Volt: the volt unit that measures energy per charge
A car battery with an electric potential of 12 volts can provide 12 joules of energy for each coulomb of charge delivered to its starter motor. That relationship identifies the volt unit: one joule per coulomb. Electric potential measures the energy available to push each unit of charge through a circuit. An outlet rated 120 volts AC has an RMS voltage of 120 volts; the instantaneous voltage swings each cycle, so 120 joules per coulomb holds only as an effective value. These examples connect a familiar battery and wall outlet to the same energy-per-charge quantity, placing the volt within the practical work of sending electrical energy through circuits.
Electricity had entered human experience long before systematic study began. Fishers handling electric fish felt shocks, and woolen garments produced the effects of static electricity. Ancient Egyptians were familiar with electric fish. Serious scientific study began around 1600, and the efforts of many researchers developed a good understanding of electricity and its uses by the end of the nineteenth century. The path led from experiences with fish, clothing, and lightning toward the study of electrical behavior. The volt unit belongs to the vocabulary that expresses that behavior numerically, giving electric potential a defined relationship between energy and charge.
The materials through which electricity moves have their own roles. Matter consists of atoms containing negatively charged electrons, positively charged protons, and uncharged neutrons. Metals are good conductors, readily permitting electrical flow because electrons in their outer orbitals can easily participate. Poor conductors are called insulators; glass is one example. Even good conductors offer some resistance, measured in ohms. Semiconductors form a further class whose response to changing conditions can turn flow on or off. Usually made of silicon with tiny amounts of other elements added, they lie at the heart of computer chips and underpin LED lights and photovoltaic cells.
A closed circuit is necessary for electricity to flow. In a simple battery-and-lamp circuit, electricity travels from the high-energy end of the battery through the lamp and back to the low-energy end. Opening the switch stops that flow. The amount of charge moving over time is current, measured in amperes: one ampere means one coulomb per second through a conductor. Current and electric potential express different quantities within the same circuit. The volt unit describes energy for each unit of charge; the ampere describes the rate of charge flow. Both belong to the working circuit, alongside the resistance of its conducting materials.
Water pressure offers an analogy for electric potential. Water coming from a pipe connected to a reservoir atop a tall building has more pressure than water from a tank just above the pipe. Electrical potential similarly expresses the energy available to move charge through a circuit. Electromotive force is the potential difference of a source when no current is flowing. A circuit can therefore be described through the energy available to each charge quantity and through the current flowing, with resistance also entering the relationship between the electrical quantities.
The energy side of that relationship uses the joule, named for James Joule, a nineteenth-century brewer who demonstrated the equivalence of mechanical work and heat. In his apparatus, gravity pulled on a weight, turning paddle wheels that mixed water inside an insulated container. Mixing warmed the water. By calculating the work done by the gravitational pull, Joule connected that mechanical work to heat. One joule is approximately the energy needed to lift a 100 g apple one meter. The unit thus measures energy across different forms, including the energy expressed per coulomb when electric potential is measured in volts.
Ohm's law connects electric potential, current, and resistance: electric potential equals current multiplied by resistance. It is written E = I x R, with E in volts, I in amperes, and R in ohms. Current passing through resistance does work that can appear as heat, as in a toaster, hair dryer, or space heater. Power expresses work over time and is measured in watts. For electricity, one watt equals one ampere multiplied by one volt. Power therefore depends on both current and electric potential, combining the charge-flow rate with the energy available to each unit of charge.
Electrical current also produces a magnetic field around a wire, a property Hans Christian Oersted discovered in 1820. A year later, Michael Faraday showed how a current-carrying wire could rotate around a magnet to make an electric motor. In 1831 he discovered magnetic induction: moving a magnet along a wire caused electrical flow in a closed circuit. This led to electric generators, with successful commercial designs appearing around 1860. A generator is essentially a motor spun by an outside force to produce induced current.