11.1 Internal energy and temperature

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Energy transfer

Between two objects energy transfer occurs if:

  • a force is exerted by one on the other, and makes it move i.e. work is done on the other one.
  • heat transfer by convection, conduction or radiation happens, due to a temperature differnece.
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Internal energy

The internal energy of an object is the sum of the all the molecules' kinetic and potential energies.

The internal energy is constant if:

  • There is no heat transfer or work done by the object
  • The heat transfer and work done 'balance' each other out

For example in a filament lamp, the work done by the electricity pushing the electrons through it, equals the energy loss by radiation and heat loss, therefore the internal energy is constant.

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Molecule- smallest particle that is characteristic of the substance. 

  • In a solid - strong forces of attraction between paticles arranged in a regular 3-D structure. The paticles cannot move out of their positions, only vibrate. When heated, they vibrate more and more until they break out of the structure: melting.
  • In a liquid - particles move randomly in contact with each other, as the intermolecular forces are still strong. It has no fixed shape and can flow easily. When heated they gain enough energy to break free as become a gas: vaporisation.
  • In a gas - move around randomly taking up a lot of space and with few intermolecular forces. If heated, the gas molecules move around with more kinetic energy.
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Temperature is a measure of how much internal energy an object has.

If two objects are the same temperature, they are in thermal equilibrium.

Absolute zero = 0K, -273 degrees Celsius.

Ice point = 273K, 0 degrees Celsius.

Steam point = 373K, 100 degrees Celsius.

An object at absolute zero has an internal energy of 0.

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