specific latent heat of fusion (melting) is not the same value as the specific latent heat of vapourisation (boiling/evapourating)

specific latent heat is different for different materials, and its different for boiling and meting

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Conduction

process where vibrating particles pass on extra kinetic energy to neighbouring particles

  • when particles are heated, atoms become more energised and vibration increases
  • vibration is spread through material - heating it up
    • metals are good conductors as their electrons are free to move. e.g. saucepan
    • non-metals have no free electrons so warm up slowly (good insulators) e.g. saucepan handle
    • heating makes electrons move faster & collide more frequently with other electrons
    • liquids and gasses heat slower than solids - particles arent held tightly preventing them from bumping each other so air is good insulator
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Convection

when more energetic particles move from hotter region to cooler region, taking their energy with them

  • when heated particles expand and become less dense
  • warmer less dense fluid rises above cooler fluid into cooler region
  • the now denser cold fluid falls into warm areas
  • end up with a circulation of fluid (convection currents)
    • radiators use convection to circulate warm air
    • cant happen in olids as particles cannot move (vibrate on spot)
    • to reduce convection you need to stop fluid moving
      • clothes/blankets/cavity wall foam insulation trap pockets of air so the air cannot move so heat is conducted slowly through air pockets and material between

(http://ts4.mm.bing.net/th?id=JN.akHER8ycHdu%2bOknhnK32UA&pid=1.1)

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Radiation

different from conduction as doesnt need a material to travel through; only happens in transparent substances; amound of radiation emmitted or absorbed depends on the surface colour of the object

  • dull/rough/dark surface- poor reflector - good absorber
    • painting radiators black help emit heat radiation
    • black solar panels for water heating to absorb heat
  • shiny/light surface - good reflector - poor absorber
    • painting fridge white help it reflect heat radiation
  • Grills/toasters - infrared radiation to heat food - heat radiated by grill is absorbed by surface particles of the food - then conducted/convected to other parts of food
    • line pan with silver foil reflecting heat back into bottom of food so more evenly cooked
  • microwaves use radiation to cook food - penetrace 1cm into outer layer where they're absorbed by water or fat molecules increasing their kinetic energy - then conducted/convected to other parts
    • dont cover food as it would be reflected away and cause dangerous sparks (ok for glass/plastic)
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Insulating Homes

paybacktime - when the money saved on energy bills equals the initial cost

    • payback time = initial cost/ annual saving
  • loft insulation - fibreglass laid accross loft floor to reduce conduction through ceiling into roof
  • hot water tank jacket - reduced conduction
  • double glazing - two layers of glass with an air gap reduces conduction
  • thick curtains - reduce conduction and radiation through the windows
  • draught-proofing - strips of foam and plastic around door and windows stop hot air going out reducting convection
  • cavity walls and insulation - two layers of brick with a gap between them reduce conduction but yu still get some energy lost by convection.
    • insulating foam into gaps traps pockets of air to minimise this convection
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Sankey Diagrams

  • some input energy is always lost or wasted (often as heat)
  • the less energy wasted, the more efficient the device is
  • efficiency = useful energy output / total energy input (x100%)
    • low energy light bulbs are roughly 4 times more efficient and last about 8 times longer - but theyre expensive
  • the thickness of the arrow on a sankey diagram represents amount of energy (on sankey diagram)

total electrical energy is 100 j, 90 j is transferred as heat energy and 10 j transferred as light energy (http://www.bbc.co.uk/staticarchive/ef1765b78bf7df43639092d398d58b646138287b.gif)

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Introduction to Waves

  • wave speed (m/s)= frequency (Hz)x wavelength(m)
  • It is common for kilohertz (kHz) 1000Hz, megahertz (MHz) 1000000 Hz and gigahertz (GHz) to be used when waves have very high frequencies

(http://www.gcsescience.com/Transverse-Wave.gif)

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Wave Properties

  • all waves can be reflected, refracted and diffacted when waves arrive at an obstical and their direction of travel changes
  • angle of incidence = angle of reflection
  • reflecting at uneven surface= reflect at different angle
  • reflecting from even surface = same angle reflection
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Diffraction

  • all waves spread out at edges when they pass through a gap/pass object
    • a gap much larger than the wavelength causes little spreading and a sharp shadow eg light through a doorway
    • a gap similar to the wavelength causes a lot of spreading with no sharp shadow eg sound through a doorway
  • hear someone through a open door even if cant see them as size of gap and wavelength of sound are roughly = so sound wave diffracts and fills the room
  • reduces the quality of images seen in microscopes and telescopes. It can cause rings or spikes around the image of the object being viewed.

