P1 - Energy for the home

P1a: Heating houses

Measuring temperature:

  • thermogram uses colour to show temperature
  • temperature = a measurement of hotness on an arbitrary scale
  • heat = a measurement of internal energy on an absolute scale

Specific Heat Capacity:

  • = amount of energy needed to change temperature of substance by 1 degrees C (in J/kg degree Celcius)
  • water = 4200 joules per kg degree celcius
  • energy transferred = mass x specific heat capacity x temperature change

Specific latent heat:

  • energy needed to melt/boil 1kg of material in joules per kilogram (j/kg)
  • energy transferred = mass x specific latent heat
  • when a substance changes state, energy is needed to break bonds of molecules 
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P1b: Keeping homes warm

Insulation homes:

  • double glazing reduces energy loss by conduction:
    • gap between two panes of glass is filled with gas or contains a vacuum
    • particles of gas are far apart so energy is difficult to transfer
    • no particles in a vacuum so impossible to transfer energy
  • loft insulation reduces energy loss by conduction and convection:
    • warm air in home rises
    • energy transferred through ceiling by conduction
    • air in loft warmed by top of ceiling and trapped in loft insulation
  • cavity wall insulation reduces energy loss by conduction and convection:
    • air in foam is good insulator
    • air cannot move by convection because it is trapped in foam
  • insulation blocks used to build new homes reduce energy transfer by radiation:
    • blocks have shiny foam on both sides 
    • energy from Sun reflected back to keep home cool in summer
    • energy from home is reflected back to keep home warm in winter
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P1b: Keeping homes warm

Energy is transferred by:

  • conduction - due to transfer of kinetic eergy between particles
  • convection - gas expands when heated and makes it less dense so it rises
    • density = mass / volume (in g/cm3)
  • radiation - energy transferred through waves not by particles, works in a vacuum

Energy efficiency

Payback time

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P1c: A spectrum of waves

Properties of waves:

  • amplitude of wave = maximum displacement of a particle from rest position
  • crest = highest point on a wave above rest position
  • trough = lowest point on a wave below rest position
  • wavelength = distance between two sucessive points
  • frequency = number of complete waves passing a point in one second
  • speed of wave = frequency x wavelength

Electromagnetic spectrum:

  • radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray

Refraction: when the speed of a wave decreases as the wave enters a more dense medium

Diffraction: spreading out of a wave as it passes through a gap

  • larger gap = less diffraction, most diffraction occurs when gap is similar size to wavelength
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P1d: Lights and lasers

Morse code:

  • dots and dashes to represent the alphabet
  • code is used by signalling lamps as a series of short and long flashes of light
  • example of a digital signal

Laser light:

  • white light = made up of different colours of different frequencies out of phase
  • laser light = one single frequency, in phase and shows low divergence
  • used to read from the surface of a CD:
    • surface of CD is pitted 
    • pits represent digital signal
    • light shone onto CD surface and difference in reflections provides info for digital signal
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P1d: Lights and lasers


When light travels from more dense material to less dense material, angle of refraction is larger than the angle of incidence


When angle of refraction = 90 degrees, angle of incidence = critical angle


When angle of incidence = bigger than critical angle --> total internal reflection


Endoscopes: allows doctors to see inside a body without need for surgery

  • light passes through one set of optical fibres and illuminates inside of body
  • light is reflected and reflected light passes up another set of fibres to an eyepiece / camera
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P1e: Cooking and communicating using waves

Cooking with infrared radiation:

  • energy is absorbed by surface of food
  • kinetic energy of surface of food particles increases
  • rest of food heated by conduction

Cooking with microwaves:

  • penetrate up to 1cm into food
  • microwave ovens cook food by microwave radiation
    • water of fat molecules in outer layer of food vibrate more, kinetic energy of food particles increases, and rest of food cooked by conduction

Properties of microwaves:

  • have wavelengths between 1mm and 30cm
  • mobile phones use longer wavelengths than microwave ovens so less energy is transferred
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P1e: Cooking and communicating using waves

Microwave radiation:

  • used to communicate over long distances
  • transmitter and reciever must be in line of sight
  • aerials are normally situated on top of high buildings

Microwave communication:

  • satellites use microwave communication
  • signal from Earth recieved, amplified and re-transmitted back to Earth
  • satellites are in line of sight because there are no obstructions in space

Signal strength for mobile phones can change over a short distance:

  • microwaves do not show much diffraction
  • adverse weather and large areas of water can scatter signals
  • curvature of Earth limits line of sight so transmitters have to be on tall buildings or close together
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P1f: Data transmission

How electrical devices are controlled:

  • pressing button on device completes circuit which sends a coded signal to LED at front
  • signal includes start command, instruction command, device code and 
  • LED transmits series of pulses that recieved by device and decoded to change channel

Analogue to digital switchover:

  • gives improved signal quality for picture and sound and greater choice of programmes
  • allows user to interact with programme
  • provides information servies such as programme guides and subtitles

Advantages of digital signal:

  • interferences also occur on digital signals but does not affect signal as digital signals have only two values, 0 and 1
  • multiplexing allows a large number of digital to be transmitted at the same time
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P1g: Wireless signals

Radio waves reflected and refracted by Earth's atmosphere:

  • amount of refraction depends on frequency
  • less refraction at higher frequencies

Radio stations broadcast signals with same frequency because radio stations are too far away to interfere:

  • Digital Audio Broadcasting provides greater choice of radio stations but not of good quality
  • DAB eliminates interference between other radio stations

Radio waves are reflected from ionosphere and undergo total internal reflection:

  • water reflects radio waves, land mass doesn't
  • reflection by ionosphere allows radio waves to be recieved from aerial not in line of sight

Microwaves pass through the ionosphere:

  • signals recieved by satellites, amplified and retransmistted back to Earth
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P1h: Stable Earth

Earthquake waves:

L waves travel round the surface very slowly.

P waves are longitudinal pressure waves:

  • travel through the Earth at between 5km/s and 8km/s
  • can pass through solids and liquids and are refracted by the core
  • path taken by P wave means that scientists can work out size of Earth's core

S waves aretransverse waves:

  • travel through the Earth at between 3km/s and 5.5 km/s
  • can only pass through solids and don't travel through liquids
  • not detected on the opposite side of Earth to an earthquake which tells scientists that Earth's core is a liquid
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P1h: Stable Earth

Tans are caused by the action of UV light on the skin:

  • cells in the skin produce melanin which then produces a tan
  • people with darker skin do not tan as easily because UV radiation is filtered out
  • use a sunscreen with high SPF to reduce risk:
    • max length of time to spend in the Sun = published normal burn time x SPF

Ozone depletion:

  • ozone is found in the stratosphere
  • helps to filter out UV radiation
  • CFCs from aerosols and fridges destroy ozone and reduce the thickness of the ozone layer which increases the potential danger to humans
  • the ozone layer is thinnest above the South Pole because ozone depleting chemicals work best in cold conditions
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