Final answer:
An alternating current is induced in a wire when it moves up and down in a magnetic field, due to Faraday's law of electromagnetic induction.
Step-by-step explanation:
When a wire moves up or down repeatedly in a magnetic field, an alternating current is induced in the wire. This phenomenon is described by Faraday's law, which tells us that a change in the magnetic environment of a coil of wire will cause a voltage (or emf - electromotive force) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The process of inducing a current by changing the magnetic field is utilized in power generation, where mechanical motion, such as the movement of water in a hydroelectric dam, is converted into electrical energy.
Instances of this include alternating current (AC), which is the type of electrical current produced by generators and utilized by most appliances. Unlike direct current (DC), where the electric charge flows in a single direction, AC electric charge changes direction periodically. The period of an AC waveform is called the cycle and is typically measured in hertz (Hz), indicating the number of cycles per second. When you move a magnet near a coil of wire, you are actually moving a magnetic field near the wire, which induces an alternating emf in the wire, and subsequently, an alternating current.