Electric Power Primer
Electric power is defined as the rate at which electrical energy is
transferred by an electric circuit. The SI unit of electric power is the watt.
Electrical power is distributed via cables and electricity poles or pylons.
When electric current flows in a circuit, it can transfer energy to do
mechanical or thermodynamic work. Devices convert electrical energy into many
useful forms, such as heat (electric heaters), light (light bulbs), motion
(electric motors), sound (loudspeaker) or chemical changes. Electric power can
be produced mechanically by generation, or chemically, or by direct conversion
from light in photovoltaic cells, also it can be stored chemically in batteries.
Electric power, like mechanical power, is represented by the letter P in
electrical equations. The term wattage is used colloquially to mean "electric
power in watts."
In direct current resistive circuits, instantaneous electrical power is
calculated using Joule's Law, which is named after the British physicist James
Joule, who first showed that heat and mechanical energy were interchangeable.
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 electric 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
electric power (also referred to as active power). That portion of electric
power flow due to stored energy, that returns to the source in each cycle, is
known as reactive power.
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:
(apparent power)2 = (real power)2 + (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
(real power) = (apparent power) * cos(theta)
(reactive power) = (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.
Electric 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 vector cross-product of the electrical and magnetic fields over
a specified area, thus:
The result is a scalar since it is the surface integral of the Poynting vector.