A: The oil-drops of Millikan's experiment should be microscopic size.
R: For larger drops the electric fields needed in the experiment will be impractically high.

Options

A. If both Assertion & Reason are true and the reason is the correct explantion of the assertion , then mark (1)
B. if both Assertion & Reason are true but the reason is not the correct explantion of the assertion , then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the assertion and reason problem regarding Millikan's oil drop experiment, we will analyze both statements step by step.

### Step 1: Understanding the Assertion
The assertion states that "The oil-drops of Millikan's experiment should be microscopic size."

- In Millikan's oil drop experiment, the objective was to measure the charge of an electron by observing the behavior of tiny oil droplets that are electrically charged.
- The size of these oil droplets must be microscopic to ensure that they can be suspended in the air by an electric field without falling due to gravity.

### Step 2: Understanding the Reason
The reason states that "For larger drops, the electric fields needed in the experiment will be impractically high."

- When an oil drop is charged and placed in an electric field, it experiences an upward electrostatic force that counteracts its weight (downward force due to gravity).
- The relationship between the electric field (E), mass (m), and charge (q) of the droplet can be expressed as:

\[ E = \frac{mg}{q} \]

- Here, \(m\) (mass) is related to the volume of the droplet and its density (\(\rho\)):

\[ m = \rho \cdot V \]

- As the volume of the droplet increases, the mass increases, which in turn increases the required electric field to balance the gravitational force. This means that for larger droplets, the electric field required becomes impractically high, making it difficult to conduct the experiment effectively.

### Step 3: Conclusion
Both the assertion and reason are true. The assertion is correct because microscopic oil droplets are necessary for the experiment to work effectively. The reason is also true because larger droplets would require an impractically high electric field to balance their weight, making the experiment unfeasible.

### Final Answer
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.

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