# Average Energy density and the Poynting vector of an EM wave

• edfink1
In summary, the problem in Griffiths Introduction to Electrodynamics involves calculating the average energy density and Poynting vector using the formulas U = 1/2 * ɛ₀ * E² + 1/2 * μ₀ * B² and S = ɛ₀ * E×B, respectively. The fields used in the formulas are the electric and magnetic fields, E and B, respectively. Substituting the given fields in the problem into the formulas yields the solutions for U and S.
edfink1
Homework Statement
Average Energy density and the Poynting vector of an EM wave
Relevant Equations
energy density of the EM fields, Poynting vector
Hi,
In Problem 9.12 of Griffiths Introduction to Electrodynamics, 4th edition (Problem 9.11 3rd edition), in the problem, he says that one can calculate the average energy density and Poynting vector as

using the formula

I don't really understand how to do this. I show my attempt below, but I don't think it is good. Can someone explain if it is really all there is to it?

Delta2
My attempt: To calculate the average energy density, we use the formulaU = 1/2 * ɛ₀* E² + 1/2 * μ₀ * B². Where E and B are the electric and magnetic fields, respectively. Substituting the fields for this problem gives usU = 1/2 * ɛ₀ * (E₁ + E₂)² + 1/2 * μ₀ * (B₁ + B₂)².To calculate the Poynting vector, we use the formulaS = ɛ₀*E×BSubstituting the fields for this problem gives usS = ɛ₀ * (E₁ + E₂) × (B₁ + B₂).

## 1. What is average energy density of an EM wave?

The average energy density of an EM wave is the amount of energy per unit volume that is carried by the wave. It is represented by the symbol u and is measured in joules per cubic meter (J/m3). This value is important in understanding the intensity of an EM wave and its effects on matter.

## 2. How is average energy density related to the Poynting vector?

The average energy density is directly related to the Poynting vector, which is a mathematical representation of the direction and magnitude of energy flow in an EM wave. The Poynting vector is calculated by taking the cross product of the electric field vector and the magnetic field vector, and its magnitude is equal to the average energy density multiplied by the speed of light.

## 3. What factors affect the average energy density of an EM wave?

The average energy density of an EM wave is affected by the amplitude of the electric and magnetic fields, the frequency of the wave, and the medium through which the wave is traveling. In a vacuum, the average energy density is constant and is equal to u = ε0 E02 = B020, where ε0 and μ0 are the permittivity and permeability of free space, and E0 and B0 are the amplitudes of the electric and magnetic fields, respectively.

## 4. How does the average energy density of an EM wave change as it propagates through different mediums?

The average energy density of an EM wave decreases as it travels through a medium with a higher refractive index. This is because the speed of light is slower in these mediums, so the Poynting vector (and therefore the average energy density) decreases. In addition, the energy of the wave may be absorbed or scattered by particles in the medium, further decreasing the average energy density.

## 5. Can the average energy density of an EM wave be negative?

No, the average energy density of an EM wave cannot be negative. This is because it is a measure of the energy carried by the wave, and energy cannot have a negative value. However, in some cases, the Poynting vector may have a negative direction, indicating that the energy is flowing in the opposite direction of the wave's propagation. This does not mean that the average energy density is negative, but rather that the energy is being transferred in a different direction.

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