(adsbygoogle = window.adsbygoogle || []).push({}); Thermodynamics--work, heat, & internal energy

1. The problem statement, all variables and given/known data

When a system is taken from state i to state f along path iaf in Figure 18-41, Q = 60 cal and W = 20 cal. Along path ibf, Q = 51 cal.

Fig. 18-41 (see attatched)

(a) What is W along path ibf?

(b) If W = -13 cal for the return path fi, what is Q for this path?

(c) If E_int,i = 7 cal, what is Eint,f?

(d) If E_int,b = 18 cal what is Q for path ib?

(e) For the same value of E_int,b, what is Q for path bf?

2. Relevant equations

[tex]\Delta E = Q - W[/tex] and is path independant

3. The attempt at a solution

(a) [tex]\Delta E_{iaf} = \Delta E_{ibf}[/tex]

hereon, iaf will be abbreviated a, and ibf will be abbreviated b.

[tex] Q_a - W_a = Q_b - W_b[/tex]

[tex] Q_a - W_b + Q_b = -W_b [/tex]

[tex] 60-20+51=-W_b=91[/tex]

So W_b = -91 cal. Wrong. And yes the problem wants the answer in calories.

On the chance that they're giving work performedonthe system rather than work performedbythe system, I changed Q-W to Q+W, and got that W_b=29 cal, which is still wrong.

I used the same exact reasoning for part b (and got it wrong).

I got part C correct [47 cal] using the fact that DeltaE is path independent, and Delta E = final energy - initial energy.

Parts d and e I just have no idea where to begin. So I guessed 18 cal for both. Surprise! Both wrong...

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Thermodynamics-work, heat, & internal energy

**Physics Forums | Science Articles, Homework Help, Discussion**