# Partial differential: partial scalar partial vector

1. Sep 30, 2009

### Havik

Hi,

I have a problem to find the meaning of a special partial differential: partial scalar partial vector.

i.e. dF/dn where F is a scalar and n is a i.e. normal vector. This is a partial diff.

n could be a vector consisting of partial differentials, (dT/dx,dT/dy)

I have looked in literature but found nothing.

Can someone help me?

Thank you very much
/Andreas

2. Sep 30, 2009

### tiny-tim

Welcome to PF!

Hi Andreas! Welcome to PF!

(have a curly d: ∂ )
No such thing … you can't have d(scalar)/d(vector) or ∂(scalar)/∂(vector).

But (for example, when calculating flux) you can have ∂F/∂n or d(F.n^)/dn, where n^ is the unit vector in the normal direction, and n is the distance in that direction.

3. Sep 30, 2009

### LCKurtz

Perhaps you are thinking about directional derivatives. If F(x,y,z) is a scalar function (perhaps the temperature at (x,y,z)), and V is a vector, then the rate of change of F in the direction of V is:

$$\frac {\partial F}{\partial \hat v} = D_{\hat v}(F) = \nabla F \cdot \hat V$$

where $$\hat V$$ is a unit vector in the direction of V.

4. Oct 1, 2009

### Havik

Hi tiny-tim and LCKurtz,

This is exactly the explaination I am looking for! I had a hard time to understand the meaning of such partial derivative. And it with $$\partial$$ it should be

It is the rate of F in the direction of some unit vector n that is normal to an arbitrary surface. I have no problem to find the growth rate of F in x and y but when it came to a other direction depending on other things, it became a problem. But now I understand how to do it!

I actually did not think of the thing that n must be a unit vector. I will make it a unit vector!

Thank you very much for your help on this problem, I have been struggling to find the answer for a long time!