Heat transfer

Whether you want to prevent heat from escaping, cook something more quickly or answer an exam question, you need to know the processes of heat transfer:

  • conduction
  • convection
  • radiation

It's also worth a look at the Thermal concepts page for information about evaporation


Summary

Conduction

While all matter is capable of allowing heat transfer by conduction, it is fastest in solids, particularly in metals with delocalised electrons.

 When the end of a solid bar is heated, the particles are made to vibrate more than before. The kinetic energy is passed from one molecule to the next along the rod by collisions.

NB: Remember that all objects above absolute zero will have at least some kinetic energy in their particles.

Convection

 We don't ask you to memorise much in Phyiscs, but it's worth learning the key words for a good convection answer...

When a fluid is heated, the molecules spread apart, causing it to be less dense. This less dense fluid rises into the surrounding cold fluid. This leaves a region of low pressure behind and so the cooler, denser fluid fills the gap, creating a convection current.

Radiation

Radiation is what you feel as you warm your hands in front of a fire.

All objects with a temperature above absolute zero emit infrared radiation. Infrared radiation is a region of the electromagnetic spectrum, and so it travels at the speed of light through a vacuum (convenient for planet Earth!) as well as being able to pass through air and other matter. 

 The amount of radiated heat depends upon the temperature of the source, it's colour and surface area. The rate of radiation absorbed by and emitted from black bodies is greater than silver ones (which are much better reflectors). Hot objects emit more radiation than cooler objects.


Test Yourself

Use quizzes to practise application of theory. 


START QUIZ!

Just for Fun

Consider a frozen chicken. To defrost the chicken, you have two options: placing it on the worktop as it is, or wrapping it in a woollen jumper. Which option will be the quicker?

MY PROGRESS

How much of Heat transfer have you understood?