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Electric power is transformed to other forms of power when electric charges move through an electric potential (voltage) difference, which occurs in electrical components in electric circuits. When electric charges move through a potential difference from a high voltage to a low voltage, the energy in the potential is converted to kinetic energy of the charges, which perform work on the device. Devices in which this occurs are called passive devices or loads; they consume electric power, converting it to other forms such as mechanical work, heat, light, etc. Examples are electrical appliances, such as light bulbs, electric motors, and electric heaters. If the charges are forced to move by an outside force in the direction from a lower potential to a higher, work is being done on the charges, so power is transferred to the electric current 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; sources of electric current, such as electric generators and batteries. Passive sign convention__ Main article: Passive sign convention In electronics, which deals with more passive than active devices, electric power consumed in a device is defined to have a positive sign, while power produced by a device is defined to have a negative sign. 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 dissipated power: P = I^2 R = \frac{V^2}{R}, where R is the electrical resistance. Alternating current__ Main article: AC power 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 power flow 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). That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power. The real power P in watts consumed by a device is given by P = {1 \over 2}V_p I_p \cos \theta = V_{rms}I_{rms} \cos \theta \, where Vp is the peak voltage in volts Ip is the peak current in amperes Vrms is the root-mean-square voltage in volts Irms is the root-mean-square current in amperes θ is the phase angle between the current and voltage sine waves Power triangle: The components of AC power 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: \mbox{(apparent power)}^2 = \mbox{(real power)}^2 + \mbox{(reactive power)}^2 Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage are both sinusoids with a known phase angle θ between them: \mbox{(real power)} = \mbox {(apparent power)}\cos(\theta) \mbox{(reactive power)} = \mbox {(apparent power)}\sin(\theta) The ratio of real power to apparent power is called power factor and is a number always between 0 and 1. Where the currents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion Electromagnetic fields__ Question book-new.svg This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2012) Electrical power 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 must be replaced by a more complex calculation, the integral of the cross-product of the electrical and magnetic field vectors over a specified area, thus: P = \int_S (\mathbf{E} \times \mathbf{H}) \cdot \mathbf{dA}. \, The result is a scalar since it is the surface integral of the Poynting vector. |
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