Question:
Two identical coaxial coils \(P\) and \(Q\) carrying equal amount of current in the same direction are brought nearer. The current in
Two identical coaxial coils \(P\) and \(Q\) carrying equal amount of current in the same direction are brought nearer. The current in
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When flux linkage increases, induced current acts to oppose the increase. This is the direct application of Lenz's law.
Updated On: May 5, 2026
- \(P\) increases while in \(Q\) decreases
- \(Q\) increases while in \(P\) decreases
- both \(P\) and \(Q\) increases
- both \(P\) and \(Q\) decreases
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The Correct Option is D
Solution and Explanation
Concept:
This question is based on mutual induction and Lenz's law. When two current-carrying coils are brought nearer, the magnetic flux linked with each coil due to the other coil changes. According to Lenz's law, the induced current opposes the change in magnetic flux.
Step 1: Understand the initial condition.
Two identical coaxial coils \(P\) and \(Q\) carry equal current in the same direction. Since current is in the same direction, the magnetic fields produced by the two coils support each other.
Step 2: Effect of bringing the coils nearer.
When the coils are brought closer, the magnetic flux of one coil linked with the other coil increases. So, for each coil: \[ \text{Magnetic flux linkage increases} \]
Step 3: Apply Lenz's law.
Lenz's law says that induced current will oppose the change that produces it. Here, the change is an increase in magnetic flux. So the induced current in each coil will oppose this increase. This opposition reduces the effective current in both coils.
Step 4: Decide the change in current.
Since the induced effect opposes the original current in both coils, the current in both coils decreases. Therefore: \[ I_P \downarrow,\quad I_Q \downarrow \]
Step 5: Check the options.
Option (A) is incorrect because both coils are affected symmetrically.
Option (B) is incorrect for the same reason.
Option (C) is incorrect because induced current opposes the increase in flux.
Option (D) is correct because currents in both coils decrease. Hence, the correct answer is: \[ \boxed{(D)\ \text{both }P\text{ and }Q\text{ decreases}} \]
This question is based on mutual induction and Lenz's law. When two current-carrying coils are brought nearer, the magnetic flux linked with each coil due to the other coil changes. According to Lenz's law, the induced current opposes the change in magnetic flux.
Step 1: Understand the initial condition.
Two identical coaxial coils \(P\) and \(Q\) carry equal current in the same direction. Since current is in the same direction, the magnetic fields produced by the two coils support each other.
Step 2: Effect of bringing the coils nearer.
When the coils are brought closer, the magnetic flux of one coil linked with the other coil increases. So, for each coil: \[ \text{Magnetic flux linkage increases} \]
Step 3: Apply Lenz's law.
Lenz's law says that induced current will oppose the change that produces it. Here, the change is an increase in magnetic flux. So the induced current in each coil will oppose this increase. This opposition reduces the effective current in both coils.
Step 4: Decide the change in current.
Since the induced effect opposes the original current in both coils, the current in both coils decreases. Therefore: \[ I_P \downarrow,\quad I_Q \downarrow \]
Step 5: Check the options.
Option (A) is incorrect because both coils are affected symmetrically.
Option (B) is incorrect for the same reason.
Option (C) is incorrect because induced current opposes the increase in flux.
Option (D) is correct because currents in both coils decrease. Hence, the correct answer is: \[ \boxed{(D)\ \text{both }P\text{ and }Q\text{ decreases}} \]
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