Electricity is responsible for many well-known physical phenomena such as lightning, electric fields and electric currents, and is put to use in industrial applications such as electronics and electric power.
Concepts in electricityEdit
In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms.
- Electric charge: a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
- Electric potential: the potential energy per unit charge associated with a static (time-invariant) electric field.
- Electric current: a movement or flow of electrically charged particles.
- Electrical energy: energy made available by the flow of electric charge through a conductor or from the forces between charged particles.
- Electric power: The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy (or converted from another form into electric energy).
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Electric charge Edit
- Main article: Electric charge
Electric charge is a property of certain subatomic particles [](e.g., electrons[] and protons[]) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces[] of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity[]" is used interchangeably with the phrases "electric charge[]" and "quantity of charge." There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.
The most common experience with electric charge in everyday life is that of static cling - when two particular types of materials are rubbed together, they tend to stick together, at least for a while. This phenomenon occurs because of the exchange of charges between the two materials — one becomes positively charged while the other becomes negatively charged, and because of their opposite signs there is a force of attraction between them. Another common experience with electric charge is that of walking on carpet and then touching a doorknob, where one experiences a shock. Walking on carpet causes the body to acquire a net charge, which then escapes from the body to the doorknob.
Electric field Edit
- Main article: Electric field
The concept of electric field was introduced by Michael Faraday[]. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, electric field is a little bit different. Gravitational force depends on the masses of two bodies, whereas electric force depends on the electric charges of two bodies. While gravity can only pull two masses together, the electric force can be an attractive or repulsive force. The criteria for the direction of the forces between two charged bodies are generally proposed as follows:
- Both charges are of same sign (e.g. both charges are positive), in which case there will be a repulsive force between the two.
- The charges are opposite, in which case there will be an attractive force between the two bodies.
- The magnitude of the force varies inversely with the square of the distance between the two bodies, and is also directly proportional to the product of the unsigned magnitudes of the two charges.
Electric potential Edit
- Main article: Electric potential
The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradients indicate the direction of heat flows. Similarly, there is an electric potential at every point in space, and its gradient in the electric field indicates where charges move.
Electric current Edit
An electric current is a flow of electric charge, and its intensity is measured in amperes. Examples of electric currents include metallic conduction, where electrons flow through a conductor such as a metal wire, and electrolysis, where ions (charged atoms) flow through liquids. The particles themselves often move quite slowly, while the electric field that drives them propagates at close to the speed of light. See electrical conduction for more information.
Devices that use charge flow principles in materials are called electronic devices.
A direct current (DC) is a unidirectional flow, while an alternating current (AC) reverses direction repeatedly. The time average of an alternating current is zero, but its energy capability (RMS value) is not zero.
For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.
Electrical energy Edit
- Main article: Electrical energy
Electrical energy is energy stored in an electric field or transported by an electric current. Energy is defined as the ability to do work, and electrical energy is simply one of the many types of energy. Examples of electrical energy include:
- the energy that is constantly stored in the Earth's atmosphere, and is partly released during a thunderstorm in the form of lightning
- the energy that is stored in the coils of an electrical generator in a power station, and is then transmitted by wires to the consumer; the consumer then pays for each unit of energy he receives
- the energy that is stored in a capacitor, and can be released to drive a current through an electrical circuit
Electric power Edit
- Main article: Electric power
Electric power is the rate at which electrical energy is produced or consumed, and is measured in watts (symbol: W).
A fossil-fuel or nuclear power station converts heat to electrical energy, and the faster the station burns fuel, assuming constant efficiency of conversion, the higher its power output. The output of a power station is usually specified in megawatts (millions of watts). The electrical energy is then sent over transmission lines to reach the consumers.
Each consumer uses appliances that convert the electrical energy to other forms of energy, such as heat (in electric arc furnaces and electric heaters), light (in light bulbs and fluorescent lamps), or motion, i.e. kinetic energy (in electric motors). Like the power station, each appliance is also rated in watts, depending on the rate at which it converts electrical energy into another form. The power station must produce electrical energy at the same rate as all the connected appliances consume it.
In electrical engineering, the concepts of apparent power and reactive power are also used. Apparent power is the product of RMS voltage and RMS current, and is measured in volt-amperes (VA). Reactive power is measured in volt-amperes-reactive (VAR).
SI electricity units Edit
| SI electromagnetic units|
|I||Current||ampere (SI base unit)||A||A|
|q||Electric charge, Quantity of electricity||coulomb||C||A·s|
|V||Potential difference||volt||V||J/C = kg·m2·s−3·A−1|
|R, Z||Resistance, Impedance, Reactance||ohm||Ω||V/A = kg·m2·s−3·A−2|
|P||Power, Electrical||watt||W||V·A = kg·m2·s−3|
|C||Capacitance||farad||F||C/V = kg−1·m−2·A2·s4|
|Elastance||reciprocal farad||F−1||V/C = kg·m2·A−2·s−4|
|ε||Permittivity||farad per metre||F/m||kg−1·m−3·A2·s4|
|Conductance, Admittance, Susceptance||siemens||S||Ω−1 = kg−1·m−2·s3·A2|
|σ||Conductivity||siemens per metre||S/m||kg−1·m−3·s3·A2|
|H||Magnetic field, magnetic field intensity||ampere per metre||A/m||A·m−1|
|Φm||Magnetic flux||weber||Wb||V·s = kg·m2·s−2·A−1|
|B||Magnetic flux density, magnetic induction, magnetic field strength||tesla||T||Wb/m2 = kg·s−2·A−1|
|Reluctance||ampere-turns per weber||A/Wb||kg−1·m−2·s2·A2|
|L||Inductance||henry||H||Wb/A = V·s/A = kg·m2·s−2·A−2|
|μ||Permeability||henry per metre||H/m||kg·m·s−2·A−2|
See also Edit
Electrical phenomena in nature Edit
- Matter: — since atoms and molecules are held together by electric forces.
- Lightning: electrical discharges in the atmosphere.
- The Earth's magnetic field[] — created by electric currents circulating in the planet's core.
- Sometimes due to solar flares[], a phenomenon known as a power surge[] can be created.
- Piezoelectricity[]: the ability of certain crystals to generate a voltage in response to applied mechanical stress.
- Triboelectricity: electric charge taken on by contact or friction between two different materials.
- Bioelectromagnetism[]: electrical phenomena within living organisms.
- Bioelectricity[] — Many animals are sensitive to electric fields, some (e.g., sharks) more than others (e.g., people). Most also generate their own electric fields.
- What is electricity?
- Merriam-Webster: Electricity
- Tyndall: Faraday as Discovery: Identity of Electricities
- US Energy Department Statistics
- Electricity around the world
- A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin, Created and Collected by Robert A. Morse (2004)
- Understanding Electricity and some Electronics in 10 minutes(Steve Rose, Maui)
- Electricity Misconceptions
- Electrical Power Calculator
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