Year 12s: what will be the first way you research your future options? >>

Year 12s: what will be the first way you research your future options? >>

Year 12s: what will be the first way you research your future options? >>

Thermal conduction

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(edited 8 years ago)

ImageUploadedByStudent Room1434211344.584304.jpg97.1KB

Original post by goodwinning

ImageUploadedByStudent Room1434211344.584304.jpg

For a) I got 2.67 W and so converting to J/hr it's 9612 is this correct?

I get

J=\frac{kA\Delta\theta}{d}=\frac{0.4 \times 20 \times 10}{0.3} \approx 267

W.

Also, for b) I calculated the temperature at the interface to be 7.5

At thermal equilibrium, the power through both materials must be the same, so taking the temperature at the boundary as T, you can write:

\frac{0.4 \times 20 \times (10-T)}{0.3} = \frac{0.004 \times 20 \times (T-0)}{0.01}

which gives me

T=7.7^{\circ}

C

but I'm confused as to what the temperature gradients are? Is it just 2.5/0.1 for plaster and 7.5/30 for brick?

More or less, except you're not using SI units: 30 cm = 0.3 m

And to calculate the new heat loss do you have to find dq/dt, which is the same for each layer - I got 2 for this so is that it or is the heat loss 4 to account for both layers (as dq/dt is the same for each layer)?

I don't understand this entirely, but once you have T at the boundary, you can sub it back into one or other equation for the power, which is the same in each layer.

(edited 8 years ago)

OP

Thanks so much, you've been a big help :smile:

:smile:

Btw when I use T at the interface (7.7)to calculate the power I get:

0.4x20(10-7.7)/0.3 = 61.33

And

0.004x20(7.7-0)/0.01 = 61.6

Are those two values meant to be the exact same?

Posted from TSR Mobile

Original post by goodwinning

Thanks so much, you've been a big help :smile:

:smile:

Btw when I use T at the interface (7.7)to calculate the power I get:

0.4x20(10-7.7)/0.3 = 61.33

And

0.004x20(7.7-0)/0.01 = 61.6

Are those two values meant to be the exact same?

Posted from TSR Mobile

Yes - that's just a bit of rounding error.

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