Conduction, Convection and Radiation notes

Thermal energy will always be transferred from hotter areas to colder areas.

• This always happens by the processes of:

  Conduction

  Convection

  Radiation

Objects will always lose heat until they are in thermal equilibrium with their surroundings: eg. a mug of hot tea will cool down until it reaches room temperature.

Conduction

Conduction is the main method of thermal energy transfer in solids.

• Metals are extremely good at conducting heat→ They have an additional method of conduction: %%free delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material and hence transfer heat through the metal very quickly.%%

• Solids are also good conductors because the particles are close together and bonds are strong, making transfer of KE more rapid.

In fact, diamond, a non-metal, is also an excellent conductor because it has very strong intermolecular bonds.

• Non-metals, liquids and gases are poor at conducting: Poor conductors are called insulators.

In fact, materials containing small pockets of trapped air are especially good at insulating, as air is a gas and so a poor conductor

• When a substance is heated, the atoms start to vibrate more.

  • They bump into each other, transferring energy from atom to atom
  • Intermolecular forces allo the atoms to pass vibrations from one to another
  • The stronger the forces, the faster the vibrations are passed

  

Convection

Convection in the transfer of heat through fluids (liquids and gases); it cannot occur in solids.

• When a fluid is heated:

  • Molecules push each other apart, making the fluid expand
  • The hot fluid becomes less dense than the surroundings
  • The hot fluid rises, and the cooler surrounding fluid moves in to take its place
  • Eventually, the hot fluid cools, contracts and sinks back down again
  • This is called a %%convection current%%

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Thermal Radiation

Thermal radiation is the transfer of heat energy by infrared (IR) waves.

• All bodies, no matter what temperature, %%emit a spectrum of thermal radiation in the form of IR waves%%.

Radiation is the only type of heat transfer that can travel through a vacuum.

Gases allow radiation through better than liquids, liquids better than solids.

The hotter object, the more infrared radiation it emits in a given time.

Colours and surfaces affect how well a body emits and absorbs thermal radiation:

• Black objects are the best at emitting and absorbing thermal radiation

• Shiny objects are the worst at emitting and absorbing thermal radiation