# Homework Help: Energy Density Units Electromagnetic Waves

1. Nov 2, 2012

### PeachBanana

1. The problem statement, all variables and given/known data

In what units is energy density measured?

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

2. Relevant equations

E = cB (as derived by Maxwell)

3. 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.

2. Nov 2, 2012

### aralbrec

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

The left side is electric field (not energy).

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. Nov 3, 2012

### PeachBanana

Hi.

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