- Heat Transfer
- Conduction works in ALL solids (especially well in metals)
- Conduction works by VIBRATIONS passed from particle-to-particle
- It works really well in metals because they contain FREE ELECTRONS which transfer heat very rapidly.
LIQUIDS don't conduct well because their particles are too spread out to pass on vibrations.
GASES (particles even further apart) and VACUUMS (no particles)
Things that use the poor conducting ability of the air to keep heat in:
Double Glazing, Woollen Jumpers and Quilts
Fibre Glass – "Fibre Glass traps AIR and the air DOESN'T Conduct!"
- Convection works in LIQUIDS and GASES (Fluids).
- Convection doesn't work in solids or vacuums because particles in a solid can't move out of position and there aren't any particles in a vacuum.
- In Convection particles nearest the heat collide more, so move apart.
- HOT AREA EXPANDS (NOT "the particles expand")
- Hot area now LESS DENSE than surrounding fluid...
- So it RISES.
- COMMON MISTAKE WARNING! "Heat rises" – Wrong!
- "HOT PARTICLES rise"
- Heat radiation travels from place-to-place by infra-red waves, which can travel through a vacuum as they don't use particles.
- An object has to be hotter than its surroundings to emit heat radiation.
- ROUGH, DULL and BLACK surfaces are best at absorbing infra-red waves.
- Also ROUGH, DULL and BLACK surfaces are best at emitting infra-red waves.
- An object HOTTER than its surroundings, a Rough, Dark, Black surface will get rid of the heat fastest. A SMOOTH, SHINY, SILVER surface will keep the heat in longer.
- An object COOLER than its surroundings, a Rough, Dark, Black surface will take heat in best and a SMOOTH, SHINY, SILVER surface reflects heat away.
"Law of Conservation of Energy"
Energy cannot be destroyed, only TRANSFORMED (converted) to other forms.
Basic Forms of Energy:
- Gravitational Potential
- Thermal (Heat)
Energy Transformation Questions:
E.g. What is the energy transformation in a light bulb?
ELECTRICAL –> HEAT + LIGHT
E.g What device will carry out this transformation?
ELECTRICAL –> SOUND
"useful" just means the type of energy we want.
"waste" energy is any form of energy which we don't want.