# heat transfer

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#1

in this question 15 the answer is D and in question 12 answer is D.Why cant conduction be a means of heat transfer for electric grill whereas conduction is a mean of heat transfer from warm air to the sheet of ice on the river.Please explain this to me bcz I am confused.These seem to contradict one another bcz in Q 15 radiation only is the means of heat transfer whereas in Q12 its both variations nd conduction.i eman shouldnt conduction also be a means of heat transfer from electric grill to bread even though it will be very slow.Please help me out here.
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2 months ago
#2
(Original post by Manahil01)

in this question 15 the answer is D and in question 12 answer is D.Why cant conduction be a means of heat transfer for electric grill whereas conduction is a mean of heat transfer from warm air to the sheet of ice on the river.Please explain this to me bcz I am confused.These seem to contradict one another bcz in Q 15 radiation only is the means of heat transfer whereas in Q12 its both variations nd conduction.i eman shouldnt conduction also be a means of heat transfer from electric grill to bread even though it will be very slow.Please help me out here.
The difference is that in 12 the air is physically in contact with the ice and this enables conduction. There needs to be contact for conduction to happen.
In 15 the heating element is not physically in contact with the bread. So no conduction.
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#3
But the air particles are
in contact with the heating element and these air particles do come in contact with the bread wouldnt that be conduction aswell?
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2 months ago
#4
(Original post by Manahil01)
But the air particles are
in contact with the heating element and these air particles do come in contact with the bread wouldnt that be conduction aswell?
In practice, no, because air does not really conduct heat. What happens with air and other fluids is convection, where the particles physically move and carry the heat. As the heating element is above the toast, you don't get transfer by convection because the hot air particles will rise (convection current) and move away from the bread. In the case of the ice, the warmer air particles are actually in contact with the ice.

I will admit though that this question is a little bit concerning, in that the situation is not as clear cut as they would like it to be, and you are right to question this.
Last edited by Stonebridge; 2 months ago
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