The word convection may produce slightly different but similar usages in different scientific or engineering contexts or purposes. The broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause. However, in thermodynamics “convection” often correlates specifically to heat transfer by convection.
Additionally, convection includes fluid movement both by bulk motion (advection) and by the action of individual particles (diffusion). However, in some cases, convection is taken to mean only advective phenomena. For instance, in the transport equation, which describes some different transport phenomena, terms are separated into “convective” and “diffusive” effects, with “convective” meaning purely advective in context.
Contents
What is convection?
Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool area, as opposed to the little transfer of heat between atoms involved with conduction. Suppose we consider heating up a local part of the air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. For reasons discussed in the previous section, being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces – this movement of hot air into a cooler region is then said to transfer heat by convection.
Heating a pot of water on a stove is an excellent example of the transfer of heat by convection. When the furnace is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually, the water starts bubbling – these bubbles are local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the more cooling water at the top by convection. At the same time, the more cooling, more dense water at the top will sink to the bottom, where it is subsequently heated. These convection currents are illustrated in the following figure.
View now two regions separated by a barrier, one at a higher pressure relative to the other, and subsequently remove the fence, as in the following figure. Certain convection currents are illustrated in the following figure.
When the barrier is removed, material in the high pressure (high density) area will flow to the low pressure (low density) area. If heating of the content created the low-pressure region, one sees that movement of material in this way is an example of heat flow by convection.
A famous example of convection currents that can be interpreted in this manner is the creation of breezes over land masses next to large bodies of water. Water has a larger heat capacity than land and subsequently holds heat better. It, therefore, takes longer to change its temperature, either upward or downward. Thus, during the day the air above the water will be cooler than that over the land. This creates a low-pressure area over the land, relative to the high-pressure area over the water, and subsequently one finds breezes blowing from the sea to the ground. On the other hand, during the night water cools off more slowly than the land, and the air above the water is slightly warmer than over the land. This creates a low-pressure area over the water relative to the high-pressure area over the ground, and breezes will blow from the land to the sea. These are illustrated in the following figure.
6 examples of convection
1 Boiling water – The heat passes from the burner into the pot, heating the water at the bottom. Then, this hot water rises and cooler water moves down to replace it, causing a circular motion.
2 Radiator – Puts warm air out at the top and draws in cooler air at the bottom.
3 Steaming cup of hot tea – The steam is showing heat being transferred into the air.
4 Ice melting – Heat moves to the ice from the air. This causes the fusion from a solid to liquid.
5 Hot air balloon – A heater inside the aircraft heats the air, and so the air moves upward. This causes the balloon to rise because the hot air gets trapped inside. When the pilot wants to descend, he releases some of the hot air, and cool air takes its place, causing the balloon to lower.
6 Frozen material thawing – Frozen food thaws more quickly under cold running water that if it is placed in water. The action of the running water transfers heat into the food faster.
What is the key difference between conduction and convection?
The substantial differences between conduction, convection are explained as under:
- Conduction is a process in which heat is transported between parts of a continuum, through direct physical contact. Convection is the principle, wherein currents transmit heat in a fluid, i.e., liquid or gas.
- Conduction shows, how heat is transferred between objects in direct contact, but Convection reflects how heat travels through liquids and gases.
- Conduction takes place as a result of the difference in temperature, i.e., heat streams from a high-temperature area to a low-temperature area. Convection happens due to the variation in density, such that the heat moves from the low-density region to the high-density region.
- Conduction usually occurs in solids, through molecular collision. Convection occurs in fluids by mass motion of molecules in the same direction.
- The transfer of heat is through the heated solid substance, in conduction, whereas in convection the heat energy is transmitted by way of an intermediate medium.
Why is the convection is important?
Without convection, the equator would stay smoldering hot, the poles would remain well in the deep freeze, and places in between would be in between. The currents move the warm air and warm water from the tropics up to the poles and move cold air and water from the poles down to the tropics. Until now, the coasts have enjoyed milder weather because of the convection currents.
Unfortunately, the currents are slowing down. The cold water of the polar caps helped push the presents along. Since they are not as severe, the flow has slowed, and the coasts are getting colder because it takes longer for the cold air and water to travel past on their way to the tropics. There is a theory that the currents will become so sluggish that a new ice age will happen. If it does, the currents will pick up speed again. We won’t live long enough to see it.
Global warming is misnamed since it does not mean everywhere will get warmer. The end effect is the global change. It also doesn’t matter a hoot in a holler whether we caused it or it’s a natural cycle. It’s here. We have to adapt–especially the people living on coastlines. Wyoming, thankfully, is high and dry and getting warmer.
