Assertion (A): Two parallel metal plates having charge +Q and -Q are facing at a distance between them. The plates are now immersed in kerosene oil and the electric potential between the plates decreases.
Reason (R ): Dielectric constant of kerosene oil is less than 1.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is True

Explanation

To analyze the assertion and reason provided in the question, let's break it down step by step.

### Step 1: Understand the Assertion
The assertion states that when two parallel metal plates with charges +Q and -Q are immersed in kerosene oil, the electric potential between the plates decreases.

### Step 2: Initial Capacitance in Air
Initially, when the plates are in air, the capacitance \( C_0 \) of the parallel plates can be expressed as:
\[ C_0 = \frac{\varepsilon_0 A}{d} \]
where:
- \( \varepsilon_0 \) is the permittivity of free space,
- \( A \) is the area of the plates,
- \( d \) is the distance between the plates.

### Step 3: Final Capacitance in Kerosene Oil
When the plates are immersed in kerosene oil, the capacitance \( C \) changes due to the dielectric constant \( k \) of the kerosene oil:
\[ C = k \cdot C_0 = \frac{k \varepsilon_0 A}{d} \]

### Step 4: Calculate Initial and Final Potential Difference
The potential difference \( V \) between the plates is given by:
\[ V = \frac{Q}{C} \]
- **Initial Potential Difference**:
\[ V_{\text{initial}} = \frac{Q}{C_0} = \frac{Qd}{\varepsilon_0 A} \]
- **Final Potential Difference**:
\[ V_{\text{final}} = \frac{Q}{C} = \frac{Qd}{k \varepsilon_0 A} \]

### Step 5: Compare Initial and Final Potential Differences
To find the relationship between the initial and final potential differences, we can take the ratio:
\[ \frac{V_{\text{final}}}{V_{\text{initial}}} = \frac{\frac{Qd}{k \varepsilon_0 A}}{\frac{Qd}{\varepsilon_0 A}} = \frac{1}{k} \]
This indicates that:
\[ V_{\text{final}} = \frac{V_{\text{initial}}}{k} \]
Since \( k > 1 \) (for most dielectrics), it follows that \( V_{\text{final}} < V_{\text{initial}} \). Thus, the electric potential decreases when the plates are immersed in kerosene oil.

### Step 6: Evaluate the Reason
The reason states that the dielectric constant of kerosene oil is less than 1. This is incorrect, as the dielectric constant of kerosene oil is actually greater than 1. Therefore, the assertion is true, but the reason is false.

### Conclusion
- **Assertion (A)**: True
- **Reason (R)**: False

### Final Answer
The assertion is true while the reason is false.

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