Edexcel IGCSE Physics 复习笔记 4.1.4 Conduction, Convection & Radiation

• Thermal energy transfers from hotter areas to cooler areas by the processes of:
  • Conduction
  • Convection
  • Radiation

   

• Objects will always lose heat until they are in thermal equilibrium (same temperature) with their surroundings
  • For example, a mug of hot tea will cool down until it reaches room temperature

   

• Conduction is the main method of thermal energy transfer in solids
  • Metals are extremely good at conducting heat

   

• Non-metals are poor at conducting heat whilst liquids and gases are extremely poor
  • Poor conductors are called insulators

   

• Materials containing small pockets of trapped air are especially good at insulating, as air is a gas and hence a poor conductor

Conduction: the atoms in a solid vibrate and bump into each other

• When a substance is heated, the atoms start to move around (vibrate) more
  • As they do so they bump into each other, transferring energy from atom to atom

   

• Metals are especially good at conducting heat as the delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material and hence transfer heat better

• Convection is the main way that heat travels through liquids and gases
  • Convection cannot occur in solids

   

• When a fluid (a liquid or a gas) is heated:
  • The molecules push each other apart, making the fluid expand
  • This makes the hot fluid 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
  • The resulting motion is called a convection current

   

A convection current caused by the heat transfer from the fire

• All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves
  • Thermal radiation is heat transfer by infrared (part of the electromagnetic spectrum)

   

• The hotter object, the more infrared radiation it radiates in a given time

The infrared emitted from a hot object can be detected using a special camera

• The colour of an object affects how well it 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

   

• The table below summarises the absorbing and emitting abilities of different colours:

Absorbing and Emitting of Difference Colours Table

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