I\'m working on some physics problems having to do with thermodynamics, all of w
ID: 1266373 • Letter: I
Question
I'm working on some physics problems having to do with thermodynamics, all of which ask me to find Q (the transfer of heat). To do this, I have to use specific energy in the formula. Q=mass x specific heat x change in temperature. The problem is, I don't know in which instances to use specific energy measured in kcal/kg*degreesC or in J/kg*degrees C. I don't want to rely on the units used for the rest of the formula (e.g. Q is in kcal so specific heat must also use kcal). I want to understand the principle behind the use of these different units so I won't accidentally slip up on them during my exam, so if anyone could shed some light on this, I would really appreciate it.
Explanation / Answer
Yes, all units must be consistent.
There are only two basic instances. The first, and most common, is...
Q measured in Joules, mass measured in kg (or sometimes grams), and specific heat will be measured in J/kg C (or J/g C) (Temp always must be in Celsius)
Just keep units consistant in the solution... Joules must match Joules, kg matches kg, and g matches g.
Mainly, if Q is in Joules, always use specific heat with the Joule
The second instance is Q measured in calories (or kilocalories). Calories and kiloCalories are basically the same with the metric prefix, but...
If Q is measured in cal or kcal, then specific heat must match at cal/g C or kcal/g C or kcal/kg C.
Again, always match units.
One further thing, the conversion between Joules and calories is 4.184 J per cal or 4184 J per kCal.
You should aready know grams to kg.
Hope this helps. If you have any further questions please ask. I used to teach Physics and can help further if needed.