In physics and engineering, a phase vector ("phasor") is a representation of a sine wave whose amplitude (A), phase (θ), and frequency (ω) are timeinvariant. It is a subset of a more general concept called analytic representation. Phasors reduce the dependencies on these parameters to three independent factors, thereby simplifying certain kinds of calculations. In particular the frequency factor, which also includes the timedependence of the sine wave, is often common to all the components of a linear combination of sine waves. Using phasors, it can be factored out, leaving just the static amplitude and phase information to be combined algebraically (rather than trigonometrically). Similarly, linear differential equations can be reduced to algebraic ones. The term phasor therefore often refers to just those two factors. In older texts, a phasor is also referred to as a sinor.
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Euler's formula indicates that sine waves can be represented mathematically as the sum of two complexvalued functions:
or as the real part of one of the functions:
As indicated above, phasor can refer to either or just the complex constant, . In the latter case, it is understood to be a shorthand notation, encoding the amplitude and phase of an underlying sinusoid.
An even more compact shorthand is angle notation:
The sine wave can be understood as the projection on the real axis of a rotating vector on the complex plane. The modulus of this vector is the amplitude of the oscillations, while its argument is the total phase ωt + θ. The phase constant θ represents the angle that the complex vector forms with the real axis at t = 0.
Multiplication of the phasor by a complex constant, produces another phasor. That means its only effect is to change the amplitude and phase of the underlying sinusoid:
In electronics, would represent an impedance, which is independent of time. In particular it is not the shorthand notation for another phasor. Multiplying a phasor current by an impedance produces a phasor voltage. But the product of two phasors (or squaring a phasor) would represent the product of two sine waves, which is a nonlinear operation that produces new frequency components. Phasor notation can only represent systems with one frequency, such as a linear system stimulated by a sinusoid.
The time derivative or integral of a phasor produces another phasor^{[2]}. For example:
Therefore, in phasor representation, the time derivative of a sinusoid becomes just multiplication by the constant, Similarly, integrating a phasor corresponds to multiplication by The timedependent factor, , is unaffected. When we solve a linear differential equation with phasor arithmetic, we are merely factoring out of all terms of the equation, and reinserting it into the answer. For example, consider the following differential equation for the voltage across the capacitor in an RC circuit:
When the voltage source in this circuit is sinusoidal:
we may substitute:
where phasor and phasor is the unknown quantity to be determined.
In the phasor shorthand notation, the differential equation reduces to^{[3]}:
Solving for the phasor capacitor voltage gives:
As we have seen, the factor multiplying represents differences of the amplitude and phase of relative to and
In polar coordinate form, it is:
Therefore:
The sum of multiple phasors produces another phasor. That is because the sum of sine waves with the same frequency is also a sine wave with that frequency:
where:
or, via the law of cosines on the complex plane (or the trigonometric identity for angle differences):
where Δθ = θ_{1} − θ_{2}. A key point is that A_{3} and θ_{3} do not depend on ω or t, which is what makes phasor notation possible. The time and frequency dependence can be suppressed and reinserted into the outcome as long as the only operations used in between are ones that produce another phasor. In angle notation, the operation shown above is written:
Another way to view addition is that two vectors with coordinates [A_{1} cos(ωt+θ_{1}), A_{1} sin(ωt+θ_{1})] and [A_{2} cos(ωt+θ_{2}), A_{2} sin(ωt+θ_{2})] are added vectorially to produce a resultant vector with coordinates [A_{3} cos(ωt+θ_{3}), A_{3} sin(ωt+θ_{3})]. (see animation)
In physics, this sort of addition occurs when sine waves "interfere" with each other, constructively or destructively. The static vector concept provides useful insight into questions like this: "What phase difference would be required between three identical waves for perfect cancellation?" In this case, simply imagine taking three vectors of equal length and placing them head to tail such that the last head matches up with the first tail. Clearly, the shape which satisfies these conditions is an equilateral triangle, so the angle between each phasor to the next is 120° (2π/3 radians), or one third of a wavelength ^{λ}/_{3}. So the phase difference between each wave must also be 120°, as is the case in threephase power
In other words, what this shows is:
In the example of three waves, the phase difference between the first and the last wave was 240 degrees, while for two waves destructive interference happens at 180 degrees. In the limit of many waves, the phasors must form a circle for destructive interference, so that the first phasor is nearly parallel with the last. This means that for many sources, destructive interference happens when the first and last wave differ by 360 degrees, a full wavelength λ. This is why in single slit diffraction, the minima occurs when light from the far edge travels a full wavelength further than the light from the near edge.
Electrical engineers, electronics engineers, electronic engineering technicians and aircraft engineers all use phasor diagrams to visualize complex constants and variables (phasors). Like vectors, arrows drawn on graph paper or computer displays represent phasors. Cartesian and polar representations each have advantages.
With phasors, the techniques for solving DC circuits can be applied to solve AC circuits. A list of the basic laws is given below.
Given this we can apply the techniques of analysis of resistive circuits with phasors to analyze single frequency AC circuits containing resistors, capacitors, and inductors. Multiple frequency linear AC circuits and AC circuits with different waveforms can be analyzed to find voltages and currents by transforming all waveforms to sine wave components with magnitude and phase then analyzing each frequency separately, as allowed by the superposition theorem.
In analysis of three phase AC power systems, usually a set of phasors is defined as the three complex cube roots of unity, graphically represented as unit magnitudes at angles of 0, 120 and 240 degrees. By treating polyphase AC circuit quantities as phasors, balanced circuits can be simplified and unbalanced circuits can be treated as an algebraic combination of symmetrical circuits. This approach greatly simplifies the work required in electrical calculations of voltage drop, power flow, and shortcircuit currents. In the context of power systems analysis, the phase angle is often given in degrees, and the magnitude in rms value rather than the peak amplitude of the sinusoid.
The technique of synchrophasors uses digital instruments to measure the phasors representing transmission system voltages at widespread points in a transmission network. Small changes in the phasors are sensitive indicators of power flow and system stability.

( 
Since this must hold for all , specifically: it follows that:

( 
It is also readily seen that:
Substituting these into Eq.1 and Eq.2, multiplying Eq.2 by and adding both equations gives:
