# Flux boundary condition on a face of a cube

1. Jul 27, 2011

### svishal03

Lets us say I have a cube and I apply to a face of the cube a heat flux of 100 watt/m^2.

Lets us say i divide the face of the cube into say 10 elements (area of each face of the element is 1 m^2).

What will be the flux on each element , will it also be 100 watt/m^2?

Sorry for a fundamental question

2. Jul 27, 2011

### nasu

If the flux through the face is uniform, any sub-area of the face will have the same flux.
But this is kind of trivial affirmation, is the definition of uniform flux.

3. Jul 27, 2011

### svishal03

Thanks, can you pelase explain the physical interpretation of the application of flux?That is, if i appply a flux of say 10W/m2 ona face, can you explain physically what is done?

4. Jul 27, 2011

### gsal

I am not sure if this will help, or whether you will come back with a 'recursive' question.

If you apply a flux of 100 W/m2, it means that you are applying a total of 100W and uniformly distribute them over a square meter.

If your face's cross sectional area is not 1 square meter, but 2, and you insist that you are putting 100 watts per square meter...then, you are actually putting in 200 watts total over 2 square meter...which is to say, 100 watts per square meter.

If you face's cross section area is 0.5 square meter, and you insist that you are putting 100 watts per square meter, then you are only putting 50 watts total over 0.5 square meters...which is to say, 100 watts per square meter.

Do you know how to calculate pressure? you know, like force divided by area? Kind of similar thing, where the total heat injected is analogous to force and when dividing by area you get the "density", if you know what I mean...

is that what you were asking ?

or are you asking how in the world you apply heat, in the first place? 'cause this you can do with an iron and just measured the power consume by it and then you know how much heat you are putting...assuming 100% efficiency on the iron and that no heat escapes between the iron and the face...

5. Jul 27, 2011

### nasu

The previous post explains already what the flux means.
"Application of flux to a face" is not really the most proper term. The flux is "through the face" rather than "to the face".
If you ask how can you produce a heat flux, then there are different ways to do it.
In order to have heat transfer you need a temperature gradient. This means regions with different temperatures.
In the case of the cube, either heating or cooling the cube relative to the ambient will result in heat flux through all its faces. The flux though may not be uniform.
Maybe a little more context will help clarify what you are after.