The Poynting vector describes the energy per unit area supplied by an electromagnetic field. Electric and magnetic fields combine to form an electromagnetic field, with the Poynting vector written as S = E x B. The equation may need to be divided by μ0 for practical applications, and theoretical physics uses an alternative metric system with the Poynting vector written as S = (c/4π)*E x B.
The Poynting vector is an important equation in physics. Describes the amount of energy per unit area supplied by an electromagnetic field. Like any vector, the Poynting vector has both magnitude and direction.
Electromagnetic fields are a combination of electric and magnetic fields. Any electrical device that is turned on emits the electromagnetic waves that make up 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 switched on to produce an electric field, it just needs to be connected. 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 the flow of electric current. When a device is turned on, 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 variables in the equation are vector quantities, as indicated by the bold font. S is the Poynting vector, E is the electric field, and B is the magnetic field.
Electric and magnetic fields are measured in terms of their energy density, which may also be referred to as their strength. The electric and magnetic fields are perpendicular to each other in an electromagnetic wave. This means that the magnitude of the electromagnetic field energy is simply the magnitude of the electric field energy density, multiplied by the magnitude of the magnetic field density.
For practical applications, the basic equation often needs to 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 Weber per Ampere per meter. Other equivalent units are also sometimes used: Newton per Ampere squared or Henries per metre.
Theoretical physics commonly uses an alternative metric system called cgs, which means it applies 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 S = (c/4π)*E x B, where c stands for the speed of light.
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