Heat loss rate directionality/sign convention

  • Thread starter adammw111
  • Start date
  • #1
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Homework Statement


An 8.0m by 12m house is built on a concrete slab 23cm thick. What is the heat-loss rate through the floor if the interior is at 20°C while the ground is at 10°C?

Homework Equations



[itex]H=-k A \frac{\Delta T}{\Delta x}[/itex]

kconcrete = 1.0 W / m K (approx)

The Attempt at a Solution



[itex]H=\frac{-1.0 (8 * 12) (10-20)}{0.23} = 4.17[/itex] kW

My problem is that the book (Essential University Physics, Wolfson) has the answer as -4.17 kW, but that would imply that heat is being transferred from the ground to the house, which it clearly cannot be from the temperature of the house being larger than the ground's temperature.

I usually have problems with the sign of this value of H, how do I know what direction they are asking for by "through" an object - it could be either. Would either value be acceptable in an exam?

Thanks.
 

Answers and Replies

  • #2
374
1
Are you sure the sign on delta X is positive? What is your coordinate system for the horizontal slab? Is x=0 the 10 or 20 C surface?
 
  • #3
1,506
18
First of all you have given K to be 1.0W/mK. which means 1000W/K to be used in the equation . This gives the answer to the equation to be 4.17kW.
As to which direction, heat flows from hot to cold.
Sometimes worrying about the signs that crop up in equations can be confusing..... they are never 'wrong' but don't forget common sense.
 
  • #4
1,506
18
Sorry! My mistake, mK is correct I misread it as milliKelvin and not as m.K
Ignore my previous response
 

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