The hotter the object; the more IR it emits at a given time
IR energy transfer does not involve particles
IR is part of the electromagnetic spectrum (just beyond the visible red light)
IR can be detected by our skin, as it makes us feel warm
IR can travel through a vacuum (like space) and thats how we get energy from the sun
Surfaces and radiation
Dark, matt surfaces are good absorbers of IR
For instace; an object that was painted a matt black and left in the sun for an hour, will get hotter
than the same object painted a shiny white.
Dark matt surfaces are also good emitters of IR
Light, shiny surfaces are good reflectors of IR
2
3
States of matter
The three states of matter are; SOLID, LIQUID AND GAS
We can make a substance change between these states by heating and cooling it.
In a solid, the particles vibrate around fixed positions, so the solid has a fixed shape
In a liquid, the particles are in contact with each other. They can move around at
random, meaning a liquid doesn't have a fixed shape and can flow.
In a gas, the particles are usually far apart
and move at random much faster. It doesn't
have a fixed shape and it can flow. The
density of a gas is much less than a solid or
liquid.
Conduction
Conduction occurs many in solids, most liquids and gasses are poor conductors.
If one end of a solid is heated, the particles at that end gain kinetic energy and vibrate more. This energy
is passed to neighbouring particles and in this way the energy is transferred through the solid.
This process occurs in metals.
Also, when metals are heated; their free electrons gain kinetic energy and move through the metal,
transferring energy by colliding with other particles. Hence all metals are good conductors.
Poor conductors are called insulators. Materials such as wool and
fibreglass are good insulators because they contain trapped air.
Convection
Convection occurs in fluids (fluids are liquids and gasses)
When a fluid is heated it expands. The fluid becomes less dense and rises. The warm fluid is replaced
by cooler, denser fluid. The resulting convection current transfers energy throughout the fluid.
Convection currents can be on a very small scale, such as heating water in a beaker, or on a very large scale; such
as heating the air above land and sea. Convection currents are responsible for onshore and offshore breezes.
Evaporation and condesation
Evaporation is when a liquid turns into a gas. Evaporation takes place because the most energetic molecules escape from the liquids surface and enter the air. Therefore,
the average kinetic energy of the remaining molecules is less. So the temperature of the liquid decreases. This means that evaporation causes cooling.
The rate of evaporation is increased by:
Increasing the surface area of the liquid.
Increasing the temperature of the liquid
Creating a draught of air across the liquids surface
Condensation is when a gas turns into a liquid, this often
takes place on cold surfaces such as windows and mirrors.
The rate of condensation is increased by:
Increasing the surface area
Reducing the surface temperature
Energy transfer by design
The greater the temperature difference between an object and
its surroundings, the greater rate at which energy is transferred.
The rate at which energy i
transferred also depends on:
The materials the object is in contact with.
The objects shape.
The objects surface area
Sometimes we want to maximise the rate of energy transfer
to keep things cool, to do this we may use things that:
Are good conductors
Are painted a dull black
Have the air flow around them maximised.
Sometimes we want to minimise the rate of
energy transfer to keep things warm by:
Using things that are good insulators
Things that are white and shiny
Things that prevent
convection currents by
trapping air in small pockets.