temperature calculation

[iwonder]

I recently took a standardized test that posed a theoretical question. What is the sum of 50 degC and 50 degF? I know its probably simple but (I'm ashamed to say) it stumped me. Would someone please set me straight. iwonder

 

[nbucska]

Convert C to F by multiplying with 9/5 and adding 32.
<nbucska@pcperipherals.com>

 

[electricpete]

The &quot;sum&quot; of two temperatures is an ambiguous concept, since one might be tempted to add the two absolute temperatures, which does not correspond to any physical problem that I know of.

But if the &quot;sum&quot; is intended to convey the concept of a temperature rise above another temperature (usually an ambient), then it is straight forward.

50F ambient + 50C*(9F/5C) rise =140F
50C ambient + 50F*(5C/9F) rise =77.8c

Note those two answers are NOT the same temperature... as it should be (they represent different temperatures).

One answer that would almost always be wrong (for any physical problem involving a temperature rise or delta-T) would be to convert temperatures between F and C using the formula that involves 32 and then add them. For example you convert the 50C rise to 50C*(9F/5C)+32=122F and then try to add this to 50F ambient and you get 172F... a meaningless number. 50C rise does NOT corresponde to 122F rise.... the 32F zeroing factor is irrelevant when converting temperature rises.

I think the final answer is that the question as you have posed it is irrelevant. More info regarding the physical problem is needed to determine what you mean by the &quot;sum&quot; of two temperatures.

 

[iwonder]

The test was multiple choice a)102degF b)21degC c)77degF d)50degC. If you convert to degF and add you get 102, but if you convert to degC and add you get 21. How do you decide which temperature scale to use?

 

[nbucska]

Hi Pete:
Look posting above... <nbucska@pcperipherals.com>

 

[dydt]

hi iwonder..

is that the complete question? or maybe its a mixture of two materials and find the total temperature?

for example...i don't think you can add the two like adding a series resistor...say i have a 100 deg.C of water in a cup and another 50 deg. of another water in another cup...say of equal volume...mixing them doesnt mean i have 150 deg of water!

and no matter how i try...i cant get any of the selections they gave you..

so my answer?...is e...none of the above..

interesting!
dydt

 

[nbucska]

Hi dydt:
Not 150 bot 150/2 --- but you have to add.
<nbucska@pcperipherals.com>

 

[ggeng]

Hello iwonder,
This is not a simple question. I see it as a question to test your temp conversion knowledge and &quot;best logic&quot;. The question may not have had a correct answer, but may be looking for the &quot;best&quot; or closest answer of the group, as many test questions did that I wrote taking engineering at University. The logical solution would have been to convert,
50C = 122F and 50F = 10C, so the combinations are,
122+50 =172F or 50C +10C = 60C,but 172F does not equal 60C and also these were not choices. If I convert the 4 possible answers to celcius, I get, 38.88,21,25,50. Since the answer definitely cannot be less than 50 C, by elimination I would pick 50C? Hope you passed!
Gary

 

[iwonder]

Well I don't feel so ashamed now. Thanks for all the help and support. In case anyone was curious I did choose d)50 deg C. iwonder-(if I passed this test too!)

 

[electricpete]

Nick and ggeng - I stick by my answer that for any meaningful real-world problem, we have an initial/base/ambient temperature and a rise/differential/ delta temperature. When you convert the rise/differential/delta you can NOT use a formula that includes 32 in it.

For example if have a 50C rise above a 50F ambient the resulting temperature is NOT 50F +[50C*(9F/5C)+32F]=50F+122F=172F. It is 50F +[50C*(9F/5C)]=140F.

iwonder - I agree with dy/dt.... none of the above.