Diffraction through a wide gap (http://www.bbc.co.uk/staticarchive/d21d166e660376051165990052889074ad80ae4b.jpg)       Diffraction through a narrow gap (http://www.bbc.co.uk/staticarchive/31d29a61aab276481d40a6f7faa903eb36a59af3.jpg)

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Refraction

waves travel at different speeds in substances which have different densities so when cross boundary between 2 substances (glass and air) it changes speed

  • Refraction doesn't happen if they cross the boundary face on/ at an angle of 90° (called the normal) - in that case they carry straight on but slows down (shorter wavelength but same frequency)
  • if hits a different medium at an angle, part of the wave hits the denser material first and slows down while other part carries on at the first faster speed for a bit - so wave changes direaction

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Internal Reflection

internal reflection depends on the critical angles - it only happens when the light ray travels through dense material towards less dense substance. if the angle of incidence is big enough, the ray doesnt come out at all, but reflects back into the material. ---> big enough means bigger then the critical angle - every material has its own critical angle

    • incidence less then critical = refracted into outer layer but some is internally reflected
    • incidence equal to critical = ray go along surface with a lot of internal reflection
    • incidence greater then critical = no light comes out its all internally reflected
  • Fibre optic cables use internal reflection for communication
  • pulses of light or infrared bounces waves off the sides of very narrow core which is protected by outer layers as when it enters, it hits the boundary between the core and the outer layer at an angle greater then the critical angle.
  • an endoscope also uses fibre optics (produces image on inside of body through tiny key hole incision rather then cutting patient open)
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EM Waves

  • as frequency and wavelength of EM radiation changes its interation matter changes (absorbed, reflected or transmitted)
  • about 1/2 EM radiation from sun is visible light, most of rest is infrared and UV (gives suntan)
  • all travel at same speed though a vacuum
  • each end of the spectrum waves pass through material, nearer the middle is absorbed
  • higher frequency are more dangerous to living cells - have more energy
  • when absorbed it causes heating and ionisation - can be dangerous
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Signal Commmunication and their advantages

as well as cooking and keeping house warm, EM waves are used for communication

  • using light, radio and electrical signals is good as travels really fast
  • electrical wires & optical fibres carry lots of info very quick
  • info sent through optical wire and electrical wire are pretty secure
    • inside cable so cant easily get tapped into; radio signals travel through air so they can be intercepted more easily (issue for wireless internet networks)
  • cables can be difficult to repair
  • wireless communication is portable
  • wireless relies on aeril to pick up signal and signal strength depends on location
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Communicating With Light

  • Morse Code - digital signal with light pulses meaning different letters
    • flashing a light on and off in a way that can be decoded
    • alphabet is represented by different sequence of dots and dashes
    • speed up communication over long distances
  • Optical Fibres - carry data over long distances as pulses of light/infrared radiation by internal reflection
    • bouncing waves off narrow core protected by outer layers
    • ray enters fibre so hits boundary between core and outer layer at angle greater then critical angle - internal reflection
    • reflected until emerges at other end
      • used for telephone and broadband internet cables, medical purposes (see inside body without operation)
  • advantage- quick way of communication; multiplexing means lots of different signals can be transmitted down one optical fibre at same time (dont need as many cables); little interference
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Lasers

  • all waves have same frequency and wavelength - monochromatic (single colour)
  • waves are in phase, troughs and crests are in line increasing amplitude - intense beam
  • coherent as they have fixed phase difference
  • low divergence - beam is narrow, stays narrow even at long distances from source - useful for guiding weapons to their targets
  • Some lasers are capable of heating materials- useful in surgery and for cutting metals
  • CD Players use lasers to read digital information
  • The shiny surface of a CD contains billions of microscopic pits in a spiral track. The pattern of pits contains digital information. The laser light is reflected off the surface of the CD as the disk spins, and is detected by:
    • the change in the amount of reflected light caused by entering or leaving a pit is interpreted as a 1
    • no change in the amount of reflected light is interpreted as a 0.
  • reflected signals then changed into electrical signal
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Infrared

  • used in cooking (grills and toasters)
  • infrared sensors used in security systems (detect heat from intruders' body)
  • used instead of visible light to carry info through optical fibres
  • used to monitor temperatures
    • hotter the object, the more infrared radiation given out, so can detect heat loss through sensors
    • also detected by night-vision equipment - turns into electrical signal shown on display screen
  • remote controlls emit pulses of IR to send info to TV and DVD players
    • pulses act as digital on/off code (like morse code)
    • device will detect and decode the pattern and follow coded instruction
  • transmit ingo between mobile phones or PCs (only over short distances)
    • like how remote controls transfer info to device
    • disadvantage:
      • need to be close as IR beam from small low powered control is weak
      • point beam straight at device as IR waves travel in straight lines
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Radio Waves

  • good at long distance communication - dont get absorbed by atmosphere as much as waves in middle of spectrm or those with high frequency
  • long-wave radio (1-10km) can be transmitted from one place and received halfway round the world as they diffract around curved surfaces of earth
    • short wavelengths used for FM radio/TV (10cm-10m) - to get reception have to be in direct sight of the transmitter as the signal doesnt bend around hills or go through buildings
    • short wave radio signals (10m-100m) can be received at long distances from transmitteer due to reflection in ionosphere.
    • medium wave signals can also reflect from the ionosphere, depending on atmospheric conditions and time of day
  • DAB - Digital Audio Broadcasting - is a digital system for transmitting radio programmes. FM, Frequency Modulation, is an analogue system for transmitting radio programmes. Both have advantages and disadvantages. For example:

