Question

Two identical coaxial coils P and Q carrying equal current in the same direction are brought nearer. The current in:

• A. P increases while in Q decreases
• B. Q increases while in P decreases
• C. Both P and Q increase
• D. Both P and Q decrease

Answer 1

The Correct Option is D

When two identical coaxial coils P and Q are carrying equal currents in the same direction and are brought closer together, they interact through their magnetic fields. The principle at play here is Lenz's Law, which states that the direction of the induced electromotive force (emf) is such that it opposes the change in magnetic flux that produced it. 

As the coils are brought closer, the magnetic flux linkage through each coil increases due to the presence of the other's magnetic field. To oppose this increase in magnetic flux, the system will act to reduce the current flowing in each coil. This is due to Lenz's Law, which effectively reduces the magnetic interaction by decreasing the currents.

The correct choice, in this scenario, is that both P and Q decrease in current. This is consistent with induced electromagnetic effects seeking to counteract changes in their magnetic environment:

• The mutual inductance between the coils increases as they approach each other.
• This increased mutual inductance results in a greater opposing emf, reducing the net current in each coil.

Therefore, when such an interaction occurs, the result is both P and Q experience a decrease in current.

Answer 2

When two identical coaxial coils carrying equal current in the same direction are brought closer:

Key Physics Principles:

1. Mutual Induction: As the coils move closer, their mutual inductance increases.
2. Lenz's Law: The induced current opposes the change that produced it.
3. Energy Conservation: The system resists the change in configuration.

What Happens to the Current:

• As coil P approaches coil Q, the changing magnetic flux through Q induces an opposing current in Q
• Simultaneously, the changing flux through P induces an opposing current in P
• This mutual induction effect reduces current in both coils
• The closer the coils, the stronger this effect becomes

Therefore, the current in both P and Q decreases when they are brought nearer.

Visualization:

Coil P Current (→) → Induced field in Q opposes (←) → Q's current decreases
Coil Q Current (→) → Induced field in P opposes (←) → P's current decreases