Unit Question  [ENDORSED]

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Unit Question

Postby Teddy_Rosenbluth » Sun Jan 22, 2017 2:46 pm

In some of the textbook questions, I end up getting L*atm and the answer in the book is in J. What are the units that pressure and volume need to be in for the work equation?

Zulfiqar Lokhandwala 1H
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Re: Unit Question

Postby Zulfiqar Lokhandwala 1H » Sun Jan 22, 2017 3:00 pm

The equation for work is work=P*deltaV and the units that come from that is
P*deltaV= kg*m^-1*s^-2(m^3)= kg*m^2*s^-2 which equates to Joules
So if im not mistaken, the units for work should come out in Joules

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Re: Unit Question  [ENDORSED]

Postby Christina_F_3F » Sun Jan 22, 2017 3:38 pm

I had the same question, Teddy. I think that depending on the units of pressure and volume given, you may have to use a conversion to get to joules/kilojoules. If the pressure is given in kilograms per meter and the volume in meters cubed per second squared, then you don't really need to do any conversion because the units will cancel to give joules/kilojoules.
However, pressure and volume are not always given with these units. For example, if you are given liters for volume and atmospheres for pressure, you will need to convert the answer you get in L*atm into joules by multiplying by 101.325 J/1 L*atm to get an answer in joules (I got this number from page 275 of the textbook -- and on page 180, problem 8.9 of the solutions manual, it is suggested that this conversion is found by using the equivalency of the ideal gas constants, 8.314 J*K*mol^-1 and 0.08206 L*atm*K^-1*mol^-1. This makes sense because once you cancel out the units you are left with joules).
In short, I think that the unit for work is joules, and if you are given units that do not equate right away to joules, then you need to find a conversion factor to get to joules (whether or not these values will be provided, however, I have no clue).
I hope this helps! Let me know if my response is too confusing.

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Re: Unit Question

Postby samuelkharpatin2b » Sun Jan 22, 2017 8:38 pm

By definition, 1 L*atm is equivalent to 101.3 Joules. Units for both work (w) and heat (q) should either be in either Joules (J) or kilo-Joules (kJ).

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Re: Unit Question

Postby Cris-1C » Sun Jan 22, 2017 8:49 pm

If you end up getting L*atm as the units for your final answer, then I believe you are using the correct units for both the Volume and Pressure since volume is expressed in atm and volume in L. However, work is recorded as Joueles, meaning a conversion of units will be required after you obtain the answer in L*atm. After looking at some of the examples in the book, 1 L = 101.325 J, in which the L*atm answer must be multiplied by (101.325 J / 1 L*atm). In doing so, the units of L*atm cancel out and you are left with Joueles as your final answer. I hope this helps!

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