If a second coil is placed in the field of the first coil that is connected to an AC source, the rising and collapsing magnetic field of the first coil will cut across the second coil and will induce an AC voltage in the second coil as well. This is known as ______ induction.

• A. Mutual
• B. Self
• C. Electrical
• D. Capacitive

Answer: (A)

Mutual induction is at work when a changing magnetic field from one coil induces voltage in a second coil placed in that field. Here, the first coil is fed by an AC source, so its current and magnetic field vary with time. The magnetic flux linking the second coil changes as the first coil’s field rises and falls, and Faraday’s law says that a voltage is induced in the second coil to oppose that change (Lenz’s law). The induced voltage in the secondary is proportional to the rate of change of the primary current, with the proportionality given by the mutual inductance M (v_secondary = M di_primary/dt). This transformer-like coupling is what characterizes mutual induction.

Self-induction would involve the coil’s own changing current inducing emf in itself, not in another coil. Capacitive effects involve electric field coupling, not magnetic coupling, and “electrical induction” isn’t the standard term for this phenomenon.