Question 33

[li5504552]

-why is the answer not convection? Thanks.

[mcat_premed3832]

In a pocket of air, you can have convection and you can have radiation but you can't have conduction. Conduction is the only one that requires physical contact.

[li5504552]

-but why is answer c convection wrong? Thanks.

[admin]

The question is asking which type of heat transfer do the air pockets limit. So we are looking for what does NOT work in the air pockets. Convection and radiation DO occur in the air pockets so they are the wrong answer.

[kazuma]

(not pertaining to the question, well sort of) radiation is just electromagnetic waves, so it does not require a medium. I just wanna clarify this concept thx

[admin]

[kbshah6109]

The question is asking what is "limited".
Conduction does NOT occur in air pockets, so how can nothing be limited.
Convection and Radiation DO occur, so one of these must be limited more than the other.

I would have chosen Conduction because I know that conduction cannot occur in an air pocket bc there is no physical contact between materials/molecules. I ruled out this choice because it does NOT occur, so I figured that how can something that doesnt exist be limited.
So I chose convection because it deals with the physical transfer of waves through a gas/fluid.

Is this question poorly worded - is what I'm thinking right?

thanks

[admin]

It is incorrect to say that air does NOT conduct, actually, air (after all, it does contain molecules in motion) is a very poor conductor.

"Air and water are relatively poor conductors": www.ucar.edu/learn/1_1_1.htm

"Some materials are better conductors of heat than others, metals are good conductors, air is poor.": www.aos.wisc.edu/~aos152/lesson1/content.html

[newno558362]

Hmm, I disagree with this answer

Heat transfer due to convection occurs when a fluid moves OVER a (warm) surface.... thus removing the heat energy with it.

Wearing a down filled jacket, in effect, creates a layer of air around you, thus limiting convection. Thats the point of the puffy jacket in winter.

Conduction still occurs immediately, to heat up the air around you or in the jacket. Generally, the small air pockets are limiting the FLOW of air.... again, limiting convection.


What if this question were the opposite? Without a down filled jacket, heat would be lost most through what? I'd say convection!

Heat loss through convection, is minimized through the jacket.
I think the answer to this question should be convection or at least both convection AND conduction should be correct.

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I don't know... please help

[ddd51192765]

I agree with newno558362.

The point of a jacket or blanket is that it prevents the air around you from moving, and eventually that air equilibrates with your body temperature - thus preventing CONVECTION. I think the right answer should have been D.

[nedaa.asba6809]

Going back to the definition of heat convection; it is the transfer of the thermal energy among neighbouring molecules in a substance. Conduction needs matter and not require any bulk motion of matter.
While Convection refers to the movement of molecules within fluids, and radiation describes any process in which energy emitted by one body travels through a medium or through space.
Then answer (D) can't be true. The only possible answer is (B) because unlike conduction, convection and radiation require a medium to transmit energy.

[ladywait223593]

nedaa - radiation does not require a medium to transmit energy, it can be transmitted through a vacuum. It might be best to think of these in their classical forms, stove top(conduction) vs CONVECTION oven vs Sun (radiation). Convection and conduction do require a "medium".

What types of heat transfer are involved in evaporation during sweating? I'm thinking all three are going on, maybe not all related to the process of evaporation but when a person is overheated all three are occurring. Any thoughts?

[mcat_premed3832]

ladywait of course you are correct that radiation requires no medium since it is electromagnetic radiation. And, if you have ever seen any of the Predator movies, you would have seen that humans can be tracked because we all emit infrared radiation.

And yes, convection and conduction are also means by which humans can interact with their environment including through sweating discussed more below.

But first I would like to address some earlier points: