Specific power
Encyclopedia
In physics
and engineering
, surface power density or sometimes simply specific power is power
per unit area
.
where
The above equation yields units of W/m2 . The units of mW/cm2, are more often used when making surveys. One mW/cm2 is the same power density as 10 W/m2. The following equation can be used to obtain these units directly:
The simplified relationships stated above apply at distances of about two or more wavelengths from the radiating source. This distance can be a far distance at low frequencies, and is called the far field. Here the ratio between E and H becomes a fixed constant (377 Ohms) and is called the characteristic impedance of free space. Under these conditions we can determine the power density by measuring only the E field component (or H field component, if you prefer) and calculating the power density from it.
This fixed relationship is useful for measuring radio frequency or microwave (electromagnetic) fields.Since power is the rate of energy transfer, and the squares of E and H are proportional to power, E2 and H2 are proportional to the energy transfer rate and the energy absorption of a given material.
s away from the source is called the far field. As the source emits electromagnetic radiation
of a given wavelength, the far-field electric component
of the wave E, the far-field magnetic component
H, and power density are related by the equations: E = H × 377 and Pd = E × H.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
, surface power density or sometimes simply specific power is power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
per unit area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
.
Applications
- The intensity of electromagnetic radiation can be expressed in W/m2. An example of such a quantity is the solar constantSolar constantThe solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit...
. - Wind turbineWind turbineA wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
s are often compared using a specific power measuring watts per square meter of turbine disk area, which is
, where r is the length of a blade. This measure is also commonly used for solar panelPhotovoltaic moduleA solar panel is a packaged, connected assembly of solar cells, also known as photovoltaic cells...
s, at least for typical applications. - RadianceRadianceRadiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...
is surface power density per unit of solid angle (steradians) in a specific direction. Spectral radiance is radiance per unit of frequency (Hertz) at a specific frequency.
Electromagnetic wave
As an electromagnetic wave travels through space, energy is transferred from the source to other objects (receivers). The rate of this energy transfer depends on the strength of the EM field components. Simply put, the rate of energy transfer per unit area (power density) is the product of the electric field strength (E) times the magnetic field strength (H).- Pd (Watts/meter2) = E × H (Volts/meter × Amperes/meter)
where
- Pd = the power density,
- E = the electric field strength in volts per meter,
- H = the magnetic field strength in amperes per meter.
The above equation yields units of W/m2 . The units of mW/cm2, are more often used when making surveys. One mW/cm2 is the same power density as 10 W/m2. The following equation can be used to obtain these units directly:
- Pd = 0.1 × E × H mW/cm2
The simplified relationships stated above apply at distances of about two or more wavelengths from the radiating source. This distance can be a far distance at low frequencies, and is called the far field. Here the ratio between E and H becomes a fixed constant (377 Ohms) and is called the characteristic impedance of free space. Under these conditions we can determine the power density by measuring only the E field component (or H field component, if you prefer) and calculating the power density from it.
This fixed relationship is useful for measuring radio frequency or microwave (electromagnetic) fields.Since power is the rate of energy transfer, and the squares of E and H are proportional to power, E2 and H2 are proportional to the energy transfer rate and the energy absorption of a given material.
Far field
The region extending farther than about 2 wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s away from the source is called the far field. As the source emits electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
of a given wavelength, the far-field electric component
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
of the wave E, the far-field magnetic component
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
H, and power density are related by the equations: E = H × 377 and Pd = E × H.
- Pd = H2 × 377 and Pd = E2 ÷ 377
- where Pd is the power density in watts per square meter (one W/m2 is equal to 0.1 mW/cm2),
- H2 = the square of the value of the magnetic field in amperes squared per meter squared,
- E2 = the square of the value of the electric field in volts squared per meter squared.