Which of the following laws of thermodynamics defines the term internal energy?

Options

A. Zeroth law
B. First law
C. Second law
D. Third law

Answer

To determine which law of thermodynamics defines the term internal energy, we can analyze each of the four laws and their implications regarding energy.

### Step-by-Step Solution:

1. **Understanding the Laws of Thermodynamics**:
- **Zeroth Law**: Establishes the concept of temperature and thermal equilibrium but does not define internal energy.
- **First Law**: States that energy cannot be created or destroyed, only transformed. It relates heat transfer (ΔQ), work done (ΔW), and internal energy (ΔE).
- **Second Law**: Deals with the direction of processes and the concept of entropy, not internal energy.
- **Third Law**: States that as temperature approaches absolute zero, the entropy of a perfect crystal approaches a constant minimum, unrelated to internal energy.

2. **Focusing on the First Law**:
- The First Law of Thermodynamics is mathematically expressed as:
\[ \Delta Q = \Delta E + \Delta W \]
- Here, ΔE represents the change in energy of the system, which includes internal energy (U), kinetic energy (K), and potential energy (P).

3. **Neglecting Kinetic and Potential Energy**:
- In many thermodynamic processes, especially when considering systems at constant volume or when the motion is negligible, we can neglect changes in kinetic energy and potential energy.
- Thus, we can simplify the equation to:
\[ \Delta E = \Delta U \]
- This leads to the modified First Law equation:
\[ \Delta Q = \Delta U + \Delta W \]
- From this, we can conclude that internal energy (U) is defined as part of the energy change in the system.

4. **Conclusion**:
- Since the First Law of Thermodynamics directly relates to internal energy through the equation mentioned, we can conclude that the correct answer is the **First Law of Thermodynamics**.

### Final Answer:
The law of thermodynamics that defines the term internal energy is the **First Law of Thermodynamics**.