Question

A gas absorbs 500 J heat and does 200 J work. Change in internal energy is:

• A. 300 J
• B. 500 J
• C. 700 J
• D. -300 J

Hint:

Think of it like a bank account: 500 J is "deposited" (heat absorbed), and 200 J is "spent" (work done). The balance remaining in the account is the change in internal energy (300 J).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question is a direct application of the First Law of Thermodynamics.
The first law is essentially a version of the law of conservation of energy. It states that energy can neither be created nor destroyed; it can only be converted from one form to another.
For a thermodynamic system, the total heat energy supplied to the system is used to increase the internal energy of the system and to do external work.
Key Formula or Approach:
The mathematical statement of the First Law is:
\[ \Delta U = Q - W \]
Where:
- \(\Delta U\) is the change in internal energy.
- \(Q\) is the heat added to the system.
- \(W\) is the work done by the system.
Sign Conventions:
- Heat absorbed (in) = +ve
- Work done by gas (expansion) = +ve
Step 2: Detailed Explanation:
From the problem statement, identify the given values:
Heat absorbed (\(Q\)) = +500 J.
Work done (\(W\)) = +200 J.
Substitute these values into the first law equation:
\[ \Delta U = 500 \text{ J} - 200 \text{ J} \]
\[ \Delta U = 300 \text{ J} \]
The internal energy of the system increases by 300 J. This usually manifests as an increase in the temperature of the gas.
Step 3: Final Answer:
The change in internal energy is +300 J.
This corresponds to option (A).