Question

When a system is taken from a state $i$ to $f$ along the path $iaf$ (as shown in the figure). $Q=50cal$ and $W=20cal$; along path $ibf$, $Q=36cal$
(i) What is $W$ along path $ibf$?
(ii) If $W=13$cal for path $fi$, what is $Q$ for the path $f_i$?
(iii) Take $E_{int,i}=10cal$ then what is $E_{int,f}$?

• A. $30,20,40cal$
• B. $6,-43,40cal$
• C. $10,-20,30cal$
• D. $15,35,25cal$

Answer 1

The correct option is B $6,-43,40cal$

For path $iaf$
$Q=50cal$
$W=20cal$
According to Law of thermodynamics, $dQ=dU+dW$
or $dU=dQ-dW=50-20=30cal$

(i) For path $iaf$
$Q=36cal$
$W=?$
$dU=30cal$ (since internal energy depends only on the initial and final positions of the system)
$\therefore$ $W=Q=dU=36-30=6cal$

(ii) $W=-13cal$
$dU=-30cal$
$Q=?$
$\therefore$ $Q=dU+W=-43cal$

(iii) $E_{int,f}=E_{int,i}+\Delta U=10cal+30cal=40cal$