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Physics Question 36 – JEE-MAIN 2025

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Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : Magnetic monopoles do not exist. Reason (R) : Magnetic field lines are continuous and form closed loops. In the light of the above statements, choose the most appropriate answer from the options given below :

Recall Gauss's Law for Magnetism and the properties of magnetic fields.

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Ninja Strategy•Fundamental Concepts Check

Verify the correctness of both Assertion and Reason based on fundamental physics principles, then assess their causal relationship.

Step 1: Evaluate Assertion (A)✦ Active

Assertion (A) states that magnetic monopoles do not exist. This is a fundamental principle in classical electromagnetism, often expressed by Gauss's Law for Magnetism, which states that the net magnetic flux through any closed surface is zero (∮B→⋅dA→=0). This implies that isolated magnetic North or South poles cannot exist. Therefore, Assertion (A) is correct.

Step 2: Evaluate Reason (R)○ Expand

Reason (R) states that magnetic field lines are continuous and form closed loops. This is also a correct statement. Magnetic field lines originate from the North pole and terminate at the South pole outside a magnet, but they continue from the South pole to the North pole inside the magnet, forming continuous closed loops. They do not have starting or ending points like electric field lines do on charges.

Step 3: Determine if (R) explains (A)○ Expand

The fact that magnetic field lines form continuous closed loops (Reason R) is a direct consequence and explanation for the non-existence of magnetic monopoles (Assertion A). If magnetic monopoles existed, magnetic field lines would originate from a North monopole and terminate at a South monopole, similar to electric field lines originating from positive charges and terminating at negative charges. The closed-loop nature of magnetic field lines signifies the absence of isolated sources or sinks for the magnetic field, hence no magnetic monopoles. Thus, (R) is the correct explanation for (A).

💡 Teacher's Secret Hint

Remember that electric field lines originate and terminate on charges, while magnetic field lines always form closed loops.

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