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.

• 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.