Heat Transfer Flashcards
how can energy be transferred by heating?
conduction, convection and radiation.
what do conduction and convection involve?
transfer of energy by PARTICLES.
the bigger the temperature difference…
… the faster energy is transferred between the thermal energy stores of a body and its surroundings
describe the process of CONDUCTION
- in a solid, the particles are held tightly together.
- so when one particle vibrates, it COLLIDES with other particles nearby and the vibrations quickly pass from particle to particle.
- This process continues throughout the solid and gradually some of the energy is passed all the way through the solid, causing a rise in temperature at the other side of the solid.
- it’s then transferred to the surroundings or anything touching the object.
describe the process of THERMAL RADIATION
- consists of purely electromagnetic waves at a certain range of frequencies.
- IR
- all objects are continuously emitting and absorbing IR radiation
- an object that’s hotter than its surroundings emits more radiation than it absorbs (as it cools down). an object that’s cooler does the opposite.
- some colours and surfaces absorb or emit radiation better than others.
describe a practical to investigate conduction
- attach beads at regular intervals to one half of a long metal bar using wax
- hold the metal bar in a clamp stand and using a bunsen burner, heat the side of the bar with no beads from the very end
- as time goes on, energy is transferred along the bar by conduction and the temperature increases along the rod
- the wax holding the beads in place will gradually melt and the beads will fall as the temp increases, starting with the bead closest to the point of heating.
when does convection occur?
convection occurs when the more energetic particles in a fluid move from the hotter region to the cooler region and transfer energy as they do.
how does an immersion heater work?
- energy is transferred from the heater coils to the thermal energy store of the water by conduction (particle collisions)
- the particles around the coil get more energy so they start moving around more. this means there’s more distance between particles, i.e. the water expands and becomes LESS DENSE.
- this reduction in density means the hotter water tends to rise above the denser, cooler water.
- as the hot water rises it displaces the colder water out of the way, making it sink towards the heater coils.
- this cold water is then heated and rises and you end up with a convection current circulating energy through the water.
practical to demonstrate convection
coloured crystals in a beaker
how can you reduce the rate of heat energy transfer.?
- to reduce energy transfers by conduction, use materials with LOW THERMAL CONDUCTIVITY
- to reduce convection, you need to stop the fluid moving and prevent convection currents from forming
- INSULATION uses both of these techniques to reduce energy transfers
how does insulation work?
- clothes, blankets and foam all work by TRAPPING POCKETS OF AIR.
- the air can’t move so the energy has to conduct very slowly through the pockets of air as well as the material in between (both have low thermal conductivity)
which colours and surfaces ABSORB and EMIT IR better?
- a black, matt surface will be best at absorbing and emitting radiation best.
- a white, shiny surface is not as good
what are emission and absorption of radiation related to?
surface and temperature
how would you REDUCE energy transfers away from an object by thermal radiation?
use a surface that is a poor emitter eg. shiny and white
