Question

When a system is taken through the process $abc$ shown in the figure, $90\text{J}$ of heat is absorbed by the system and $40\text{J}$ of work is done by it. If the system does $30\text{J}$ of work during the process $adc$, how much heat flows into it during the process?

• A. $70\text{J}$
• B. $80\text{J}$
• C. $50\text{J}$
• D. $60\text{J}$

Answer 1

The correct option is B $80\text{J}$

From $1^{st}$ law of thermodynamics.
$\Delta Q=\Delta U+\Delta W$
$Q_{ab}+Q_{bc}=(U_c-U_a)+W_{abc}$
$\Rightarrow Q_{abc}=(U_c-U_a)+W_{abc}$
$\Rightarrow 90=(U_c-U_a)+40$
$\Rightarrow U_c-U_a=50\text{J}$
Now for process $adc$,
$Q_{adc}=(U_c-U_a)+W_{adc}$
$=50+30$
$=80\text{J}$
So $80\text{J}$ of heat flows into the system.