The transfer of heat by radiation is self evident if you stand outside on a sunny day. You can easily feel the warmth of the sun on your skin. Solar radiation is transmitted in the form of rays called infrared waves. The actual way that energy is transferred by waves is a bit complex. It's worth mentioning here that we usually think of heat as being transmitted in the radiation process, strictly speaking this is not true. The heat of a radiating body is transformed, at the instant of radiation, into radiant energy. The heat from the sun, for example, is transformed into radiant energy and travels in that form throughout space; it is retransformed into heat when it strikes an object.The only thing that is important is that you understand that the energy can be transmitted through empty space, a vacuum (well almost anyway). This is different then conduction or convection in that both of these forms use an existing medium to transfer heat.
Note: Since heat is energy, it's important to understand The Law of Conservation of Energy. In it's simplest form it states that; energy can never be created or destroyed only transferred.
Conduction is the ability of a material to allow the transfer of heat energy directly through it. How well a substance does this has to do with the electrical bonding of it's molecular structure. Substances with a 'loose" outer electron, conduct heat (and electricity) well. Metals are the best conductors of heat and electricity for this reason. We could use a spoon and hot cup of coffee to demonstrate this.... If you put the spoon in the cup and hold the handle you will slowly feel the handle "warming" up. It will eventually get quite hot! If a material possesses this ability it is commonly referred to has a conductor, if not, it is referred to has an insulator. Some good conductors are copper, brass and iron. Some good insulators are wood, cork, water and air. In homes that use steam or hot water heat baseboard or radiators, people sometimes confuse radiation as the method of heat transfer. This is not the case. The energy that is contained in the steam or hot water is transferred to the "radiators element" by conduction. These are usually made from cast-iron, copper or aluminum. How the heat gets from the radiator to warm a room is explained in convection.
The most common places for a convection current to occur is in masses of air (gases) and water (liquids). The medium, air or water, needs to come in contact with the heat source in order for the heat to be transferred. Lets take the radiator as an example. Once the immediate volume of air that surrounds the radiator makes contact, heat is transferred, you guessed it, by conduction. This air then begins to expand (the molecules of air move farther apart from another) and hence become less dense then the air around them.This air mass rises because colder air moves under and forces the column of warm air to rise. This colder air then makes contact with the radiator surface and heat is transferred once again, repeating the cycle. It's also important to remember that the expansion of air is a cooling process. As warm air rises it cools and "falls" to continue the current. This will continue as long as there is a heat source and some air.
+ نوشته شده توسط بهزادی در جمعه نوزدهم آذر ۱۳۸۹ و ساعت 13:45 |