# Surface power density

In physics and engineering, surface power density, or sometimes simply specific power[disputed ] is power per unit area.

## Applications

• The intensity of electromagnetic radiation can be expressed in W/m2. An example of such a quantity is the solar constant.
• Wind turbines are often compared using a specific power measuring watts per square meter of turbine disk area, which is $\pi r^{2}$ , where r is the length of a blade. This measure is also commonly used for solar panels, at least for typical applications.
• Radiance 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.

## Background

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 RMS electric field strength in volts per meter,
H = the RMS magnetic field strength in amperes per meter.

The above equation yields units of W/m2 . In the USA 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 wavelengths 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.

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 RMS squared per meter squared,
E2 = the square of the value of the electric field in volts RMS squared per meter squared.