17 It is possible to think of the flow of electrons in a wire as the flow of water in a pipe. If we open the valve on one end to let water into the pipe, water flows to the end of the pipe because the incoming water pushes what is already in the pipe towards the end. All these measuring units get their names from the scientists who discovered the phenomena they measure. Search the Internet for information about Amp re, Volta, and Ohm. DC AND AC CURRENT What is electric current? Electric current is electric charge in motion produced by a source such as a generator, a battery, a solar cell, or a fuel cell. To start electric current in a wire, electrons must be pushed into it by some force : the force that pushes electrons is called potential difference or voltage, and it is measured in volts; the amount of current flowing in a wire is measured in amp res. While flowing in a material, current always meets a certain amount of opposition from the material itself. This opposition to the flow of current is called resistance and it is measured in ohms . How does current flow? Electrical current flows in two ways: The rate at which alternating current changes direction is called frequency and is measured in Hertz (Hz). The direction of the current is reversed 60 times per second in the USA, 50 times per second in Europe. in a single direction, which is the case of Direct Current, or DC. In this case, electrons move steadily forward from the negative to the positive pole of the source; the resulting current is in the opposite direction; swiftly changing its direction, as happens with Alternating Current, or AC. In this case, electrons move back and forth, continuously reversing the direction of the current . DC current DC current is mainly generated by batteries or solar cells: they allow the current voltage to remain relatively constant, so that electrons can steadily follow their route. This type of current is mainly used in portable devices such as mobiles and laptops, because they have to store power to be used off the mains. When their batteries need recharging, adapters are used to convert high-voltage AC taken from a domestic socket to the low voltage DC needed by the equipment. AC current AC current is delivered to buildings along transmission lines . It is produced by means of a generator, which consists of a coil of wire rotating in a magnetic field. When the coil rotates in the magnetic field, a potential difference is induced in one direction; as the rotation goes on, the induced potential difference reverses its direction, and an alternating current is produced. UNDERSTANDING ELECTRICITY back and forth: avanti e indietro coil: bobina fuel cell: cella carburante mains: rete elettrica to reverse: invertire socket: presa steadily: costantemente swiftly: velocemente 52 elecTriciTy