Liquids and gases are fluids; their particles are not fixed in place; they move about the bulk of the sample of matter. The model used for explaining heat transfer through the bulk of liquids and gases involves convection. Convection is the process of heat transfer from one location to the next by the movement of fluids.
The moving fluid carries energy with it. The fluid flows from a high temperature location to a low temperature location. To understand convection in fluids, let's consider the heat transfer through the water that is being heated in a pot on a stove. Of course the source of the heat is the stove burner. The metal pot that holds the water is heated by the stove burner. As the metal becomes hot, it begins to conduct heat to the water. The water at the boundary with the metal pan becomes hot.
Fluids expand when heated and become less dense. So as the water at the bottom of the pot becomes hot, its density decreases. Differences in water density between the bottom of the pot and the top of the pot results in the gradual formation of circulation currents. Hot water begins to rise to the top of the pot displacing the colder water that was originally there. And the colder water that was present at the top of the pot moves towards the bottom of the pot where it is heated and begins to rise.
These circulation currents slowly develop over time, providing the pathway for heated water to transfer energy from the bottom of the pot to the surface. Convection also explains how an electric heater placed on the floor of a cold room warms up the air in the room. Air present near the coils of the heater warm up. As the air warms up, it expands, becomes less dense and begins to rise. As the hot air rises, it pushes some of the cold air near the top of the room out of the way.
The cold air moves towards the bottom of the room to replace the hot air that has risen. As the colder air approaches the heater at the bottom of the room, it becomes warmed by the heater and begins to rise.
Once more, convection currents are slowly formed. Air travels along these pathways, carrying energy with it from the heater throughout the room. Convection is the main method of heat transfer in fluids such as water and air.
It is often said that heat rises in these situations. The more appropriate explanation is to say that heated fluid rises. For instance, as the heated air rises from the heater on a floor, it carries more energetic particles with it. As the more energetic particles of the heated air mix with the cooler air near the ceiling, the average kinetic energy of the air near the top of the room increases. This increase in the average kinetic energy corresponds to an increase in temperature.
The net result of the rising hot fluid is the transfer of heat from one location to another location. The convection method of heat transfer always involves the transfer of heat by the movement of matter. This is not to be confused with the caloric theory discussed earlier in this lesson. In caloric theory, heat was the fluid and the fluid that moved was the heat. Our model of convection considers heat to be energy transfer that is simply the result of the movement of more energetic particles.
The two examples of convection discussed here - heating water in a pot and heating air in a room - are examples of natural convection. The driving force of the circulation of fluid is natural - differences in density between two locations as the result of fluid being heated at some source. Some sources introduce the concept of buoyant forces to explain why the heated fluids rise.
We will not pursue such explanations here. Natural convection is common in nature. The earth's oceans and atmosphere are heated by natural convection. In contrast to natural convection, forced convection involves fluid being forced from one location to another by fans, pumps and other devices.
Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations. This is an example of forced convection. The movement of the fluid from the hot location near the furnace to the cool location the rooms throughout the house is driven or forced by a fan.
Some ovens are forced convection ovens; they have fans that blow heated air from a heat source into the oven. Some fireplaces enhance the heating ability of the fire by blowing heated air from the fireplace unit into the adjacent room. This is another example of forced convection. A final method of heat transfer involves radiation.
Radiation is the transfer of heat by means of electromagnetic waves. To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin. Finally I found what I was looking for long ago!!! God Bless!!! Email Us: Blastinfo theblast. Our Other Stations. Read More. Blast Backs. Poker Night 2. Companion Cube Pin. Quantum Conundrum.
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