  • DAB makes more radio stations available and suffers from less interference from other broadcasts. On the other hand, DAB may have a poorer audio quality than FM, and not all areas of the UK are currently covered by DAB broadcasts

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Radio Waves Continued

  • Diffraction
  • amount of diffraction depends on the wavelength of the wave relative to the size of the gap -
    • longer wavelengths diffract a lot as they are large compared to gap, so they can bend round corners and obstacles - so can travel long distances between transmitter and receiver without having to be in line of sight of eachother
    • short wavelength radio and microwaves dont diffract a lot so transmitters need to be high up to avoid obstacles - some areas have trouble getting these wave radio signals e.g. foot of a mountain will have poor signal
  • diffraction can occur at edges of dishes used to transmit signal - result in signal loss as wave is more spread out so is weaker
  • Refraction - UV radiation from sun creates layers of ionised atoms in earths atmosphere. these electrically charged layers are called the ionosphere
  • radio waves travel faster through ionised parts then non-ionised parts causing refraction
  • short-wave (10m-100m) and medium(300m) radio signals refracted most in ionosphere (bounced back/refelected to earth) so can be recieced a long way from transmitter
  • amount refracted depends on frequency and angle elevation - high freq/short wave dont refract as much as medium waves
  • refraction not always good - can disrupt signal by bending it away from receiver dish
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Microwaves

  • satellite communication
    • signal from a transmitter is trnasmitted to space where is picked up by satellite's receiver dish orbiting thousands of km above earth. satellite transmits signal back to earth in a different direction where its received by satellite dish on groudn.
  • Mobile Phones
    • shorter wavelength then radio waves so they dont diffract much so are affected by curvature of earth as they dont bend round it like long wavelengths - also blocked by large obstacles (hills as cannot bend round them)
    • microwave transmitters need to be in line of sight so they can "see each other" and positioned close together, if there is an obstacle between, it results in poor/no signal
    • microwaves partially absorbed by water so when raining or near lakes there can be loss of signal through absorption/scattering
    • interfrerence also effects signal strength
      • mobiles masts can be dangerous as if microwaves absorbed by living tissue, cells may burn/die;  emitted into your body could damage health
      • no conclusive proof - any potential dangerous would be increased with prolonged exposure so we have to balance potential risks and the benefits until we know more
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Analogue and Digital Communication

signals sent along telephone wires or carried on EM waves are either analogue or digital. analogue - take any value within certain range. amplitude and frequency of an analogue wave vary continuously whereas digital can only take 2 values, usually on/off or 1/0 e.g. sending data along optical fibres as short pulses of light

  • both signals weaken as they travle so they might need to be amplified along their route - also pick up interference or noise from electrica disturbances/other signals
  • when you amplify analogue signals, noice is amplified too so loses quality. noise is easier to remove or ignore with digital so signal has higher quality
    • interference = 2 or more waves of a similar frequency meet, they can create one combined signal with a new amplitude (get it when radio stations transmit on similar frequencies)
  • digital - transmit several signals at the same time using one cable/EM wave (multiplexing)
    • multiplexing happens in phone wires - in between voice signals being transmitted, thousands of other people's voice signals can be slotted in (multiplexed). the samples are separated out again at the other end so the person you called can hear you and only you (happens so quick you dont even notice)
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Humans and the Environment

    • ultraviolet radiation causes sunburn, skin cancer, eye cataracts, premature ageing of the skin
    • darker skin gives more protecion as it absorbs more UV radiation. this prevents some of the damaging radiation from reaching the more vulnerable tissues deeper in the body
    • take care of the skin by using sunscreen higher SPF - SPF 15 means you can spend 15 times as long as you otherwise could in the sun without burning
    • risks of exposure to UV is made public through media, adverts and the government telling people how to keep safe to improve public health
    • prolonged exposure to sunbeds can also cause a lot of health problems
  • ozone is high up in the atmosphere, absorbing some of the UV rays from the sun so it reduces amount reaching earths surface, however it has become thinner due to CFCs - gases that react with ozone to break them up. this depletion allows more UV rays to reach us at the surface of the Earth
  • scientists discovered that ozone levels over the Antarctic were reduced and after much research scientists confirm that CFCs were causing the issue so they were banned
    • montreal protocol
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Seismic Wave

  • the S waves are deflected (http://www.bbc.co.uk/staticarchive/8f851e1acaeb9d0bdcb73d205d11781126f55fd9.gif)S-waves =

    S-waves travel through solids only. They cannot travel through the liquid outer core, but they can travel through the mantle and crust.

  • P waves travel through (http://www.bbc.co.uk/staticarchive/ac32d722bb7fd256311da114820001253813a95f.gif)P-waves =  Unlike S-waves, P-waves can pass through the liquid outer core. When P-waves pass from solid to liquid, then from liquid to solid, there are sudden changes in direction. The waves are refracted.
  •  Earthquakes produce shock waves. These travel through the Earth and can be detected using a device called a seismometer.
  • changes speed as the properties of the mantle and core change, causing waves to change direction (refraction). path changes abruptly so the path has a kink instead of changing slowly and having a nice curved path
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