# Dissipation Rate of a Propagating EM Wave

• uawildcat2008
In summary, the conversation discusses the concept of determining a source voltage at a distance using a loop antenna and Faraday's Law of Induction. The question is posed regarding how to determine the voltage of a lightning strike from a distance using equations and proper unit notation. The response mentions the relationship between voltage and magnetic flux, as well as the use of surface area and orientation to calculate the rate of change of lightning current. However, it is noted that this may not provide all the necessary information.
uawildcat2008
Hello,

I have a loop antenna I have been messing around with for a few years and I understand most of the physics behind it, especially with regards to Faraday's Law of Induction; however, I'm trying to work backwards now to determine a source voltage at a distance. For example, if a propagating EM wave comes in contact with my antenna in the horizontal creating 2 volts of induced charge while a lightning strike occurs, say, 20 km away in the vertical, how do I determine the lightning's voltage? Do I simply use 1/r^2 for linearity and 1/r^3 for point charge determinations or another equation? Also, what would be the proper unit notation - V/m or V/km or something completely different?

Many Thanks,
- uawildcat2008

The voltage in your circuit is equal to the rate of change of magnetic flux through the circuit, the lightning can be modeled as an instantaneous electric dipole current, which creates a pulse of magnetic field. So what you can determine is the rate of change of the lightning current, if you know the surface area of your circuit and its orientation relative to the dipole current. I don't think you can determine anything else from your measurement.

## 1. What is the dissipation rate of a propagating EM wave?

The dissipation rate of a propagating EM wave refers to the amount of energy lost per unit time as the wave travels through a medium. It is typically measured in watts per meter squared (W/m²) or decibels per meter (dB/m).

## 2. How is the dissipation rate of a propagating EM wave affected by the medium it travels through?

The dissipation rate of a propagating EM wave is affected by the properties of the medium it travels through. These properties include the density, conductivity, and permittivity of the medium. A higher density or conductivity can lead to a higher dissipation rate, while a higher permittivity can lead to a lower dissipation rate.

## 3. What factors can cause the dissipation rate of a propagating EM wave to change?

The dissipation rate of a propagating EM wave can change due to a variety of factors. These include the distance the wave travels, the frequency of the wave, and any obstacles or objects in the path of the wave. Temperature and humidity can also affect the dissipation rate.

## 4. How is the dissipation rate of a propagating EM wave calculated?

The dissipation rate of a propagating EM wave can be calculated using the wave's intensity and the properties of the medium it is traveling through. The formula for calculating dissipation rate is: P = I x α, where P is the dissipation rate in watts per meter squared, I is the wave's intensity in watts per meter squared, and α is the absorption coefficient of the medium.

## 5. Why is understanding the dissipation rate of a propagating EM wave important?

Understanding the dissipation rate of a propagating EM wave is important for various reasons. It allows us to predict how much energy a wave will lose as it travels through a medium, which can impact the strength and quality of the signal. It also helps in the design and optimization of wireless communication systems and can aid in the study of how EM waves interact with different materials and environments.

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