Question

In a certain process, $400\text{cal}$ of heat is supplied to a system and at the same time $105\text{J}$ of mechanical work was done on the system. The increase in its internal energy is

• A. $425\text{cal}$
• B. $375\text{cal}$
• C. $505\text{cal}$
• D. $295\text{cal}$

Answer 1

The correct option is A $425\text{cal}$

Given,
$\Delta Q=400\text{cal};\Delta W=-105\text{J}=-\frac{105}{4.2}=-25\text{cal}$
From 1st law,
$\Delta Q=\Delta U+\Delta W$
$\Rightarrow 400=\Delta U-25$
$\Rightarrow \Delta U=425\text{cal}$