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Assertion: A thin aluminium disc, spinni...

Assertion: A thin aluminium disc, spinning freely about a centre pivot, is quickly brought to rest when . placed between the poles of a strong U-shaped magnet
Reason:A current induced in a disc rotating in a magnetic field produces a force which tends to oppose the disc's motion.

A

A. both Assertion and Reason are both are both wrong

B

B. Assertion is right, Reason is wrong

C

C. both Assertion and Reason are correct and Reason explains Assertion

D

D. both Assertion and Reason are correct, but Reason does not explain Assertion

Text Solution

AI Generated Solution

The correct Answer is:
To solve the question, we need to analyze the assertion and the reason provided: **Assertion**: A thin aluminium disc, spinning freely about a center pivot, is quickly brought to rest when placed between the poles of a strong U-shaped magnet. **Reason**: A current induced in a disc rotating in a magnetic field produces a force which tends to oppose the disc's motion. ### Step-by-Step Solution: 1. **Understanding the Assertion**: - The assertion states that when a thin aluminium disc is placed in a magnetic field created by a strong U-shaped magnet, it comes to rest quickly. This implies that there is a significant interaction between the disc and the magnetic field. **Hint**: Consider how magnetic fields interact with conductive materials. 2. **Understanding the Reason**: - The reason explains that when the disc rotates in the magnetic field, an electric current is induced in the disc. This is due to electromagnetic induction, which occurs when a conductor moves through a magnetic field. **Hint**: Recall Faraday's law of electromagnetic induction, which states that a changing magnetic field within a closed loop induces an electromotive force (emf). 3. **Induced Current and Eddy Currents**: - The induced current in the disc is known as an eddy current. Eddy currents are loops of electric current that are induced within conductors by a changing magnetic field in the conductor. They flow in closed loops within the conductor and create their own magnetic fields. **Hint**: Think about how the motion of the disc in the magnetic field creates a change in magnetic flux. 4. **Application of Lenz's Law**: - According to Lenz's Law, the direction of the induced current will be such that it opposes the change that produced it. In this case, the change is the motion of the disc. Therefore, the eddy currents generated in the disc will create a magnetic field that opposes the disc's rotation. **Hint**: Remember that Lenz's Law is a consequence of the conservation of energy. 5. **Conclusion**: - Since the induced current creates a magnetic field that opposes the motion of the disc, it results in a torque that acts against the rotation of the disc. This opposing force causes the disc to come to rest quickly. **Hint**: Consider how forces acting in opposite directions affect motion. ### Final Answer: Both the assertion and the reason are true. The reason correctly explains the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason explains the assertion.
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