Physics P1A

THERMAL RADIATION

Thermal radiation is energy transferred by electromagnetic waves (infra-red waves)

All objects emit thermal radiation

The hotter an object is the more thermal radiation it emits

Heat radiation can travel through a vacuum

SURFACES AND RADIATION

Dark matt surfaces are better emitters of radiation than light shiny surfaces

Dark matt surfaces are better absorbers of radiation than light shiny surfaces

A matt surface absorbs more radiation because it has lots of cavities

If :- 

• an object is hotter than its surroundings it will cool down - lose heat energy
• cooler than its surroundings it will warm up - gain heat energy

CONDUCTION

Conduction in a metal is due mainly to free electrons transferring energy inside the metal and heat energy is transferred when the electrons collide with other particles in addition to when the atoms vibrate

Non-metals are poor conductors because they do not contain free electrons and heat energy only transfers when the atoms vibrate

Conduction occurs mostly in solids, liquids and gases are poor conductors

Materials such as fibreglass are good insulators because they contain pockets of trapped air

Trapped air is a poor conductor so it is, conversely a good insulator

CONVECTION

Convection only takes place in gases and liquids (fluids)

Convection is due to hot liquid or gas rising

Convection is due to circulation currents within the fluid.

why do fluids expand when heated:-

• particles are less dense in gases and liquids and can move about
• when a gas or liquid is heated the particles move about more and the fluid becomes less dense
• the density decreases because the mass of the fluid occupies a greater volume
• So heating a fluid makes it less dense and causes it to rise

Convection does not take place in a solid because particles are not free to move

HEAT TRANSFER BY DESIGN 

A radiator has a large surface area so it can lose heat easily

Small objects lose heat more easily than large objects - because they have a larger surface area in relation to their mass

Examples of heat transfer by design

Car engine

• Cooling system of the car transfers thermal energy from the engine to the radiator
• the radiator is flat so it has a large surface area
• This increases heat loss throght convection in the surrounding air and through radiation
• A cooling fan switches on when the car is too hot - this increases the flow of air across the radiator

Vacuum flask

• The vacuum between the outer walls cuts out heat transfer from convection or conduction between the two walls
• glass is a poor conductor of heat so there is little conduction through the glass
• the outer glass surface is silvery to reduce radiation through the outer wall
• the plastic cap stops heat loss through  convection
• The plastic cap and outer protection are poor conductors so there is little heat loss from the cap or the outer cover

House

• Draft excluder - stops heat loss by convection
• Loft insulation - stops heat…