- Infrared Radiation is energy transfer by electromagnetic waves
- All objects emitt infrared radiation
- the hotter an object is the more infrared radiation it emitts at a given time
- It does not involve particles
- therefore it can travel through a vaccum: we know this because it can travel from the sun to earth
All objects above the temperature of absolute zero emitt infrared radiation.
The sun is 93 million mile away from earth and it takes approximately 8 minutes for infrared raidation to reach earth from the sun.
Surfaces and Radiation...
- Dark, Matt surfaces are good emitters of infrared radiation
- Dark, Matt surfaces are good absorbers of infrared radiation
- Light, Shiny surfaces are good reflectors of infrared raiation
Houses in hot countries are often painted white because white surfaces are the worst absorbers of infrared radiation and thus the house is kept cool.
The pipes at the back of a fridge are often painted black because dark surfaces are the best emitters of infrared radiation. therefore the heat is transferred away from the fridge quickly, keeping it cool.
States of Energy...
States of energy...
- The particles in a solid are closley spaced and vibrate in fixed positions. This means solids have a fixed shape.
- The particles in a liquid move about at random but are in contact with each other. This means a liquid dosent have a fixed shape and can flow.
- The particles in a gas are spaced farther apart and move around quickly and randomly. Therefore a gas can flow. The density of a gas is also much less than that of a liquid or solid.
Conduction occurs mainly in solids. Most liquids and all gases are poor conductors.
If one end of a solid is heated the particles in that end vibrate more vigorously as they gain kinetic energy. The energy is passed to neighbouring particles by vibration. In this way energy is transfer through the solid.
This process occurs in metals. In addition the free electrons in metals gain kinetic energy and move through the metal passing energy on by means of collision.
Poor conductors are called insulators. Materials that contain air are good insulators.
- Metals are the best conductors
- Non-metals are poor conductor because they do not contain free electrons
Convection occurs in fluids. Convection is the movement of energy through fluid by the movement of the fluid itself.
When a fluid is heated it expands. Therefore there is the same mass in a bigger volume, meaning it becomes less dense. This warmer, less dense fluid rises and is replaced by the cooler more dense fluid. When this is continuous it creates a convection current, which transfers the energy throughout the fluid.
Convection currents are responsible for off shore and on shore breezes:
- during the day the air above the land is warmer so it rises. it is replace by the cooler air from the sea, creating an off shore breeze.
- during the night the air above the sea is warmer so it rises. it is is replaced by the cooler air from the land, creating an on shore breeze
Evaporation and Condensation...
Evaporation is when a liquid turns into a gas. It takes place when the most energetic liquid molecules escape to the air. This leaves the average kinetic energy less than before. Therefore the temperature of the liquid decreases. This means 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 across the liquids surface
Condensation is when a gas turns into a liquid. It often takes place on surface such as mirrors and windows.
The rate of condensation is increased by:
- increasing the surface area
- reducing the surface temperature
Energy Transfer by Design...
The rate of energy transfer depends on:
- The temperature between the object and its surroundings
- materials that the object is in contact with
- shape, size and material of the object
To maxmise the rate of energy transfer use things that:
- are good conductors, are painted dull black and have air flow around them maxmised.
To minimise the rate of energy transfer use things that:
- are good insulators, are white and shiny and that prevent convection currents by trapping air.
Specific Heat Capacity...
The Specific Heat Capacity of a substance is the amount of energy need to heat 1kg of that substance by 1C.
The formula is: E = m c 0
E - energy transferred ( J )
m - mass ( kg )
c - specific heat capacity ( J/kgC )
0 - temperature change ( C )
Heating and Insulating Buildings...
Heating and Insulating Buildings...
Most people want to minimise energy transfer from their homes to reduce fuel bills. This can be done by fitting:
- Fibreglass loft insulation to reduce energy transfer by conduction
- Cavity wall insulation to prevent convection currents (traps air in small pockets)
- Double glazing to reduce energy transfer by conduction
- Draught proofing to reduce energy transfer by cinvection
- Aluminium foil behind radiators to reflect infrered radiation back to a room
The U-value of a material tells us how much energy, per second, passes through it. The lower the U-value the better an insulator the material is.
Solar Heating panels contain water which is heated by infrared radiation from the sun. this water is then used to heat buildings or for domestic hot water. They are cheap to run but expensive to buy and install. Also they do not heat water at night. It is roughly 20 years payback time.