Electric power is the rate at which
electrical energy is transferred by an
electric circuit. The
SI unit of
power is the
watt, one
joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force appli ...
per
second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of watts are called kilowatts, megawatts and gigawatts respectively.
A common misconception is that electric power is bought and sold, but actually electrical energy is bought and sold. For example, electricity is sold to consumers in
kilowatt-hours (kilowatts multiplied by hours), because energy is power multiplied by time.
Electric power is usually produced by
electric generators, but can also be supplied by sources such as
electric batteries. It is usually supplied to businesses and homes (as domestic
mains electricity) by the
electric power industry through an
electrical grid.
Electric power can be delivered over long distances by
transmission lines and used for applications such as
motion,
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
or
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
with high
efficiency.
[
]
Definition
Electric power, like
mechanical power, is the rate of doing
work, measured in
watts, and represented by the letter ''P''. The term ''wattage'' is used colloquially to mean "electric power in watts." The electric power in
watts produced by an
electric current ''I'' consisting of a
charge of ''Q'' coulombs every ''t'' seconds passing through an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
(
voltage) difference of ''V'' is
where
*''Q'' is electric charge in
coulombs
*''t'' is time in
seconds
*''I'' is electric current in
amperes
*''V'' is electric potential or voltage in
volts
Explanation
Electric power is transformed to other forms of energy when
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
s move through an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
difference (
voltage), which occurs in
electrical component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are n ...
s in electric circuits. From the standpoint of electric power, components in an electric circuit can be divided into two categories:
Active devices (power sources)
If the charges are moved by an 'exterior force' through the device in the direction from the lower electric potential to the higher, (so positive charge moves from the negative to the positive terminal),
work will be done ''on'' the charges, and energy is being converted to electric
potential energy from some other type of energy, such as
mechanical energy or
chemical energy. Devices in which this occurs are called ''
active'' devices or ''power sources''; such as
electric generators and
batteries
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
.
Some devices can be either a source or a load, depending on the voltage and current through them. For example, a
rechargeable battery acts as a source when it provides power to a circuit, but as a load when it is connected to a battery charger and is being recharged.
Passive devices (loads)
When electric charges move through a potential difference from a higher to a lower voltage, that is when
conventional current (positive charge) moves from the positive (+) terminal to the negative (−) terminal,
work is done by the charges on the device. The
potential energy of the charges due to the voltage between the terminals is converted to
kinetic energy in the device. These devices are called ''
passive'' components or ''loads''; they 'consume' electric power from the circuit, converting it to other forms of energy such as
mechanical work, heat, light, etc. Examples are
electrical appliances, such as
light bulbs,
electric motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate f ...
s, and
electric heaters. In
alternating current (AC) circuits the direction of the voltage periodically reverses, but the current always flows from the higher potential to the lower potential side.
Passive sign convention
Since electric power can flow either into or out of a component, a convention is needed for which direction represents positive power flow. Electric power flowing ''out'' of a circuit ''into'' a component is arbitrarily defined to have a positive sign, while power flowing ''into'' a circuit from a component is defined to have a negative sign. Thus passive components have positive power consumption, while power sources have negative power consumption. This is called the ''
passive sign convention''.
Resistive circuits
In the case of
resistive (Ohmic, or linear) loads, Joule's law can be combined with
Ohm's law (''V'' = ''I·R'') to produce alternative expressions for the amount of power that is dissipated:
where ''R'' is the
electrical resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...
.
Alternating current without harmonics
In
alternating current circuits, energy storage elements such as
inductance and
capacitance may result in periodic reversals of the direction of energy flow. The portion of energy flow (power) that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as
real power (also referred to as active power).
The amplitude of that portion of energy flow (power) that results in no net transfer of energy but instead oscillates between the source and load in each cycle due to stored energy, is known as the absolute value of
reactive power.
The product of the RMS value of the voltage wave and the RMS value of the current wave is known as
apparent power. The real power ''P'' in watts consumed by a device is given by
where
*''V''
p is the peak voltage in volts
*''I''
p is the peak current in amperes
*''V''
rms is the
root-mean-square voltage in volts
*''I''
rms is the
root-mean-square current in amperes
*''θ'' = ''θ''
v − ''θ''
i is the
phase angle by which the voltage sine wave leads the current sine wave, or equivalently the phase angle by which the current sine wave lags the voltage sine wave
The relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the ''power triangle''. Using the
Pythagorean Theorem, the relationship among real, reactive and apparent power is:
Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both
sinusoids
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
with a known phase angle θ between them:
The ratio of real power to apparent power is called
power factor and is a number always between −1 and 1. Where the currents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion.
