ways of heating

• Whenever an object is heated, energy is transferred to its thermal energy store & its temperature increases.
• The process of heating can take place in 3 different ways depending on the medium involved:

  • Conduction
  • Convection
  • Radiation

  
• • Conduction takes place in solids
  • Convection takes place in fluids
  • To get through empty space, heat has to be transferred in the form of radiation.

  
• Conduction:


  • As one end of a solid object is heated, energy is transferred to the kinetic energy stores of the particles in that end.
  • This causes the particles to vibrate faster & collide with their neighbouring particles more often & with more energy.
  • As the process repeats, energy is passed along the object from one particle to the next.
  • Until it’s spread out evenly & it will be the same temp. everywhere.
  • Even though it's kinetic energy that's being passed between particles, when considering the object as a whole we say that it's heat (or thermal energy) that's being transferred.

  
• Conduction occurs in solids, as the particles in a solid are held together really closely.

  • Meaning there’s lots of collisions that pass on the energy
  • But in liquids & gases, the particles are all further apart, so they don’t collide as much.

  
• Thermal conductivity:


  A measure of how well a material conducts energy when it is heated.
• • Metals have a high thermal conductivity, they transfer heat energy rapidly.
  • Plastics have a low thermal conductivity, they're used as insulators.
  • Most fluids have a low thermal conductivity.

  
• A fluid:


  Any substance that can flow, so the particles are free to move around. These include liquids & gases.

  
• Convection:


  • As a fluid is heated, particles gain KE & move around faster.
  • Random diffusion causes energetic particles to move away from the warmer region, towards cooler regions.
  • So higher energy particles in the warmer region are a lot more spread out than those in the cooler region.
  • The fluid in the warmer region expands as it heats up & becomes less dense than the cooler fluid.
  • So it will rise above any cooler fluid that hasn't been heated, because that fluid is more dense.
  • As the fluid cools down it will become more dense again & sink back down.

  
• If convection takes place in a limited space, like a container or a room, it can create a convection current.
• • Heating a container of fluid would mean, the particles near the heat source would gain KE & spread out becoming less dense.
  • As they’re less dense, these particles rise above the cooler, less dense particles above them.
  • At the same time, these cooler particles sink down & take their place.
  • While this is happening, the hot particles will lose their energy & cool down
  • The cool particles, now at the bottom, would heat up.
  • Because of this, the cycle would keep on repeating for as long as the fluid was being heated.
  • This cycle is called a convection current.

  
• Convection current can be seen like from the oceans or inside buildings, where radiators warm the nearby air & set off the cycle.

  
• To reduce heat loss by convection, stop the free flow of fluids like:


  • Covering yourself with a blanket at night, it stops the warm air from escaping
  • Closing your windows

  
• Conduction & convection both involve particles gaining kinetic energy:

  • But in conductions, only the energy is transferred between the different particles
  • While in convection, the particles move

  
• Heat energy can be transferred without particles (it could travel through a vacuum)

  • This happens when energy is transferred by radiation & specifically when the energy is carried by infrared waves (a form of electromagnetic radiation).

  
• All objects constantly absorb & emit radiation, they do both at the same time.

  • The hotter the object, the more radiation it emits.

  
• When hot putting your hand over a barbecue, it feels hot, even if you’re not touching it.

  • The very hot metal & coal is emitting infrared radiation, which is absorbed by the hand.

  
• Warm fluids rise because they are less dense than cooler fluids.