What is Electricity and How Electricity is Generated and Used

The wheel, electricity, telecommunications and the computer have each changed daily life. This introduction to electricity uses electricity as the broad term for phenomena involving electric charge, electric fields, voltage, current and electrical energy. Electric current is one part of that subject, not a complete definition of electricity.

Ordinary matter contains positive protons and negative electrons. A neutral object has zero net charge because its total positive and negative charge balance. Electrons in atoms occupy quantum states rather than fixed planetary orbits. In conductors some electrons can move through the material, while liquids, plasmas and semiconductors can carry charge through ions or holes as well. These different charge carriers all take part in electricity.

An object is negatively charged when it has excess negative charge and positively charged when it has a deficit of electrons relative to its positive charge. Charging normally transfers electrons; it does not create charge. Charge is conserved, although it can move between objects or separate within a material.

An electric field exerts force on mobile charge. When a conducting path and sustained potential difference are present, charge can move through the path as current. Touching two conductors at different potentials can cause a brief redistribution of charge, but that transient example is not the definition of electricity.

The related terms in electricity

1. Electric Charge: Charge is a conserved property of matter measured in coulombs. Electrons and protons have electric charge of equal magnitude and opposite sign. The net charge of an object is the algebraic sum of its positive and negative charge, not simply a count of particles.
2. Electric Current: Current is the rate at which net charge crosses a chosen surface. The electric current I equals dQ/dt. Its SI unit is the ampere, and one ampere is one coulomb per second. A conductor’s current depends on the applied field and the material response; voltage alone does not set every electric current.
3. Electric Potential: electric potential is electric potential energy per unit charge at a point relative to a chosen reference. A difference in Electric potential between two points is voltage. The difference of electric potential can drive current when a closed conductive path exists. For an ideal ohmic element, current is proportional to voltage; other devices have nonlinear or time-dependent behaviour. The absolute electric potential depends on the reference, while the measurable potential difference does not.
4. Electric Field: An electric field gives the force per unit positive test charge at each point in space. Its direction is the force direction for positive charge. A source charge contributes to the electric field around it, and the fields from multiple sources add as vectors.

Charge, current, potential and field describe related parts of electricity. Electrical energy and power describe how energy is transferred and converted.

How is Electricity Generated

Power systems do not create energy. They convert mechanical, chemical, radiant or other energy into electrical energy. Three common conversion routes produce electricity for an external circuit.

1. Electromechanical Process: A changing magnetic flux through a conductor loop induces an emf. Relative motion between a conductor and a magnetic field is one way to change the linked flux lines. Rotating electrical generators convert mechanical work through this principle. Conventional DC generators, alternators and dynamos differ in how their terminals deliver the induced voltage.
2. Electrochemical Process: In types of battery, oxidation and reduction reactions separate charge and establish terminal voltage. A closed external circuit lets chemical energy convert to electrical energy.
3. Solid State Electric Generation: Photovoltaics are one form of direct electricity generation. Absorbed photons can create electron-hole pairs in a semiconductor. The built-in field near a PN junction separates some carriers, producing photovoltage. When the PN junction is connected to a load, charge flows through the external circuit. Conventional PV solar cells use this process to convert radiant energy to electrical energy.

Types of Electricity

1. Alternating current reverses direction periodically. A rotating coil generator naturally induces alternating emf. In conventional DC generators, a commutator mechanically rectifies the armature output at the terminals. In alternators, slip rings or a stationary armature arrangement deliver AC without that commutation.
2. Direct current has a consistent reference direction, although its magnitude need not be constant. Batteries and solar cells supply DC at their terminals; power electronics can convert between DC and AC.

Generation Transmission and Distribution of Electricity

An electrical power plant converts primary energy into electrical power. For AC networks, a step up transformer often raises generator voltage before long-distance transmission. The generation of electricity uses a voltage suited to the generator and plant equipment. For a given transmitted power, higher voltage reduces current and therefore reduces resistive I²R losses in electrical transmission. Substations use step down transformers to supply the voltage levels required for electrical distribution.

The generation of electricity, transmission of electricity and distribution of electricity commonly use three-phase AC for bulk power, but network designs vary. DC transmission can be preferred for long distances, submarine or underground cables and links between unsynchronised AC systems. Customer service depends on the local network and load: homes often receive single-phase AC, while many larger loads use a three phase system. Commercial, industrial and residential customers are not classified by one universal phase rule.