Mutual inductance in a coil is caused by what phenomenon?

Mutual inductance occurs when a changing magnetic field generated by one coil induces an electromotive force (EMF) in another coil located nearby. This phenomenon relies on the principle of electromagnetic induction, where a variable magnetic flux, originating from the first coil, penetrates the second coil. As the magnetic field changes over time, it induces an electrical current in the second coil due to Faraday's Law of Electromagnetic Induction.

This induction is highly dependent on both the strength of the magnetic field and the rate at which it changes, making variable flux the key factor in mutual inductance. The geometric configuration and the materials used in the coils can also affect the efficiency of this induction, but it's the presence of changing flux that is fundamental to the phenomenon itself.

The other options do not adequately describe the process of mutual inductance. A static flux would not create any change to induce current in another coil. Resistance changes between coils pertain to electrical resistance rather than magnetic induction. Capacitive coupling relates to the interaction between electric fields rather than magnetic fields, which is not applicable in the context of mutual inductance.