# Griffiths Electrodynamics gradient of charge distribution

by Sparky_
Tags: charge, distribution, electrodynamics, gradient, griffiths
 P: 166 I do not understand the following from Griffiths’ Electrodynamics – page 424 Equation 10.21. $$\nabla p = \dot{p} \nabla {tr} = …$$ I’m not sure how much of this applies (I think my question is on the math) but p is the charge distribution, tr is the retarded time. Is this an application of the chain rule? With the gradient being a derivative with respect to spatial location (x,y,z), why is the time derivative showing up in the gradient? I initially want to say if something is dependent upon t but not on x, then its derivative with respect to x is zero. The result looks like the chain rule applied – I don’t see why the time dependent portion shows up. Can you help clear this up for me? Thanks Sparky_
 P: 266 continuity equation?
 P: 166 I do not see it yet. I see later on the same page $$\nabla \dot{p} = \ddot{p} \nabla {tr} = …$$ Can you explain further? Somehow the gradient is giving an additional time derivative. Thanks Sparky_
P: 502

## Griffiths Electrodynamics gradient of charge distribution

$\rho$ has arguments like this:
$\rho (\vec{r}', t_r(\vec{r}, \vec{r}', t))$

The gradient is being applied w.r.t to the coordinates of $\vec{r}$ ( not $\vec{r}'$ which gets integrated away). The coordinates that we would be taking the derivative with respect to in order to obtain the gradient are only found in the parameters of $t_r$. So this result is from the chain rule. Here is one component of the gradient, for example.
$(\nabla \rho)_x = \frac{\partial \rho (\vec{r}', t_r(x, y, z, \vec{r}', t))}{\partial x} = \dot{\rho}\frac{\partial t_r}{\partial x}$
 P: 266 Oh I see, did he specify that the dot derivative is with respective to retarded time?
 PF Patron Sci Advisor Thanks P: 3,975 It won't matter. ##\partial _{t_{r}} = \frac{\partial t_{r}}{\partial t}\partial _{t} = \frac{\partial }{\partial t}(t - \frac{\mathfrak{r}}{c})\partial_{t} = \partial_{t}##. Anyways, as noted above ##\rho = \rho(r',t_{r}) ## and ##r'## is no longer a variable after integration but ##t_{r} = t_{r}(t,x,y,z,r')## so ##\nabla \rho = \partial _{t_{r}}\rho \nabla t_{r} = \partial_{t}\rho \nabla t_{r}##. Not sure what that has to do with the conservation of 4-current (continuity equation) ##\partial_{a}j^{a} = 0##.
 P: 166 Thank you!! I went back in this section of the text and reread. I see that p (charge density) is specified p(r’, tr). That is actually the point of this topic (the nonstatic case). You confirmed that this is an application of the chain rule and p is a function of position and tr. Thank you for the help! Sparky_

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