Physics - P1.1 - Energy Transfer By Heating

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P1.1.1 - Infared Radiation

  • Infrared waves - part of the electromagnetic spectrum, just beyond red visible light
  • Can feel it - makes us feel warm
  • All objects emit infrared radiation
  • Hotter the object = more infrared radiation emitted in a given time
  • Can travel through a vacuum - eg. space - how we get heat from the sun
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P1.1.2 - Surfaces and Radiation

  • Absorbers of infrared radiation - black, matt surfaces are better absorbers than shiny, white surfaces - it will become hotter if left in the sun
  • Emitters of infrared radiation - black, matt surfaces are better emitters than shiny, white surfaces - it will transfer energy more quickly  and cool down faster
  • Reflectors of infrared radiation -  shiny, white surfaces are better reflectors than black, matt surfaces
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P1.1.3 - States of Matter

  • Three states of matter: solid, liquid and gas - can make objects change between them by heating or cooling
  • Solid - particles vibrate in fixed position: fixed shape
  • Liquid - particles in contact but can move at random: not a fixed shape and can flow
  • Gas - particles far apart and move at random much faster: not a fixed shape and can flow - density of a gas < density of a liquid or solid
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P1.1.4 - Conduction

  • Conduction occurs mainly in solids - liquids and gases are poor conductors
  • One end of a solid heated -> particles gain kinetic energy there: vibrate more -> energy passed to neighbouring particles -> energy is transferred through the solid - this process occurs in metals
  • In metals, when heated - free electrons gain kinetic energy and move through the metal, transferring energy by colliding with other particles: all metals are good conductors
  • Poor conductors = insulators - eg. wool, fibreglass - contain trapped air: good insulators
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P1.1.5 - Convection

  • Convection occurs in fluids - liquids and gases
  • Fluid heated -> expands -> becomes less dense -> rises -> cooler denser fluid replaces warm fluid - resulting convection current transfers energy through the fluid
  • Convection currents - occur on small scale, eg. heating water in a beaker, or large scale, eg. heating air above land and sea - convection currents responsible for onshore/offshore breezes
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P1.1.6 - Evaporation and Condensation

  • Evaporation - liquid -> gas - most energetic molecules escape from liquids surface and enter air - average kinetic energy of remaining molecules is less = liquid temperature decreases - evaporation causes cooling
  • Rate of evaporation increased by - increased surface area, increased temperature of liquid, draught across liquids surface
  • Condensation - gas -> liquid - often on windows or mirrors
  • Rate of condensation increased by - increased surface area, reduced surface temperature
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P1.1.7 - Energy Transfer by Design

  • Greater temperature difference between object and surroundings = greater rate at which energy is transferred
  • Rate at which energy is transferred depends on - materials object is in contact with, objects shape, objects surface area
  • To maximise rate of energy transfer (to keep things cool) use: good conductors, things dull black, air flow around maximised
  • To minimise rate of energy transfer (to keep things warm) - need to minimise transfer of energy by conduction, convection and radiation - use: good insulators, things white and shiny, trap air in small pockets (prevent convection currents)
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P1.1.8 - Specific Heat Capacity

  • Heat a substance = transfer energy to it which increases temperature - specific heat capacity = amount of energy require to heat 1kg by 1C
  • Different substance = different specific heat capacity, greater specific heat capacity = more energy required to heat, greater mass = more energy required to heat
  • Equation for specific heat capacity:
    E = m x c x θ
    E - energy transferred - J
    m - mass - kg
    c - specific heat capacity - J/kg°C
    θ - temperature change - °C
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P1.1.9 - Heating and Insulating Buildings

  • To minimise rate of energy transfer, fit
      - fibreglass loft insulation - reduces enrgy transfer by conduction
      - cavity wall insulation - traps air in small pockets - reduces energy transfer by convection
      - double glazing - reduces energy transfer by conduction
      - draught proofing - reduces energy transfer by convection
      - aluminium foil behind radiators - reflect infared radiation back into room
  • U-value - how much energy/second passes through an object - U-value used to compare materials - lower U-value = better insulator
  • Solar heating panels - contain water heated by radiation from sun, water used to heat buildings or provide domestic hot water, cheap as don't use fuel, expensive to buy and install, water isn't heated at night
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