isotherm is a

Options

A. P-V curve at constant temperature
B. P-T curve at constant volume
C. V-T curve at constant pressure
D. P-V at constant internal energy

Answer

To solve the question "Isotherm is a...", we need to analyze the provided options and understand the concept of isotherms in thermodynamics.

### Step-by-Step Solution:

1. **Understanding Isotherm**:
- An isotherm is defined as a line or curve that represents points of equal temperature on a graph. In thermodynamics, it is specifically used to describe processes that occur at a constant temperature.

2. **Identifying the Correct Option**:
- We are given four options:
1. PV curve at constant temperature
2. PT curve at constant volume
3. VT curve at constant pressure
4. PV at constant internal energy

3. **Analyzing Each Option**:
- **Option 1: PV curve at constant temperature**: This option describes a graph where pressure (P) and volume (V) are plotted, and the temperature remains constant. This is indeed the definition of an isotherm, as it represents the relationship between pressure and volume at a constant temperature.
- **Option 2: PT curve at constant volume**: This option describes a graph of pressure versus temperature at constant volume. This does not represent an isotherm, as the temperature is not constant.
- **Option 3: VT curve at constant pressure**: This option describes a graph of volume versus temperature at constant pressure. Again, this does not represent an isotherm, as the temperature is not constant.
- **Option 4: PV at constant internal energy**: This option describes a graph of pressure versus volume at constant internal energy. This is not an isotherm either, as it does not involve constant temperature.

4. **Conclusion**:
- The only option that correctly defines an isotherm is **Option 1: PV curve at constant temperature**.

### Final Answer:
Isotherm is a PV curve at constant temperature.