A thick conducting slab is introduced between the plates of an isolated charged capacitor to fill the free space partially and without touching any plate. Which of the following is wrong statement regarding the points inside the capacitor

Options

A. Electric field at some points will change and at some points remains unchanged
B. Electric field is zero at all the points
C. Electrostatic energy density at some points will change and that at some points remains unchanged
D. Total electrostatic energy of the capacitor will decrease

Answer

To solve the problem, we need to analyze the situation of a thick conducting slab placed between the plates of an isolated charged capacitor. We will evaluate the statements regarding the electric field and electrostatic energy in this configuration.

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have an isolated charged capacitor with two plates: one positively charged and the other negatively charged.
- A thick conducting slab is placed between the plates, filling part of the space but not touching either plate.

2. **Effect of the Conducting Slab**:
- The conducting slab will induce charges on its surfaces due to the electric field created by the capacitor. The inner surface of the slab will acquire a charge opposite to that of the plate it faces, while the outer surface will acquire a charge equal to that of the plate it faces.

3. **Electric Field Inside the Conducting Slab**:
- Inside a conductor, the electric field is always zero. Therefore, the electric field at points within the conducting slab itself will be zero.

4. **Electric Field in the Capacitor**:
- Outside the conducting slab, the electric field in the regions of the capacitor (above and below the slab) will remain unchanged. The presence of the conductor does not affect the electric field in these regions.

5. **Electrostatic Energy Density**:
- The electrostatic energy density is related to the electric field. Since the electric field is zero inside the conducting slab, the energy density will also be zero in that region. However, the energy density outside the slab remains unchanged.

6. **Total Electrostatic Energy of the Capacitor**:
- The total electrostatic energy of the capacitor is given by the formula \( U = \frac{1}{2} C V^2 \). When the conducting slab is introduced, it effectively changes the capacitance \( C \) of the system. Since the capacitor is isolated, the charge \( Q \) remains constant, and the potential \( V \) decreases as the capacitance increases. Thus, the total energy \( U \) will decrease.

7. **Identifying the Wrong Statement**:
- The wrong statement among the options provided is that the electric field is zero at all points inside the capacitor. This is incorrect because while the electric field is zero inside the conducting slab, it is not zero in the regions outside the slab.

### Conclusion:
The wrong statement regarding the points inside the capacitor is that the electric field is zero at all points inside the capacitor.