Electric power is the rate of transfer ofelectrical energy within acircuit. ItsSI unit is thewatt, the general unit ofpower, defined as onejoule persecond. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of watts are called kilowatts, megawatts and gigawatts respectively.
Electric power, likemechanical power, is the rate of doingwork, measured inwatts, and represented by the letterP. The termwattage is used colloquially to mean "electric power in watts". The electric power inwatts produced by anelectric currentI consisting of acharge ofQ coulombs everyt seconds passing through anelectric potential (voltage) difference ofV is:[2][3]where:
An often confusing aspect of the terminology is that the direction of electric current (conventional current) is defined as the direction that positive charge flows, but the actual mobilecharge carriers in circuits areelectrons, which have a negative charge. But a flow of positive charge in one direction is equivalent to an equal flow of negative charge in the other direction. So the electrons in the circuit flow in the opposite direction to the direction ofconventional current.
From the standpoint of electric power, components in an electric circuit can be divided into two categories:[2][3]
If conventionalelectric current (positive charge) is forced to flow through the device in the direction from the lower electric potential to the higher (this is equivalent to the negatively charged electrons moving from the positive terminal to the negative terminal),against the opposing force of theelectric field between the terminals, work will be doneon the charges, and energy is being converted to electricpotential energy from some other type of energy, such asmechanical energy orchemical energy. Devices in which this occurs are calledactive devices orpower sources; such aselectric generators and batteries. Some devices can be either a source or a load, depending on the voltage and current through them. For example, arechargeable 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.
Ifconventional current flows through the device in a direction from higher potential to lower potential (equivalent to the negative electrons moving from the negative terminal to the positive terminal), in the same direction as the force of theelectric field, work is done by the charges on the device. Thepotential energy of the charges due to the voltage between the terminals is converted tokinetic energy in the device. These devices are calledpassive components orloads; they 'consume' electric power from the circuit, converting it to other forms of energy such asmechanical work, heat, light, etc. Examples areelectrical appliances, such aslight bulbs,electric motors, andelectric heaters. Inalternating current (AC) circuits the direction of the voltage periodically reverses, but the current always flows from the higher potential to the lower potential side.
Since electric power can flow either into or out of a component, a convention is needed for which direction represents positive power flow.[2][3] Electric power flowingout of a circuitinto a component is arbitrarily defined to have a positive sign, while power flowinginto 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 thepassive sign convention.
In the case ofresistive (Ohmic, or linear) loads, the power formula (P =I·V) andJoule's first law (P =I^2·R) can be combined withOhm's law (V =I·R) to produce alternative expressions for the amount of power that is dissipated:whereR is theelectrical resistance.
Inalternating current circuits, energy storage elements such asinductance andcapacitance 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 asreal power (also referred to as active power).[4] 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 ofreactive power.[4][5][6] The product of the RMS value of the voltage wave and the RMS value of the current wave is known asapparent power. The real powerP in watts consumed by a device is given bywhere
θ =θv −θi is thephase 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 thepower triangle. Using thePythagorean 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 bothsinusoids with a known phase angle θ between them:
The ratio of real power to apparent power is calledpower 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.
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 equationP =IV may be replaced by a more complex calculation. The closedsurface integral of thecross-product of the electric field intensity and magnetic field intensityvectors gives the total instantaneous power (in watts)out of the volume:[7]
The fundamental principles of much electricity generation were discovered during the 1820s and early 1830s by the British scientistMichael Faraday. 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 amagnet.
Abattery is a device consisting of one or moreelectrochemical cells that convert stored chemical energy into electrical energy.[9] Since the invention of the first battery (or "voltaic pile") in 1800 byAlessandro Volta and especially since the technically improvedDaniell 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 generatesUS$48billion in sales each year,[10] with 6% annual growth. There are two types of batteries:primary batteries (disposable batteries), which are designed to be used once and discarded, andsecondary batteries (rechargeable batteries), which are designed to be recharged and used multiple times. Batteries are available in many sizes; from miniaturebutton cells used to powerhearing aids and wristwatches to battery banks the size of rooms that provide standby power fortelephone exchanges and computerdata centers.
The electric power industry provides the production and delivery of power, in sufficient quantities to areas that needelectricity, through agrid connection. The grid distributes electrical energy to customers. Electric power is generated by centralpower stations or bydistributed generation. The electric power industry has gradually been trending towards deregulation – with emerging players offering consumers competition to the traditional public utility companies.[11]
Electric power, produced from central generating stations and distributed over an electrical transmission grid, is widely used in industrial, commercial, and consumer applications. A country's per capita electric power consumption correlates with its industrial development.[12] 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 inelectric 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 energy source for building space heating. The use of electric power for pumping water ranges from individual household wells to irrigation and energy storage projects.
^abThomas, Roland E.; Rosa, Albert J.; Toussaint, Gregory J. (2016).The Analysis and Design of Linear Circuits (8 ed.). Wiley. pp. 812–813.ISBN978-1-119-23538-5.
^Fraile Mora, Jesús (2012).Circuitos eléctricos (in Spanish). Pearson. pp. 193–196.ISBN978-8-48-322795-4.
^IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions. IEEE. 2010. p. 4.doi:10.1109/IEEESTD.2010.5439063.ISBN978-0-7381-6058-0.
^Hayt, William H.; Buck, John A. (2012).Engineering Electromagnetics (8 ed.). McGraw-Hill. p. 385.ISBN978-0-07-338066-7.
