Internal pressure of a perfect gas (ideal gas) is:

Options

A. Zero
B. infinite
C. Calculated by `PV=nRT`
D. Can not say directally

Explanation

**Step-by-Step Solution:**

1. **Understanding Internal Pressure**: Internal pressure is defined as the change in internal energy (U) with respect to volume (V) at constant temperature. Mathematically, it can be expressed as:
\[ \pi = \left( \frac{\partial U}{\partial V} \right)_T \]

2. **Characteristics of an Ideal Gas**: An ideal gas is a theoretical gas composed of many particles that are in constant random motion and do not interact with each other except during elastic collisions. This means that there are no intermolecular forces acting between the gas molecules.

3. **Internal Energy and Volume**: For an ideal gas, the internal energy (U) is a function of temperature (T) only and is independent of volume (V). This implies that:
\[ U = U(T) \]
Therefore, the change in internal energy with respect to volume at constant temperature is zero:
\[ \left( \frac{\partial U}{\partial V} \right)_T = 0 \]

4. **Conclusion on Internal Pressure**: Since the change in internal energy with respect to volume is zero, we can conclude that the internal pressure of an ideal gas is:
\[ \pi = 0 \]

5. **Final Answer**: The internal pressure of a perfect gas (ideal gas) is **0**.

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