Passive sign convention
The passive sign convention states that in components in which the conventional current variable i is defined as entering the device through the terminal which is positive as defined by the voltage variable v,[2][4] the power p and resistance r are given by[5][6][7] In components in which the current i is defined such that positive current enters the device through the negative voltage terminal, power and resistance are given by With these definitions, passive components (loads) will have p > 0 and r > 0, and active components (power sources) will have p < 0 and r < 0.
A simple component (shown in these diagrams as a rectangle) is connected to the circuit by two wires, through which electric current passes through the device.
A capacitor or an inductor acts as a load when it is storing energy in its electric or magnetic field from the external circuit, respectively, but as a source when it is releasing into the external circuit the stored energy from the electric or magnetic field.
The passive sign convention arbitrarily defines power flowing into the component (out of the circuit) as positive,[2] so passive components have "positive" power flow.
In addition, components with reactance (capacitance or inductance) store energy temporarily, so they act as sources or sinks in different parts of the AC cycle.
For example, in a capacitor, when the voltage across it is increasing, the current is directed into the positive terminal, so the component is storing energy from the circuit in its electric field, while when the voltage is decreasing, the current is directed out of the positive terminal, so it is acting as a source, returning stored energy to the circuit.
In a steady-state AC circuit, all the energy stored in reactances is returned within the AC cycle, so a pure reactance, a capacitor or inductor, neither consumes nor produces net power, neither a source nor a load.
[8][9] If the actual current is in the opposite direction, the variable i will have a negative value.
It is unnecessary to know whether a component produces or consumes power when analyzing the circuit; reference directions can be assigned arbitrarily, directions to currents and polarities to voltages, then the PSC is used to calculate the power in components.
There are two choices: In practice, assigning the voltage and current variables in a circuit is not necessary to comply with the PSC.
[5] A current entering the negative terminal is equivalent to a negative current entering the positive terminal, so in such a component[5][7] One advantage of defining all the variables in a circuit to comply with the PSC is that it makes it easy to express conservation of energy.
Since with the PSC, the power dissipation in sources is negative, and power dissipation in loads is positive, the algebraic sum of all the power dissipation in all the components in a circuit is always zero[7] Since the sign convention only deals with the directions of the variables and not with the direction of the actual current, it also applies to alternating current (AC) circuits, in which the direction of the voltage and current periodically reverses.
In an AC circuit, even though the voltage and current reverse direction during the second half of the cycle, at any given instant, it obeys the PSC: in passive components, the instantaneous current flows through the device from the positive to the negative terminal, while in active components it flows through the component from the negative to the positive terminal.
The power consumption or production of a component depends on its current–voltage characteristic curve.
Whether the component acts as a source or load may depend on the current i or voltage v in it, which is not known until the circuit is analyzed.
