Physics Revision

Physics Revision

Infared Radiation
1. IR is energy transfer by electromagnetic waves
  1. ALL objects emit IR
    1. The hotter the object; the more IR it emits at a given time
2. IR energy transfer does not involve particles
3. IR is part of the electromagnetic spectrum (just beyond the visible red light)
4. IR can be detected by our skin, as it makes us feel warm
5. IR can travel through a vacuum (like space) and thats how we get energy from the sun
6. Surfaces and radiation
  1. Dark, matt surfaces are good absorbers of IR
    1. For instace; an object that was painted a matt black and left in the sun for an hour, will get hotter than the same object painted a shiny white.
    2. Dark matt surfaces are also good emitters of IR
  2. Light, shiny surfaces are good reflectors of IR
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States of matter
1. The three states of matter are; SOLID, LIQUID AND GAS
  1. We can make a substance change between these states by heating and cooling it.
  2. In a solid, the particles vibrate around fixed positions, so the solid has a fixed shape
  3. In a liquid, the particles are in contact with each other. They can move around at random, meaning a liquid doesn't have a fixed shape and can flow.
  4. In a gas, the particles are usually far apart and move at random much faster. It doesn't have a fixed shape and it can flow. The density of a gas is much less than a solid or liquid.
2. Conduction
  1. Conduction occurs many in solids, most liquids and gasses are poor conductors.
    1. If one end of a solid is heated, the particles at that end gain kinetic energy and vibrate more. This energy is passed to neighbouring particles and in this way the energy is transferred through the solid.
      1. This process occurs in metals.
  2. Also, when metals are heated; their free electrons gain kinetic energy and move through the metal, transferring energy by colliding with other particles. Hence all metals are good conductors.
  3. Poor conductors are called insulators. Materials such as wool and fibreglass are good insulators because they contain trapped air.
3. Convection
  1. Convection occurs in fluids (fluids are liquids and gasses)
    1. When a fluid is heated it expands. The fluid becomes less dense and rises. The warm fluid is replaced by cooler, denser fluid. The resulting convection current transfers energy throughout the fluid.
      1. Convection currents can be on a very small 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.
4. Evaporation and condesation
  1. Evaporation is when a liquid turns into a gas. Evaporation takes place because the most energetic molecules escape from the liquids 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.
    1. The rate of evaporation is increased by:
      1. Increasing the surface area of the liquid.
      2. Increasing the temperature of the liquid
      3. Creating a draught of air across the liquids surface
  2. Condensation is when a gas turns into a liquid, this often takes place on cold surfaces such as windows and mirrors.
    1. The rate of condensation is increased by:
      1. Increasing the surface area
      2. Reducing the surface temperature
5. Energy transfer by design
  1. The greater the temperature difference between an object and its surroundings, the greater rate at which energy is transferred.
  2. The rate at which energy i transferred also depends on:
    1. The materials the object is in contact with.
    2. The objects shape.
    3. The objects surface area
  3. Sometimes we want to maximise the rate of energy transfer to keep things cool, to do this we may use things that:
    1. Are good conductors
    2. Are painted a dull black
    3. Have the air flow around them maximised.
    4. Sometimes we want to minimise the rate of energy transfer to keep things warm by:
      1. Using things that are good insulators
      2. Things that are white and shiny
      3. Things that prevent convection currents by trapping air in small pockets.

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	Created by Willow Chaddock about 9 years ago