Problem 21 - Master Resource Book in JEE Main Physics

Question

Question Nos. 21 to 30 are Assertion-Reason type. Each of these contains two Statements: Statement 1 (Assertion), Statement Il (Reason). Each of these questions also has four alternative choice, only one of which is correct. You have to select the correct choices from the codes (a), (b), (c) and (d) given below (a) If both Assertion and Reason are true and the Reason is correct explanation of the Assertion (b) If both Assertion and Reason are true but Reason is not correct explanation of the Assertion (c) If Assertion is true but Reason is false (d) If Assertion is false but the Reason is true Assertion While operating Wheatstone bridge [PO box], in starting, the key of the battery is closed first and the key of the galvanometer is closed later and when the circuit is to be switched off then switches are released in the reverse order. Reason This is done to avoid the damage of galvanometer due to induced emf.

Step-by-Step Solution

Answer

Option (a): Both statements are true and the reason correctly explains the assertion.

Step 1: Understand the Assertion

The assertion states that while operating a Wheatstone bridge, the key of the battery is closed first and the key of the galvanometer is closed later, and the reverse order is followed while switching off the circuit. This process is a practical method followed to ensure the protection of certain components in the circuit.

Step 2: Understand the Reason

The reason provided is that this procedure is followed to prevent damage to the galvanometer due to induced electromotive force (emf). Induced emf can occur due to sudden changes in current, potentially damaging sensitive instruments like galvanometers.

Step 3: Analyze the Relationship Between Assertion and Reason

We need to establish if the sequence of operations (assertion) is genuinely to protect the galvanometer from induced emf (reason). The procedure described in the assertion does serve the purpose of safeguarding the galvanometer, supporting the reason, because closing the battery key first stabilizes the circuit before introducing the galvanometer, thereby minimizing any potential current spikes or induced emf.

Step 4: Choose the Correct Option

Since both the assertion and the reason are true, and the reason correctly explains the assertion, the correct choice is (a).

Key Concepts

Assertion-Reason Questions

Assertion-reason questions are a unique and challenging format used in various academic assessments. These questions test your logical reasoning and understanding of scientific concepts. Typically, you are given two statements: an assertion and a reason. Your task is to determine the validity of each statement and whether the reason provides a correct explanation for the assertion.

In the context of the Wheatstone Bridge, the assertion is about the sequence of operations when switching the circuit on and off. The reason attributed to this sequence is related to the protection of the galvanometer from induced electromotive force (emf). To solve a question like this, check if both statements are true first. Then, assess if the reason directly explains why the assertion operates the way it does.

The answer options usually range from both being true and the reason explaining the assertion, to scenarios where either the assertion or reason is false. This requires critical thinking to evaluate each part separately before understanding their interrelation:

• If both statements are true and the reason explains the assertion, you'll choose the option confirming that the relationship holds.
• If both are true but independent, you'll note that the reason doesn't explain the assertion.

Galvanometer Protection

A galvanometer is a sensitive device used to measure small currents in circuits, such as those in a Wheatstone Bridge. Because it is delicate, protecting it during circuit operations is crucial. The circuit's starting and stopping sequences, as described in our assertion, are specifically designed to shield the galvanometer.

When you first close the battery key, you allow the circuit to stabilize before introducing the galvanometer. This prevents sudden current changes from affecting it. As galvanometers are finely tuned, any abrupt surge can lead to their damage or malfunction. By opening the galvanometer key only after the circuit settles, you reduce potential spikes.

Upon switching off, the reverse order ensures any lingering current in the circuit dissipates without the galvanometer being directly exposed. This methodical approach helps in avoiding any mishap, thereby prolonging the life of the instrument and ensuring accurate readings.

Effective galvanometer protection involves:

• Minimizing exposure to current surges and induced emf.
• Following a specific sequence during operation and shutdown.

Induced Electromotive Force

Induced electromotive force (emf) is a voltage generated by changes in magnetic fields or current flows within a circuit. In our exercise, the sudden changes in current when turning the Wheatstone Bridge on or off could create this induced emf. This occurs due to rapid alterations in circuits, which can produce unexpected voltages.

The phenomenon of induced emf is governed by Faraday's Law of Electromagnetic Induction, which states that a change in magnetic flux through a circuit induces a voltage in the circuit. While crucial in numerous applications, this induced emf can pose risks to sensitive devices like galvanometers if not managed properly.

Understanding induced emf involves:

• Recognizing its potential to create unwanted current spikes.
• Implementing measures to prevent harm to circuit components.
• Designing circuits that mitigate its effects, like following specific switching sequences.

By recognizing when and how induced emf occurs, you can effectively design procedures (such as our assertion-reason format suggests) that protect sensitive circuit elements, ensuring both safety and accuracy.