Question about energy in EM waves

[mathplease]

The instantaneous energy density of a region of space of an EM wave is:

u = $$\epsilon$$₀E² [J/m^3]

hence the average energy density is:

u_avg = (1/2)$$\epsilon$$₀E₀² [J/m^3]

u_avg = <S> / c [J/m^3]

Is this equal to the wave's average http://en.wikipedia.org/wiki/Radiation_pressure" measured in [Pa] if the radiation is fully absorbed?

P_rad= <S> / c [Pa]

My physics lecturer said these are not the same as one is measured in [J/m^3] and one is in [Pa], which seems clearly wrong to me but I'm worried I may have a misunderstanding in these concepts. Any confirmation or clarity is appreciated.

 

[Dale]

My physics lecturer said these are not the same as one is measured in [J/m^3] and one is in [Pa], which seems clearly wrong to me but I'm worried I may have a misunderstanding in these concepts. Any confirmation or clarity is appreciated.

I don't know if you are wrong or right, but the argument given doesn't work:
1 Pa = 1 J/m³ = 1 kg/(m s²)
So the units do work correctly.

 

[Bill_K]

Yes, for an electromagnetic wave the energy density is equal to the radiation pressure.