convection, conduction

[Oh_Gee]

from GS-5 PS

"Down-filled winter clothing reduces heat losses by incorporating pockets of air into the material. Which type of heat transfer process do the air pockets limit?

a Radiation
b Conduction
c Convection
d Conduction and convection

answer in white ink. highlight below to read:
Correct Answer: B

Air is a poor conductor of heat. Both radiation and convection typically need air, or some other fluid, as a medium to transmit energy/heat.


i thought it would be C because the air pockets are stopping warm air from leaving. what do you guys think?

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[Cawolf]

I made the correct choice before viewing and agree with the explanation. I don't really understand what you mean by "stopping warm from leaving". All forms of heat transfer involve the movement of energy and changes in temperature.

 

[Oh_Gee]

I made the correct choice before viewing and agree with the explanation. I don't really understand what you mean by "stopping warm from leaving". All forms of heat transfer involve the movement of energy and changes in temperature.

sorry i meant warm air. could you explain your thought process for this question?

 

[Cawolf]

Sure, I just thought about what was changing. So introducing air pockets means that instead of there being skin/fabric/outside there is now skin/air/fabric/outside.

So how does this change things? The heat now has to pass through air to reach the fabric.

Radiation - This is not a significant source of heat loss, but the surface of the coat prevents EM waves from transferring heat from our skin - this is not changing by adding the air pockets.

Convection - Heat loss by the movement of air - also not changed as we are still insulated the same amount from air currents.

Conduction - Transfer of heat through mediums in direct contact as dictated by the thermal conductivity of the material. Air is a notoriously poor conductor, and now the heat has to conduct through the air to reach the outer jacket layer, where before it only had to conduct through the jacket layer.

This makes choice B the best answer.

 

[Oh_Gee]

Sure, I just thought about what was changing. So introducing air pockets means that instead of there being skin/fabric/outside there is now skin/air/fabric/outside.

So how does this change things? The heat now has to pass through air to reach the fabric.

Radiation - This is not a significant source of heat loss, but the surface of the coat prevents EM waves from transferring heat from our skin - this is not changing by adding the air pockets.

Convection - Heat loss by the movement of air - also not changed as we are still insulated the same amount from air currents.

Conduction - Transfer of heat through mediums in direct contact as dictated by the thermal conductivity of the material. Air is a notoriously poor conductor, and now the heat has to conduct through the air to reach the outer jacket layer, where before it only had to conduct through the jacket layer.

This makes choice B the best answer.

so because of the air pockets, its harder for the heat from our bodies to get through the jacket to the outside environment?

 

[Cawolf]

Yes - the air acts as a boundary of sorts. Heat conducts through material at known levels and air is notoriously poor at conducting. So the heat leaves the skin slower than it would just against the material of the jacket - because it must pass through the air pockets first.

This question is a matter of knowing what each of the terms mean, and then seeing if the proposed change has an effect. As I laid out in the list, conduction is the only possible change. So you don't have to known that air has a low thermal conductivity to successful answer this question, if you eliminate the wrong choices.

 

[Oh_Gee]

Yes - the air acts as a boundary of sorts. Heat conducts through material at known levels and air is notoriously poor at conducting. So the heat leaves the skin slower than it would just against the material of the jacket - because it must pass through the air pockets first.

This question is a matter of knowing what each of the terms mean, and then seeing if the proposed change has an effect. As I laid out in the list, conduction is the only possible change. So you don't have to known that air has a low thermal conductivity to successful answer this question, if you eliminate the wrong choices.

for radiation, does our body emit EM waves?

 

[Cawolf]

Yes. The skin is a highly effective thermal radiator, and could therefore lose heat in a vacuum.

We lose heat in the IR spectrum, but hotter materials (ie "red hot" metals) lose heat in the visible spectrum.

If you are interested in more I would suggest reviewing the Stefan-Boltzmann Law - though that is way beyond the scope of the MCAT.