Tuesday, February 14, 2012

Transmission of heat energy explained

How does heat energy travel through a metal?

  If a steel poker is pushed into the fire and left there for a time the handle becomes warm. Heat travels through the metal by a process called conduction.

  This process is complex. It differs between metals and non-metal, and only a brief explanation can be attempt here.
  When a metal is heated the free electrons which it contains begin to move faster, i.e., their kinetic energy increases. The hot electrons then drift towards the cooler parts of the metal and at the same time there is a drift of slower-moving ( cooler) electrons in the reverse direction.
  To a much less extent, heat energy is transmitted through a metal by vibrations of the atoms themselves which pass on energy from one to the other in the form of waves. These waves are of very high frequency and are transmitted in tiny energy packets called "phonons".
  In non-metals which have no free electrons heat energy is conducted entirely by phonons.

Comparison between good and bad conductors of heat

  Most metals are good conductors of heat; silver and copper are exceptionally good. on the other hand, substances such as cork, wood, cotton and wool are bad conductors. Both good and bad conductors have their uses. The "bit" of a soldering iron is made of copper, so that when its tip is cooled through contact with the work, heat is rapidly conducted from the body of the bit to restore the temperature of the tip and maintain it above the melting-point of solder.

Some applications of bad conductors

  Bad conductors have a very wide application. Beginning with our own personal comfort, we prevent loss of heat from ourselves by a covering of poorly conducting material. Textiles are bad conductors of heat, since they are full of tiny pockets of air enclosed by the fibres of the material. Air, in common with all gases, is a very bad conductor of heat. It is usual to say that wool is warmer than cotton. Technically, of course, we imply that it has a lower thermal conductivity than cotton.

Stone versus carpet. Which one is colder?

  A stone floor feels cold to the bare feet, but a carpet on the same floor feels warm. This difference arises from the fact that stone is a better conductor of heat than a carpet.
  To begin with both the stone floor and the carpet are at the same temperature. This may be verified by placing a thermometer in contact with each in turn. Since the feet are warmer than either, heat tends to flow from the feet. Stone, being the better conductor, conveys heat away from the feet more rapidly than the carpet. Consequently, the feet feel cold on the stone but warm on the carpet.
  Precisely the same effect is experienced when handling a garden fork in winter. The iron part of the fork feels cold, but the wooden handle warm.

Lagging: To reduce loss of heat by conduction

  Loss of heat by conduction through the walls of an oven is reduced by constructing it with double walls. The space between is packed with slag wool or glass fibre. These substances are not only very poor conductors but also have the merit of being non-inflammable. Material of low thermal conductivity used for the purpose of preventing heat loss is called lagging. Another example is the covering of hot-water storage tanks and pipes with a layer of plaster mixed with asbestos. Similarly, cold-water pipes are lagged with strips of felt or sacking to prevent freezing during very cold weather.

Ignition point of a gas: Gauze experiment - Davy safety lamp


     An inflammable gas will burn if its temperature reaches a value known as the "ignition point". The effect of a good conductor in the neighbourhood of a flame can be shown by placing a wire gauze about 5 cm above a Bunsen-burner. If the gas is turned on and lighted underneath the gauze it is found that the flame does not pass through the gauze. The wires of the gauze conduct the heat of the flame away so rapidly that the hot gases passing through the gauze are cooled below the ignition temperature.

  The gas is now turned out and after the gauze has cooled, the gas is again turned on and lit above the gauze. This time the flame continues to burn above the gauze. As in the previous case, the wires conduct heat rapidly away, with the result that the temperature of the gas in contact with the underneath surface of the gauze is not raised to its ignition point. The flame will pass through the gauze only if it should become red hot. As we shall now show, this experiment illustrates the principle of the Davy safety lamp.

The miner's safety lamp

  The enormously increased output of coal for industrial purposes towards the end of the eighteenth century brought with it a corresponding increase in the number of fatal mine accidents. An inflammable gas called methane or fire-damp is often found in coal-mines. This, when mixed with the air of the mine, exploded when it came into contact with the naked flames of the candles which, at that time, were used for illumination.

  In 1813 a society was formed to study methods for preventing these explosions, and Sir Humphry Davy was approached for advice. Davy investigated the problem and eventually found a remedy in the safety lamp. In its original form this consisted of a simple oil burner completely surrounded by a cylinder of wire gauze. The gauze, however, threw undesirable shadows, and later a thick cylindrical glass window was added, still keeping the gauze above, but encased in a brass shroud to protect it from damage.
  Should the atmosphere surrounding the lamp contain methane, its presence will be indicated by the flame becoming surrounded by a bluish haze. This is caused by the methane burning when it comes into contact with the flame. The flame cannot extend beyond the gauze and cause an explosion, since the wires of the gauze rapidly conduct the heat away. The temperature of the gauze, therefore, never rises to the ignition point of the gas-air mixture in the mine.
  Although Davy lamps have long been replaced by electric lamps they will always be remembered as an important application of science in the interests of human safety.