Energy - Transfer of Energy 01

Different forms of energy can be transformed from one form to another. Energy transfers play a major part in everyday life, for example every time we put the kettle on an energy transfer takes place. It is an essential part of a GCSE physics syllabus that you are able to understand what energy transfers take place in certain situations and how those transfers occur.

As well as conduction, convection and radiation, evaporation and condensation transfer heat energy from place to place. Evaporation takes place from the surface of a liquid. The particles in liquids are in constant motion, changing places and colliding with one-another. The kinetic theory predicts that the energy of majority of particles will lie within certain limits but there will always be some with higher than average and lower than average energies (higher tier candidates may have seen a diagram showing this).

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When they are near the surface of a liquid, particles with higher than average energies can overcome the forces holding the particles together as a liquid and pass into the air. As this happens, the number of particles in the liquid becomes smaller so the liquid volume gradually decreases. The total energy contained within the liquid is decreased and therefore the temperature of the liquid drops. Some liquids naturally evaporate faster than others, for example, ethanol will evaporate faster than water. Evaporation is a heat transfer process that transfers the thermal energy into the surroundings from a liquid. The faster that a liquid evaporates, the cooler it will become. There are several factors that affect the rate of evaporation of a liquid other than the type of liquid - you need to revise them for the exam.

The opposite of evaporation is condensation. This is the process where vapour turns back into a liquid. A vapour is made from billions of particles, if they are slowed down, they will whizz around less, become closer together and the forces they exert on each other will increase. In other words, the vapour becomes a liquid during condensation. To slow down the speed of movement of particles, it is necessary to remove heat energy, this occurs where a vapour meets a cold surface or is mixed with cooler vapour (for example when you breathe out on a cold day the water vapour in your breath condenses as it loses its energy to the cold air around you). Condensation therefore transfers thermal energy from a vapour into the surroundings. For the GCSE, you need to know the factors that affect the amount of condensation that occurs.

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  1. What effect causes energy to be transferred at a faster rate between an object and its surroundings?
    As a hot object cools down, the rate of cooling decreases as the temperature gets closer to the temperature of the surroundings. That is why you get a 'cooling curve' and not a 'cooling straight line' when you plot a graph of temperature vs time for cooling something down
  2. What factors affect the amount of condensation that occurs?
    The colder the surface is, the more quickly the hot vaporised liquid particles cool and return to liquid form on the surface. A larger concentration of liquid vapour in the atmosphere increases the number of interactions between the liquid in the atmosphere and the surface. Again, having a surface with a larger surface area also increases the number of interactions between the surface and the liquid
  3. Which material would be best to use as an insulator against heat loss?
    Expanded polystyrene contains a lot of inert gas trapped in its structure making it a good insulator because gases are poor conductors of heat
  4. How does convection occur?
    Where the fluid (remember a fluid can be either a liquid or a gas) is heated, the thermal energy increases the movement of particles, so the particles in the heated portion are further apart. Because there are fewer particles now in the same volume, the heated part of the fluid is less dense and rises to float on the more dense part. As the heated area cools, the density increases and it begins to sink again
  5. Which material is best at conducting heat?
    All metals are good conductors of heat and electricity. The free electrons and tightly packed structure of all metals aid the conduction of heat throughout the material
  6. What does the rate at which an object transfers energy by heat depend on?
    Knowing how heat energy is transferred from one place to another is useful for heating and insulating our homes
  7. What is condensation?
    Energy is transferred from higher energy particles to the surface on which condensation is happening
  8. What factors affect evaporation?
    Clothes dry better on warm windy days when they are spread out as much as possible
  9. Which of the following statements best describes our understanding of how conduction of heat occurs?
    In a solid, the particles can only vibrate. In substances that conduct heat, the vibrations are passed from one particle to the next
  10. Which of the following best explains evaporation?
    There is less overall energy left in the liquid which you can detect as a temperature drop

Author: Martin Moore

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