In an AC circuit, the voltage and current keep changing direction.
Voltage is the energy per charge, and we can visualize the energy as boxes attached to the charged particles.
The higher the voltage, the more energy per particle.
Here, we have an alternating voltage, but there is no current, and hence there is no flow of energy.
Here, we have an alternating current, but there is no voltage difference, hence there is again no net flow of energy.
This is because the number of energy boxes flowing to the left is exactly equal to the number of energy boxes flowing to the right.
To have a net flow of energy, we need both a voltage difference and a current.
Notice now that although the flow of the particles keeps changing directions, there is always a net flow of energy boxes from the left to the right.
So far, the net flow of power has been equal to the RMS voltage multiplied by the RMS current.
However, this assumes that the voltage and current are in phase.
Consider now what happens when the voltage and current are 90 degrees out of phase.
Now, over time, the net flow of energy boxes to the right is exactly equal to the net flow of energy boxes to the left.
In other words, even though we have an AC voltage and an AC current, there is no net flow of real power.
We say that the "Apparent Power" is equal to the RMS voltage multiplied by the RMS current.
But for the "Real Power" to be at its maximum for a given voltage and current, the voltage and the current need to be in phase with one another.
As the phase difference changes, the "Apparent Power" stays the same, but the "Real Power" changes.
The phase difference between the voltage and the current is equal to the angle of this triangle.
When the phase difference is 90 degrees, the "Real Power" is zero.
When the phase difference is zero, the "Real Power" is at a maximum.
If the voltage is zero, then both the Real and the Apparent Power are zero.
Similarly, if the current is zero, both the Real and the Apparent Power are zero.
The "Apparent Power" is equal to the RMS voltage multiplied by the RMS current.
The "Real Power" is equal to "Apparent Power" multiplied by the cosine of the angle representing the phase difference.
The cosine of the angle is what we call the "Power Factor."
The vector on the right of this triangle is what we refer to as the "Reactive Power."
The actual real power is the length of the bottom vector in this triangle.
When the phase difference is 90 degrees, the "Real Power" is zero.
Without changing the magnitude of the current or the magnitude of the voltage, we can increase the net flow of energy by making the voltage and the current more "in phase" with each other.
Having the voltage and current be more "in phase" with each other is desirable, because some of the energy is lost in the resistance of the wires.
The energy lost in the resistance of the wires depends on the resistance and on the magnitude of the current, regardless of the phase difference between the voltage and the current.
Therefore, if the current and voltage are more "in phase", we deliver more energy for the amount of energy lost.
In other words, the more the voltage and the current are "in phase", the more efficiently the electric power is delivered.
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