Electromagnetic fields
Electrical energy flows wherever electric and magnetic fields exist together and fluctuate in the same place. The simplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, the simple equation ''P'' = ''IV'' may be replaced by a more complex calculation. The closed
surface integral
In mathematics, particularly multivariable calculus, a surface integral is a generalization of multiple integrals to integration over surfaces. It can be thought of as the double integral analogue of the line integral. Given a surface, one ...
of the
cross-product
In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and is ...
of the electric field intensity and magnetic field intensity
vectors gives the total instantaneous power (in watts) ''out of'' the volume:
The result is a scalar since it is the ''
surface integral
In mathematics, particularly multivariable calculus, a surface integral is a generalization of multiple integrals to integration over surfaces. It can be thought of as the double integral analogue of the line integral. Given a surface, one ...
'' of the ''
Poynting vector''.
Production
Generation
The fundamental principles of much electricity generation were discovered during the 1820s and early 1830s by the British scientist
Michael Faraday
Michael Faraday (; 22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic inducti ...
. His basic method is still used today: electric current is generated by the movement of a loop of wire, or disc of copper between the poles of a
magnet.
For
electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electricity
transmission,
distribution Distribution may refer to:
Mathematics
* Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations
*Probability distribution, the probability of a particular value or value range of a vari ...
, and electrical energy storage and recovery using
pumped-storage methods are normally carried out by the
electric power industry.
Electricity is mostly generated at a
power station by electromechanical
generators, driven by
heat engine
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower stat ...
s heated by
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
,
geothermal power or
nuclear fission. Other generators are driven by the
kinetic energy of flowing water and wind. There are many other technologies that are used to generate electricity such as
photovoltaic solar panels.
A
battery
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
is a device consisting of one or more
electrochemical cells that convert stored chemical energy into electrical energy.
["battery" (def. 4b)](_blank)
''Merriam-Webster Online Dictionary'' (2009). Retrieved 25 May 2009. Since the invention of the first battery (or "
voltaic pile") in 1800 by
Alessandro Volta and especially since the technically improved
Daniell cell in 1836, batteries have become a common power source for many household and industrial applications. According to a 2005 estimate, the worldwide battery industry generates
US$48
billion in sales each year, with 6% annual growth. There are two types of batteries:
primary batteries (disposable batteries), which are designed to be used once and discarded, and
secondary batteries
A rechargeable battery, storage battery, or secondary cell (formally a type of energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to a disposable or prima ...
(rechargeable batteries), which are designed to be recharged and used multiple times. Batteries are available in many sizes; from miniature
button cell
A button cell, watch battery, or coin battery is a small single-cell battery shaped as a squat cylinder typically in diameter and high — resembling a button. Stainless steel usually forms the bottom body and positive terminal of the cell; in ...
s used to power
hearing aid
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers s ...
s and wristwatches to battery banks the size of rooms that provide standby power for
telephone exchanges and computer
data centers.
Electric power industry
The electric power industry provides the production and delivery of power, in sufficient quantities to areas that need
electricity, through a
grid connection. The grid distributes electrical energy to customers. Electric power is generated by central
power stations or by
distributed generation. The electric power industry has gradually been trending towards deregulation – with emerging players offering consumers competition to the traditional public utility companies.
Use
Electric power, produced from central generating stations and distributed over an electrical transmission grid, is widely used in industrial, commercial and consumer applications. The per capita electric power consumption of a country correlates with its industrial development.
[Ignacio J. Pérez-Arriaga (ed), ''Regulation of the Power Sector'', Springer Science & Business Media, 2014 , page 8 ] Electric motors power manufacturing machinery and propel subways and railway trains. Electric lighting is the most important form of artificial light. Electrical energy is used directly in processes such as extraction of aluminum from its ores and in production of steel in
electric arc furnaces. Reliable electric power is essential to telecommunications and broadcasting. Electric power is used to provide air conditioning in hot climates, and in some places electric power is an economically competitive source of energy for building space heating. Use of electric power for pumping water ranges from individual household wells to irrigation projects and energy storage projects.
See also
*
EGRID
*
Electric energy consumption
*
Electric power system
*
High-voltage cable
*
Power engineering
*
Rural electrification
References
Bibliography
Reports on August 2003 Blackout, North American Electric Reliability Council website*
*
External links
GlobTek, Inc. Glossary of Electric power Power Supply Terms
{{Authority control
Power
Temporal rates