AIIMS 2007 Physics Measuring Instruments Assertion Reason Question

To analyze the assertion and reason in the given question, let’s delve into the fundamental concepts pertaining to voltmeters and their operation in electrical circuits.

Assertion: Voltmeter is connected in parallel with the circuit.

This statement is true. A voltmeter is designed to measure the potential difference (voltage) across two points in a circuit. To accurately measure this potential difference without significantly altering the circuit, the voltmeter must be connected in parallel with the component or section of the circuit across which the voltage is to be measured.

Reason: Resistance of a voltmeter is very large.

This statement is also true. Voltmeters are designed with a very high internal resistance, typically in the range of megaohms (MΩ), to ensure that they draw minimal current from the circuit. This characteristic prevents the voltmeter from significantly affecting the circuit it is measuring. If a voltmeter had low resistance, it would allow a significant amount of current to flow through it, altering the voltage across the component being measured and leading to inaccurate readings.

Why the Correct Answer is A

Both the assertion and the reason are true, and the reason correctly explains why the assertion is true. The high resistance of the voltmeter allows it to be connected in parallel without affecting the total resistance of the circuit significantly, thus ensuring an accurate voltage measurement. Therefore, the correct answer is:

A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Clarification of Other Options

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

  • This option is incorrect because the reason provided directly supports the assertion. The high resistance of the voltmeter is indeed the reason why it can be connected in parallel without disturbing the circuit significantly.

• C) Assertion is true but Reason is false.

  • This option is incorrect because both the assertion and the reason are true. The assertion regarding the connection of the voltmeter in parallel is correct, and the reason about the high resistance is also correct.

• D) Both Assertion and Reason are false.

  • This option is incorrect as it contradicts the facts presented. Both statements are true in the context of how voltmeters function in electrical circuits.

Relevant Concepts

1. Ohm's Law: This law states that the current $I$ through a conductor between two points is directly proportional to the voltage $V$ across the two points and inversely proportional to the resistance $R$ of the conductor: $V = IR$

2. Ideal Voltmeter: An ideal voltmeter is one with infinite resistance, meaning it does not draw any current from the circuit, and thus does not affect the voltage across the points it measures.

3. Practical Consideration: In real-world applications, while voltmeters have very high resistance, they do not have infinite resistance. This is why they need to be used carefully to avoid loading effects, which could lead to inaccuracies in voltage readings.

In conclusion, the assertion and reason provided in the question are both valid, and the reason accurately explains why voltmeters must be connected in parallel. This understanding is crucial for students preparing for NEET/JEE, as it directly relates to the principles of current electricity and the use of measuring instruments within circuits.