Convection/Conduction

[AA|FCB|DOC]

---

Members don't see this ad.

Can anyone better describe these two to me. I seem to understand them when I look at the concept, but when given a real world example where I have to distinguish between the two sometimes I get confused. For example, in a convection oven the coil heats up the cool air which is the heated and carries the heat via fans to the food and gives it off to the food. My question is that is the first step where the coil molecules are cooliding with the air molecules, is that convection or conduction? I know the physical carrying of the heat via the air is convection for sure, but is the heating of the air by the coil also convection or is it conduction? Thanks in advance

 

[dudewheresmymd]

Can anyone better describe these two to me. I seem to understand them when I look at the concept, but when given a real world example where I have to distinguish between the two sometimes I get confused. For example, in a convection oven the coil heats up the cool air which is the heated and carries the heat via fans to the food and gives it off to the food. My question is that is the first step where the coil molecules are cooliding with the air molecules, is that convection or conduction? I know the physical carrying of the heat via the air is convection for sure, but is the heating of the air by the coil also convection or is it conduction? Thanks in advance

In a convection oven, the coil heats up the cool air (you have collisions between coil molecules and air molecules) --> energy transfer dependent on the heat that travels from the heating element to the air because a medium (air) is involved and because the heating element is not directly touching your food in the oven, thus it is not considered conduction. It is most likely considered convection or radiation (depending on the oven used..either the heating element is emitting IR waves that are absorbed by the food or heating up the air molecules via convection that will then cook the food by convection as well, you could also consider that the food is being "cooked by conduction," but since air is such a poor insulator...this is most likely negligible.

Two more examples:

In an electric heater, there is an electrical resistor in the heater that basically heats up (avg ke ~ temperature)...as more electrons flow through the resistor, the resistor heats up, and this heat is dissipated into the air via convection

In a radiative heater, the heating element emits IR radiation that is absorbed by surrounding elements in the environment-->example of radiation in which the waves do not need a medium to propagate to transfer heat

In conclusion, for conduction to occur you need collisions between molecules and physical CONTACT between two things (ie.pan directly on the electrical stove heating element), for radiation to occur (you need usually E&M radiation to be absorbed by the thing that's being heated) and for convection to occur (you need a fluid (air or liquid) that transfers heat from one object to another object).

Hope this helps.

See:
http://img257.imageshack.us/img257/2626/heattransfermodesanalog.gif

and

http://www.washingtonpost.com/wp-dyn/content/article/2007/03/27/AR2007032700665.html

 

[AA|FCB|DOC]

thank you for the post! Just to make sure again, so in the article is says that the air is a poor conductor of heat as you also mentioned in your post. I guess that is what was confusing me. So the entire process of the air molecules heating up, flowing, and distributing the heat to the food would be considered convection? Would it be safe to say then that conduction only happens between solid objects, since this case there is "direct contact" between the resistor and air but we are still calling it convection. Because I definitely understand that the fluid medium is carrying the heat over to the food which makes sense as convection. I am just trying to make sure if we break down the process into steps as some MCAT questions sometimes do, then the first step of the resistor heating up the air is still convection and NOT conduction, correct? Thanks again