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Theories and Laws

Heat

When an object is heated above the temperature of its surroundings it will lose heat to the surroundings.

Heat is transferred in three ways.

1.   CONDUCTION

  • If one end of a metal bar is heated then heat is transferred by conduction to the cold end.
  • Good electrical conductors such as copper and gold are  good conductors of heat.
  • Poor electrical conductors, such as wood and paper, are poor heat conductors.
  • Heat can be conducted between two objects if they are in close contact.
  • For example between a soldering iron and a soldering terminal; or between a power transistor and its heatsink.

2.   CONVECTION

  • Here, heat is transferred by the movement of a gas or a liquid.
  • Hot air rises and cold air falls. Liquids behave in a similar manner.
  • A hot resistor causes convection, transferring heat from the resistor to the surrounding air.
  • Hot water in a pan rises to the top while the cold water  falls to the bottom.
  • These movements are called convection currents (nothing to do with electric currents).
  • The above process is called NATURAL CONVECTION.
  • If a fan is used to aid convection it is called FORCED CONVECTION.

3.   RADIATION

  • This does not need a gas or liquid to transfer the heat.
  • Heat is expelled  mostly in the form of infrared radiation.
  • This is a form of light and travels at the speed of light.
  • It can travel through a vacuum. 
  • This is why we can feel the heat of the sun even though it has to travel through the vacuum of space to reach earth.
  • Polished surfaces are poor radiators but good reflectors of heat. That is why electric fires have shiny reflectors.
  • Black objects are good radiators.

4.  THE EFFECTS OF HEAT

  • Heat causes solid objects to expand.
  • That is why they have gaps in railway lines and bridges to allow for summertime temperatures.
  • Different metals expand at different rates.
  • A temperature switch can be made from two strips of dissimilar metals fixed together.
  • As the temperature increases, one strip grows longer than the other, causing the strips to curve. This in turn breaks (or makes) a circuit.

Heat

  • Increasing temperatures also cause liquids to expand. This behaviour  is used in the thermometer.
  • Gases also expand with temperature increases.

5.   HEAT AND ELECTRONICS

  • Heat is one of the biggest enemies of electronics, causing components to fail.
  • To minimise the effects some action can be taken.
  • Increasing the surface area increase convection and radiation. High wattages resistors are larger than low wattage ones.
  • Using holes and louvres in the casing increases natural convection.
  • Using fans provides forced convection.
  • Using heat sinks with fins increases surface area thus providing increased convection and radiation.
  • Painting heat sinks blacks increases radiation.
  • Using "heat sink compound", which is a good conductor, between transistors and their heatsinks, improves heat conduction.
  • Fitting components onto the metal chassis aids the dissipation of heat.

Keywords : Heat, Science, Electronic, Circuits, Tutorials, beginner
Writer : delon  |
20 Oct 2006 Fri   
|  7.665 Views
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