Energy Transfer By Heating

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INFRARED RADIATION

Infrared waves are part of the electromagnetic spectrum; they are just beyond visible red light.  We detect infrared radiation on our skin as it makes us feel warm.  All objects emit infrared radiation. The hotter an object is, the more infrared radiation it emits in a given time.  It can travel through a vacuum - this is how we get heat from the Sun.  The transfer of energy by infrared radiation does not involve particles.

SURFACES AND RADIATION

Dark, matt surfaces are good absorbes of infrared radiation.  They are also good emitters of it.  In contrast, white, shiny surfaces are good reflectors and poor emitters of infrared radiation.  So this means that if we have two objects - one dark/matt and the other white/shiny, the dark object will get warmer than the white object.  However, if both objects begin warm and are not being heated, the dark object will be cooler than the white object.

States of matter:

  • Solid - particles vibrate around fixed positions.  A solid has a fixed shape and a high density
  • Liquid - the particles are in contact with each other but can move about at random.  A liquid can flow
  • Gas - the particles are further apart and move at random much faster.  A gas can also flow

CONDUCTION

Conduction occurs mainly in solids, because liquids and gases are poor conductors.  If one end of a solid is heated, the particles at that end gain kinetic energy and vibrate more.  This energy is passed to neighbouring particles and in this sense energy is transferred through the solid.  In addition, metals have free electrons that also gain kinetic energy and transfer energy through the solid by colliding with other free electrons. They also collide with the metal ions. This is why metals make good conductors.  Poor conductors are called insulators. Materials that trap air, such as wool and fibreglass, make good insulators.

CONVECTION

Convection occurs in fluids (liquids and gases).  A fluid expands when it is heated.  Because there is now the same amount of matter in a larger space, the density of the fluid decreases. The fluid becomes less dense and rises.  The warm fluid gets replaced by cooler, denser fluid and the resulting convection current transfers energy through the fluid. Convection doesn't take place in solids as the particles are not free to move

EVAPORATION AND CONDENSATION

Evaporation occurs when a liquid turns into a gas.  It happens because the particles with the most energy escape

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