Heat transfer unit

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  • Created by: Youngjean
  • Created on: 01-04-16 10:42

Thermal radiation

Thermal radiation travels by waves.Thermal radiation is energy tranfer by electromangetic waves.
Every object emits thermal radiation.The hotter the object the more thermal radiation it emits.

The sun emits radiation in all parts of the electromagentic spectrum. The Earth's atomsphere blocks most of the radiation,such as ultraviolet rays.Which would reallt harm us.
Gases in the atomsphere such as water vapour,methane, carbon dixode, they trap thermal radiation given off from the Earth. The particular gasses make the Earth warmer.

But the Earth is becoming too warm.It will cause seas level to rise making it hard for the use of fossil fuel.

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Surface and radiation

Some subtstances are better than other at absoring radiation.

Dark,matt surfaces are good at emtting and aborsbing radiation. This show why solar panles are black because the solar panel can absorber heat quicker and better.

However on the other hand,

white light, shiny surfaces are poor at absoring and emtting heat. Despite this they are very goood reflectors which is good for reflecting heat. This is why a lot of thermos flask ask coated with a shiny white layer to reflect heat and keep your drink at the same temp.

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Conduction

Conduction mainly occurs in metals due to the free electrons which travel within the metal.
-Metals conduct heat better than non-metals.
-Copper is a better conductor of heat than steel.
-Wood conducts heat better than glass.
Conduction in metals.
When a metal is heated, the free electrons at the hot end gain kinetic energy and move faster.These electrons diffuse and collide with other free electrons and ions in the cooler parts of the metal.So energy is transferred everywhere in the metal.
In non metallic soild, all the electrons are held in the atom. Energy transfer only take place because the atoms vibrate and shake each other. This much less effective than energy transfer by free electrons. This is why metals are better conductors.Materials such as firbeglass are good insulators because they contain pockets of trapped air.

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Convection.

Convection only takes place in liquids and gases(fluids).Convection occurs whenever we heat fluids. A fluid is either a gas or a liquid.

How convection works.
Only happens in fluids. The circulation are because the fluids rise where they are heated(Heating makes them less dense). Then eventually fall where they cool down (cooling makes them more dense). Convection currents transfer thermal energy from hotter parts to cooler parts.

The reason fluid rise is because fluids expand when they are heated, this is because the partilces move more , taking up more space.Therefore the denity decreases because the same mass of fluid occupies a bigger volume. So heating a part of a fluid makes it less dense and thefore rises.

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Heating transfer by design

 vaccum flask.(http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCSE/bytesize%20images/reduceHeatLoss1.gif)

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