physics

> thermal radiation is energy transfer by electromagnetic waves

> all objects emit thermal radiation

> the hotter an object is, the more thermal radiation it emits

thermal radiation

> dark matt surfaces are betters emitters of thermal radiation than light shiny surfaces

> dark matt surfaces are better absorbers of thermal radiation than light shiny surfaces

surfaces and radiation

> conduction in a metal is due mainly to free electrons transferring energy inside the metal

> non-metals are poor conductors because they do not contain free electrons

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

conduction

> convection takes place only in liquids and gases (fluids)

> heating a liquid or a gas makes it less dense

> convection is due to a hot liquid or gas rising

convection

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

> small objects lose heat more easily than large objects

> heat loss from a building can be reduced by:

• aluminium foil behind radiators
• cavity wall insulation
• double glazing
• loft insulation

heat transfer by design

> energy exists in different forms

> energy can change (transform) from one form into another form

forms of energy

> energy can be transformed from one form to another or transferred from one place to another

> energy cannot be created or destroyed

conservation of energy

> useful energy is energy in the place we want it and in the form we need it

> wasted energy is energy that is not useful

> useful energy and wasted energy both end up being transferred to the surrounding, which become warmer

> as energy spreads out, it gets more and more difficult to use for further energy transfers

useful energy

> energy is measured in joules

> the efficiency of a device =

useful energy transferred by the device

    total energy supplied to the device

> wasted energy cause inefficiency

energy and effciency

> electrical energy is energy transfer due to an electric current

> uses of electrical devices include heating, lighting, making objects move ( using an electric motor) and creating sound and visual images

electrical devices

> the unit of power is the watt (w), equal to 1 J/s

> 1 killowatt (KW) = 1000 watss

> power (in watts) =

               energy transferred (in joules)

                  time taken (in seconds

electrical power