Transfer of Heat:. If you have experience the campfire while camping, you have noticed that the heat is transferred from the campfire to you and you feel warm glow on your face. If an iron utensil is placed on the fire, it becomes hot. Even someone far away of about 20 feet,also feel the warmth of the fire. In each of these cases the transfer of heat from fire to the iron utensil ,to your face and the person 20 feet away has occurred.But the principle method of heat transfer involved in each was different . The heat transfer is relating to two things ,one is the temperature ,and the other is the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place. There are three basic methods to transfer the heat, conduction,convection and radiation.
Conduction:. Heat travelled from the end of the iron untensil to the handle placed on fire by the conduction. It is simple to understand that the iron utensil like other substances is made up of molecules. In this case mostly iron molecules vibrate more vigorously when placed on fire . These in turn ,strike adjacent molecules in the cooler part just outside the flame and causes them to vibrate more vigorously .This activity continues inch by inch. Till the handle also become hot. Conduction is method of heat transfer in which energy is transferred from molecule to molecule by collision. You burn your fingers when you touch a hot steel utensil (placed on fire),because the heat from the flame has started molecular activity through the hot steel utensil to your skin by conduction. Even the molecules in your skin vibrate energetically as they are heated . Since organs in your skin detect this vibration and send a special nerve message to your brain. Not all objects conduct heat equally well, because wood is a poor conductor of heat, we use it for the handles of pots and pans. In general, metals are better conductors than non metals.Liquids ,gases such as air and nonmetal solids, all are poor conductor of heat. Therefore designated as heat insulators and are used to shield our bodies or objects from heat to prevent the loss of heat .
Convection:. How was the man 20 feet away warmed by the camp fire? A layer of air directly over the fire was heated .The heat caused it to expand and thus made it lighter than the surrounding cool air. The heavier colder air around the base of the fire swept into the fire and pushed the warmer, lighter air up to the 20 feet away man. We have discussed earlier in this chapter that how the movement of air caused by the heating of sand in the beach areas. They actually cause the convection currents, which are also responsible for the heating of a room by the room heater. Warm air is heated by the room heater is pushed up and sweeps across a cooler part of the room. Cold air falls and moves towards the heater ,where it is heated.
Radiation:. In conduction heat transferred as molecule to molecule collision. In convection a whole volume of heated material gaseous or liquid circulates. The transfer of heat by radiation is quite different from the case .The transfer of heat from camp fire to your face is affected not by vibrating molecules or the circulating air but by a from of energy transfer called radiation. Radiatns are a most important method ,because it accounts for the heating of the earth by the sun. Obviously ,conduction and convection could not carry heat from the sun to the earth because most of the 93 miles between them is empty space, almost devoid of molecules . In addition ,there are ultra violent waves. Ultra Violet rays causes sun burn and tanning. These infrared rays are called heat rays, which are responsible for the heating of earth by the sun. Infrared rays themselves should not be thought of as heat .The space between the sun and the earth is not heated by these waves. Because there is practically nothing be heated. The rays might be compared to television waves originating from a broad cat station. These waves must be picked up by your TV set and convert it to picture. Similarly the infrared rays broadcast by the sun produce heat only when they strike and excite the molecules of substances. Infrared rays are invisible to the human eyes.
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