1.1 Introduction to thermal energy ('heat energy') transfer

• Thermal energy is the energy contained in a system of particles due to their kinetic energy of vibration or movement from place to place.

  • It is better to use the phrase 'thermal energy', rather than the phrase 'heat energy'.

• Energy can be transferred from one place to another by work or by heating processes.

• You  need to know and understand how this energy is transferred and which heating processes are most important in a particular situation.

•  When energy is transferred to an object by some means or other, the energy is stored in the object's energy store.

• This energy store may be thermal (dealt with here), chemical, kinetic, magnetic, gravitational potential, elastic potential or nuclear.

• Here we are interested in energy transfer by heating (conduction, convection) and infrared (radiation).

• Heat energy must always flow from hotter material at a higher temperature to cooler material at a lower temperature and the bigger the temperature difference the bigger the rate of heat energy transfer.

• eg the greater the temperature difference between a body and its surroundings like a hot object like a mug of coffee standing in a cold room, the faster the heat energy is transferred from the hotter material to the cooler material (eg surrounding air).

• Systems, thermal energy stores and states of matter

• System is a word that means a particular object or objects that is being looked at in a particular context eg boiling water in a kettle.

• When a system changes, energy is transferred into or out of the system, this may be between different objects/materials in the system or perhaps between different energy stores (same of different).

• Closed systems are systems that do not allow energy to leave or enter (lose or gain) so the ne change in the total energy is zero.

• When an object/material is heated to raise its temperature, the thermal energy store of the object is increased.

  • This thermal energy is stored throughout all the material by increasing the kinetic energy (KE) stores of the material's individual particles eg the KE particle vibration in a solid and the KE of the rapid movement of the freely moving particles in a liquid or gas.

  • The thermal KE is distributed by either the particles vibrating against each other in a solid or the collisions between the freely moving particles in a gas or liquid. Higher KE particles will on average lose energy to lower KE particles - that's the way energy 'flows'.

  • The higher the temperature of particles the greater their average kinetic energy so they will vibrate more violently in a solid and move faster in gases and liquids.

  • Once heat energy has stopped being supplied to an object, it will distribute itself evenly to give a uniform temperature throughout the material by conduction or convection. However, if object's/material's surroundings are at a lower temperature, then heat energy will drain from this thermal store until its temperature has fallen to that of the surroundings - that's the way heat flows!

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• The amount of thermal energy transferred can be calculated from the formula

  • energy transferred (J) = mass (kg) x specific heat capacity (J/kg^oC) x temperature change (^oC)

  • For more details of calculations see  

See also

Specific heat capacity: How to determine it, use of data, calculations and thermal energy stores

Particle theory models, internal energy, heat transfer in state changes, latent heat, particle motion