When a wire is moved downward between the poles of a magnet, a potential difference, or voltage, is induced across the ends of the wire. This phenomenon is known as electromagnetic induction and is governed by Faraday's law of electromagnetic induction.
Faraday's law states that a changing magnetic field induces an electric field, which in turn leads to the generation of a potential difference or voltage. In the case of a wire moving through a magnetic field, the magnetic field lines are being cut by the wire, causing a change in the magnetic flux through the wire.
As the wire moves downward, the magnetic field lines from the magnet are being cut by the wire in a perpendicular manner. This cutting of magnetic field lines results in a change in magnetic flux through the wire. According to Faraday's law, this change in magnetic flux induces an electric field within the wire.
The induced electric field causes charges within the wire to move, creating a potential difference or voltage. This potential difference drives the flow of electric current in the wire if there is a closed circuit.
Therefore, the motion of the wire in a magnetic field creates a changing magnetic flux, which induces an electric field and subsequently a potential difference across the wire's ends. This phenomenon is the basis for various applications such as generators and electric power generation.