Why Does Paper Char First Where It Covers Wood in a Composite Rod Experiment?

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Homework Statement


A composite rod is made if copper and wood joined together. a piece of paper is filled in close contact with both rods bear the junction of wood and copper. The rod is passed several times through a bunsen flame. What will be observed and why?
A: the portion of the paper in contact with the copper chars first because copper is a better conductor of heat.
B: the entire paper chars ay the same time because the rate of heat loss through copper and wood is equal.
C: the paper char first where is covers the wood because wood is not a good conductor of heat
D: the paper chars first where it covers the copper because wood is w bad conductor of heat.

Homework Equations



Q=mc(dT)

The Attempt at a Solution


The answer is C but I don't quite get it. Since copper has a lower c (mass is constant) so won't the dT be higher than of the wood since wood has a higher c than copper? So shouldn't the wood be cooler than copper? Thanks
 
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sgstudent said:

Homework Statement


A composite rod is made if copper and wood joined together. a piece of paper is filled in close contact with both rods bear the junction of wood and copper. The rod is passed several times through a bunsen flame. What will be observed and why?
A: the portion of the paper in contact with the copper chars first because copper is a better conductor of heat.
B: the entire paper chars ay the same time because the rate of heat loss through copper and wood is equal.
C: the paper char first where is covers the wood because wood is not a good conductor of heat
D: the paper chars first where it covers the copper because wood is w bad conductor of heat.

Homework Equations



Q=mc(dT)

The Attempt at a Solution


The answer is C but I don't quite get it. Since copper has a lower c (mass is constant) so won't the dT be higher than of the wood since wood has a higher c than copper? So shouldn't the wood be cooler than copper? Thanks

Hi sgstudent! :smile:

I don't think Q=mcdT is relevant here. Only the conductivities of copper and wood matter. The paper is covered on the outside junction of both the rods. So now, if you supply heat(Bunsen burn it) the part of the paper on near the copper will easily distribute its heat to the rest of the copper rod, and share the heat, so to speak. Where as, for the wooden part... :wink:
 
Infinitum said:
Hi sgstudent! :smile:

I don't think Q=mcdT is relevant here. Only the conductivities of copper and wood matter. The paper is covered on the outside junction of both the rods. So now, if you supply heat(Bunsen burn it) the part of the paper on near the copper will easily distribute its heat to the rest of the copper rod, and share the heat, so to speak. Where as, for the wooden part... :wink:

Hi Infinitum :smile:
It accumulates the heat right? So its hotter? It still doesn't make sense since they are both given the same energy Q and since the C of wood is higher so the dT will be higher? I don't quite understand why the copper will be cooler...
 
sgstudent said:
Hi Infinitum :smile:
It accumulates the heat right? So its hotter? It still doesn't make sense since they are both given the same energy Q and since the C of wood is higher so the dT will be higher? I don't quite understand why the copper will be cooler...

The flame tries to char the paper, not the temperature of the rods. Now when this flame is on the copper rod side of the system, the heat of the flame is quickly and easily spread out through the copper, hence it reduces the effect that the paper experiences. In case of wood, it rejects most of the heat given to it, meaning the paper has to face all the fury, and so, gets charred.