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# What is the Poynting Vector?

Article Details
• Written By: H.R. Childress
• Edited By: Michelle Arevalo
2003-2017
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The Poynting vector is an important equation in physics. It describes the amount of energy per unit of area that is delivered by an electromagnetic field. Like any vector, the Poynting vector has both magnitude and direction.

Electromagnetic fields are a combination of electrical and magnetic fields. Any electrical device that is powered on emits the electromagnetic waves that compose electromagnetic fields. Everyday examples of devices that produce these fields include lamps, televisions, and cell phones.

Electric fields are produced by electrical voltages. An electrical device does not have to be turned on to produce an electric field, it only has to be plugged in. The device receives voltage from an outlet whenever it is plugged in, and this produces this type of field.

Magnetic fields, on the other hand, are only produced when an electrical device is turned on. These are produced by flowing electrical current. When a device is switched on, the current begins to flow and the magnetic and electric fields combine to form an electromagnetic field.

In its simplest form, the Poynting vector is written as S = E x B. All the variables in the equation are vector quantities, as indicated by the bold font. The S is the Poynting vector, the E is the electric field, and the B is the magnetic field.

Electric and magnetic fields are measured in terms of their energy density, which may also be referred to as intensity. The electric and magnetic fields are perpendicular to each other in an electromagnetic wave. This means that the magnitude of the electromagnetic field's energy is simply the magnitude of the electric field's energy density, multiplied by the magnitude of the magnetic field's density.

For practical applications, the basic equation must often be divided by a constant called μ0 (pronounced "mu naught"). This constant represents the permeability of free space. It is equal to 1.2566 x 10-6 Webers per Ampere per meter. Some other, equivalent units are sometimes used as well: Newtons per square Ampere or Henries per meter.

Theoretical physics commonly uses an alternative metric system called cgs, which means that it applies yet another form of the Poynting vector. The cgs system has standard units of centimeters, grams, and seconds, instead of the SI metric system's standard units of meters, kilograms, and seconds. The Poynting vector for theoretical physics is written as S = (c/4π)*E x B, where c stands for the speed of light.