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P2: Energy transfer by heating (2.2: Infrared radiation (Infrared waves…
P2: Energy transfer by heating
2.1: Energy transfer by conduction
Thermal conductors are materials that allow thermal energy to move through them easily - all metals are good thermal conductors
Thermal insulators are materials that don't allow thermal energy to move through them easily - materials such as fibreglass and wool are good thermal insulators because they trap air
The rate of thermal energy transfer through a material depends on
The temperature difference across the material
The thickness of the material
The thermal conductivity of the material
To reduce the rate of thermal energy transfer, the insulating material used should
Have as low a thermal conductivity as possible
Be as thick as possible
2.2: Infrared radiation
Infrared waves are part of the electromagnetic spectrum - they're the waves with wavelengths a little longer than the wavelengths of visible red light
We can detect infrared radiation with our skin - it makes us feel warm
All objects emit infrared radiation
The hotter an object is, the more infrared radiation it emits in a given time
An object at constant temperature emits infrared at the same rate as it absorbs it
A perfect black body is an object that absorbs all the radiation that hits it - it's also the best emitter of radiation - the radiation emitted by a perfect black body is called black body radiation
An object that has a constant temperature emits radiation across a continuous range of wavelengths
The intensity of the radiation an object emits has a peak at a certain wavelength - this peak wavelength depends on the temperature of the object
2.3: More about infrared radiation
All objects absorb and emit infrared radiation
An object at constant temperature absorbs and emits infrared radiation at the same rate
If an object absorbs infrared radiation and a greater rate than it emits it, the temperature of the object increases
If an object absorbs infrared radiation at a lesser rate than it emits it, the temperature of the object decreases
An object with a light, shiny outer surface emits radiation at a lesser rate than an object with a dark, matt surface
The earth receives light and infrared radiation from the sun
some of this radiation is reflected back into space
some is absorbed by the earth's atmosphere
some is absorbed by the earth's surface
The temperature of the earth depends on
The rate at which radiation from the sun is reflected or absorbed by the Earth's atmosphere, and the Earth's surface
The rate at which radiation is emitted from the Earth's surface, and from the Earth's atmosphere, into space
2.4: Specific heat capacity
When you heat a substance, you transfer energy to it - this increases its temperature
The specific heat capacity of the substance is the amount of energy required to raise the temperature of 1kg of the substance by 1 degree Celsius
The greater the specific heat capacity of a substance, the more energy you would need to supply to the substance in order to increase its temperature by 1 degree Celsius
For a given substance, a greater mass requires more energy to increase its temperature by 1 degree celsius
Energy needed (J) = mass (Kg) x specific heat capacity (J/Kg degrees celsius) x temperature change (degrees celsius)
2.5: Heating and insulating buildings
Whichever way a house is heated, people want to minimise the rate of energy transfer from their homes in order to reduce their fuel bills
Fibreglass loft insulation reduces the rate of energy transfer through the roof
Cavity wall insulation reduces the rate of energy transfer through the outer walls
Aluminium foil behind radiators reflects infrared radiation back into the room
Double glazing reduces the rate of energy transfer through the windows
Thick bricks with low thermal conductivity reduce the rate of energy transfer through the exterior walls
Solar panels absorb infrared radiation from the sun
Solar cell panels generate electricity directly
Solar heating panels heat water directly