Energy Density Units Electromagnetic Waves

In summary, energy density is a measure of how much energy is contained in a certain volume. It is measured in joules per cubic meter, and can also be expressed as watts per cubic meter. This relates to the magnitude of the electric and magnetic fields in a plane wave, as described by the equation E = cB.
  • #1
PeachBanana
191
0

Homework Statement



In what units is energy density measured?

1. Watts
2. Watts per cubic meter
3. Newtons per meter
4. Joules per cubic meter

Homework Equations



E = cB (as derived by Maxwell)

The Attempt at a Solution



This question is so basic yet I'm confused. I thought "B" (magnetic field) had units of Tesla. I thought "c" (speed of light) had units of m/s. I was thinking the answer was Tm / s. I'm particularly confused as to where "per cubic meter" comes into play.
 
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  • #2
E = cB (as derived by Maxwell)

This relates the magnitude of the electric field and the magnetic field in a plane wave.

The left side is electric field (not energy).
PeachBanana said:

Homework Statement



In what units is energy density measured?

1. Watts
2. Watts per cubic meter
3. Newtons per meter
4. Joules per cubic meter

I'm particularly confused as to where "per cubic meter" comes into play.

You don't have to appeal to energy density of EM fields to answer this question. Think of what does 'energy density' mean? Mass density (or just density for short) tells how much mass there is in a unit volume and is measured in kg/m3. So energy density means...
 
  • #3
Hi.

Thank you. I like the way you explained it - made it much easier to understand.
 

1. What are energy density units and how are they related to electromagnetic waves?

Energy density units refer to the amount of energy per unit volume or mass of a substance. In the context of electromagnetic waves, energy density units are used to measure the amount of energy carried by the electromagnetic field. This energy is constantly being exchanged between electric and magnetic fields as the waves propagate through space, creating a self-sustaining wave.

2. How is energy density calculated for electromagnetic waves?

The energy density of an electromagnetic wave can be calculated using the formula u = ε0E^2, where u is the energy density, ε0 is the permittivity of free space, and E is the electric field strength. This formula shows that the energy density is directly proportional to the square of the electric field strength.

3. What are some common units used to measure energy density in electromagnetic waves?

Common units used to measure energy density in electromagnetic waves include joules per cubic meter (J/m^3), ergs per cubic centimeter (erg/cm^3), and watt-hours per cubic meter (Wh/m^3). These units represent the amount of energy per unit volume of the electromagnetic field.

4. How does the energy density of electromagnetic waves vary with frequency?

The energy density of an electromagnetic wave is directly proportional to the square of the frequency. This means that as the frequency increases, the energy density also increases. This relationship is demonstrated by the formula u = ε0E^2, where the electric field strength (E) is directly related to the frequency of the wave.

5. How does the energy density of electromagnetic waves compare to other forms of energy?

The energy density of electromagnetic waves is much lower than other forms of energy, such as nuclear energy or chemical energy. However, the energy density of electromagnetic waves is still significant and can be harnessed for various purposes, such as communication and generating electricity. Additionally, electromagnetic waves have the advantage of being able to travel through space without the need for a medium, making them a versatile and efficient form of energy.

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