Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Converting Watts, Distributed Over An Area, To Energy Density

  1. Mar 29, 2012 #1

    jaketodd

    User Avatar
    Gold Member

    Is there a way to get energy density from watts, with the resultant energy density not involving time, as watts do?

    Thanks,

    Jake
     
  2. jcsd
  3. Mar 29, 2012 #2
    You have not supplied enough information.

    First of all energy density is not well defined in your question. Depending on the context it could mean energy per unit volume or energy per unit mass. You stated that you are starting with power per unit area so (J/t)/m2 or J/tm2. Assuming that by energy density you mean energy per unit volume the units are J/m3 so in order to equate the 2 you need to be able to equate time to meters. In other words, you need the velocity of the energy.
     
  4. Mar 30, 2012 #3

    QuantumPion

    User Avatar
    Science Advisor
    Gold Member

    Watt is a unit of power, and is equal to a joule per second. If you have a power and want to know an energy density, you need to multiply the power by some interval of time and divide by the area or volume to which the power is distributed.
     
  5. Mar 30, 2012 #4

    jaketodd

    User Avatar
    Gold Member

    Thanks guys...

    The watts travel at the speed of light. Also, the volume is m[itex]^{3}[/itex]. Does this help get an answer?

    Thanks,

    Jake
     
  6. Mar 30, 2012 #5

    jaketodd

    User Avatar
    Gold Member

    I'm looking for a conversion of watts, given that they travel at the speed of light, over a predefined area, which I could, perhaps, multiply by. In other words, I'm trying to get the energy density.

    Thanks,

    Jake
     
  7. Mar 31, 2012 #6

    K^2

    User Avatar
    Science Advisor

    So you have either RF radiation or some other massless boson field flux through a given area. Easy.

    [tex]\rho_E = \frac{P}{A c}[/tex]

    Edit: Note that it gives you J/m³, which is what you want, if I'm not mistaken.
     
  8. Mar 31, 2012 #7

    jaketodd

    User Avatar
    Gold Member

    Excellent. However, I think I shot myself in the foot by saying "area." I meant volume. I assume the 'A' in your equation is for two-dimensional area, and not volume? And, yes, [itex]\stackrel{J}{m^{3}}[/itex] is what I want.

    EDIT: Actually I think what you gave me might work for my purposes, but it would be nice to have answered this latest query.

    Thanks!

    Jake
     
    Last edited: Mar 31, 2012
  9. Mar 31, 2012 #8

    K^2

    User Avatar
    Science Advisor

    Power flux through volume doesn't make sense. Power flux through a give surface section does. That's what you need to know to find the average energy density of whatever's flowing past.

    Think about it in terms of analogous situation with electrical current. The quantities correspond as follows.

    [tex]P \rightarrow I[/tex]
    [tex]A \rightarrow A[/tex]
    [tex]\rho_E \rightarrow \rho_q[/tex]
    [tex]c \rightarrow v[/tex]

    Where v is drift velocity, and I is total current flowing through cross section. Then you know that current is given by this expression.

    [tex]I = \rho_q A v[/tex]

    Rearrange the terms, and you get the same thing.
     
  10. Mar 31, 2012 #9

    jaketodd

    User Avatar
    Gold Member

    Like I said in the edit, I think what you have provided will suffice for me, and I thank you greatly!

    Jake
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Converting Watts, Distributed Over An Area, To Energy Density
  1. Watts to Kinetic Energy (Replies: 10)

  2. Distribution of energy (Replies: 3)

Loading...