Eddy currents are circulating currents induced in a conducting material by a:

Eddy currents are indeed circulating currents that occur in conductive materials when they are subjected to a changing magnetic field. This phenomenon can be explained through Faraday's law of electromagnetic induction, which states that a change in the magnetic environment of a coil of wire will induce a voltage in the wire. If the conductive material forms a closed loop, the induced voltage creates circulating currents, known as eddy currents.

Eddy currents have practical implications in various applications, such as induction heating and non-destructive testing techniques. When a conductor is exposed to a varying magnetic field, the direction and magnitude of the induced currents can change, leading to the formation of these loops of electrical current within the material.

While the other options reference different types of electrical and magnetic phenomena, they do not accurately describe the conditions necessary for the formation of eddy currents. Specifically, piezoelectric effects and standing wave fronts are unrelated to the induction of currents through changing magnetic fields, and direct current would not induce circulating currents since it does not exhibit any variation. Thus, the relationship between a varying magnetic field and the generation of eddy currents makes that choice the correct answer.