Changing flux through a coil: When a permanent magnet enters a coil and a voltage is induced across the coil terminals, the phenomenon is referred to as what?

Introduction / Context:
Whenever magnetic flux linking a coil changes with time, an emf is induced across the coil according to Faraday’s law. This is the basic principle behind generators, pickups, and many sensors. A magnet moving into a coil is a classic illustration used in physics and electrical engineering courses.

Given Data / Assumptions:

• A permanent magnet is moved into a stationary coil.
• The magnetic flux linking the coil’s turns changes with time.
• We observe an induced voltage across the coil terminals.

Concept / Approach:
The induced emf magnitude is proportional to the rate of change of flux linkage: e = − N * dΦ/dt, where N is the number of turns and Φ is the flux per turn. The phrase “flux linkage” emphasizes that it is the linkage (N * Φ) changing in time that causes the emf. While terms like “counter emf” may apply to coils opposing changes in their own current, the more precise description for a moving magnet scenario is changing flux linkage producing electromagnetic induction.

Step-by-Step Solution:
Move magnet into coil → Φ through turns increases with time.Flux linkage λ = N * Φ therefore changes with time (dλ/dt ≠ 0).By Faraday’s law, an emf is induced: e = − dλ/dt.Hence, the observed voltage is due to changing flux linkage.

Verification / Alternative check:
Reverse the direction of motion or magnet polarity and observe the measured polarity reversal of the induced voltage—consistent with the sign in Faraday’s law and Lenz’s law.

Why Other Options Are Wrong:
An alternation: a vague term; not the specific phenomenon.Self-inductance: pertains to a coil’s own current change, not a magnet moving nearby.Counter emf: general term for emf opposing current change; less precise here than flux linkage induced by external motion.

Common Pitfalls:
Equating all induced voltages with “back emf.” Reserve “self-inductance” and “counter emf” for current-change scenarios within the same winding; use “changing flux linkage” for external magnetic changes.

Final Answer:
flux linkage (change in flux linkage causing electromagnetic induction)