#### dRic2

Gold Member

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**1. The problem statement, all variables and given/known data**

For a configuration of charges and currents

__confined within a volume__##V##, show that

$$\int_V \mathbf J d \tau = \frac {d \mathbf p}{dt}$$

where ##\mathbf p## is the total dipole moment.

**2. Relevant equations**

...

**3. The attempt at a solution**

I have one question: since

__the configuration of charges and currents is confined within a volume__can I assume

$$∇ \cdot \mathbf J =0 $$

?

I mean, if there is no "in" nor "out", ##\frac {\partial \rho}{\partial t}## should be zero, right ?