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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Physics > Conduction and convection

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Contents 1 Conduction 2 Examples of conduction 3 Convection 4 Example of convection 5 Comments

Conduction

• In metals, the dominant method of conduction is through the movement of electrons. This method of conduction does not operate in non-metals because there are no free electrons (other than graphite). When a metal is heated, the electrons closest to the heat source vibrate more rapidly. Electrons then collide with these atoms and gain more kinetic energy (movement energy). The electrons therefore move around faster and collide with other free electrons which then gain more kinetic energy. Kinetic energy is therefore transferred between the electrons and through the metal from the point closest to the heat source towards points further away. The electrons all travel very short distances but are very fast moving therefore conduction of heat happens very quickly.
• In metals and in insulators, there is conduction of heat due to the vibration of the atoms. As atoms closest to the heat source absorb heat/thermal energy, they make their neighbouring atoms vibrate more rapidly which then in turn make their neighbouring atoms vibrate more.

Examples of conduction

• The wire gauzes used on tripods are metal therefore they are good heat conductors. Gauzes on cookers are also metal so that heat is conducted quickly and food is cooked fast.
• Poor thermal conductors (insulators) are used for saucepan handles so that they don't heat up and can still be handled.
• Metals are used for the containers which heat liquids e.g. pans, kettles on hobs
• Air is a poor conductor therefore materials that trap air are used for insulation in lofts and hot water cylinders.

Convection

• The cool particles gain kinetic energy when they are heated from the source and expands as it heats up. The particles become less dense than the surrounding cold air therefore it rises and displace the cool air. Cool particles are more dense therefore they fall and move towards the heat source to take the place of the warm particles. They then heat up and rise while other particles cool down and fall.

Example of convection

• Convection is used in fridges to cool it down. Heat is carried away, therefore the back of fridges are always warm.
• Land & sea breezes are due to convection
• Atmospheric winds.
• Hot water systems.

Comments