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This might be a stupid question, but I actually think that it is not so obvious:

When solving Maxwell equations, depending on the problem usually a charge and current density are assumed.

However, how can there be any current without charge density? I understand that in most problems where this question arises there is no net charge. So for example in a conductor where a voltage is applied across it there is no net charge, however nevertheless a current flows.

That is quite clear to me, but formally should one not have to assume the existence of charges even if the net charge vanishes. Is this problem solved in quantum mechanics where the charge and current are manifestations of a function satisfying the Dirac equation?

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    $\begingroup$ I'm not sure I understand your question. You seem to be aware that the charge density which appears in Maxwell's equations is the net charge density, and that this can be zero while the current need not be. $\endgroup$
    – J. Murray
    Commented Jan 6, 2021 at 17:40
  • $\begingroup$ From the example I understand that the net charge vanished but that there can be a current. But I was wondering whether this is just a good approximation and that actually one could take into account that there exist charges. $\endgroup$
    – eeqesri
    Commented Jan 6, 2021 at 18:00
  • $\begingroup$ Current needs moving charges, not net charge. A lot of negative charge can easily be flowing even when equally much positive charge is present, making the material neutral but carrying current. $\endgroup$
    – Steeven
    Commented Jan 6, 2021 at 18:18
  • $\begingroup$ From a moving reference frame your neutral wire looks charged, with charge sign and density depending on the RF velocity $\vec{V}$. ;-) $\endgroup$ Commented Jan 6, 2021 at 18:28

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From the example I understand that the net charge vanished but that there can be a current. But I was wondering whether this is just a good approximation and that actually one could take into account that there exist charges.

Nobody is saying that charges don't exist. If $\nabla \cdot \mathbf E = 0$, then that means that in every volume of space, there are an equal number of positive and negative charges (which could be zero or non-zero). If you consider a classical model of electrons and protons where they are simply tiny charged spheres, then this is an approximation which would break down on atomic length scales, but this would be very quickly averaged out on larger scales.

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  • $\begingroup$ Yes that makes a lot of sense. It seems more obvious in integral formulation. Thanks $\endgroup$
    – eeqesri
    Commented Jan 6, 2021 at 18:22
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This is an interesting question and points to a possible misconception induced by the usual presentation of the subject.

The facts are that, at the macroscopic level, we have two quite independent concepts: charge and current. If one would have only the knowledge of Coulomb's law and its consequences and Ampere's law and its consequences, it would be clear that there are two separate families of phenomena induced by two different sources. However, we know much more and we know that there is a relation, encoded in the continuity equation: the divergence of the current density is equal to the time derivative of the charge density. This is directly connected to the experimental fact that we can charge or discharge conductors by means of currents. Still, it is important to recall that a constant current alone does not generate an electric field.

The reason for our belief about the strict relationship between charges and currents originate from the classical model for a current due to the motion of pointlike charges. We can assign to each charge $q$ at the point $\bf r'$, moving with velocity $\bf v$, a current density $$ {\bf j}({\bf r}) = q {\bf v} \delta ({\bf r}-{\bf r'}). $$ if we have two fluxes of an equal number of oppositely charged particles moving in opposite directions, we have a system globally neutral, but with a non-zero current.

Things become even more interesting (or puzzling) in a quantum-mechanics, where we cannot assign to each particle position and velocity at the same time.

Therefore, from the point of view of QM, it becomes useful to maintain a larger decoupling between the concept of current and charge than in the classical case. Even in the case of a single particle the presence of a current becomes decoupled from the ill-defined concept of motion. For instance, we are sure that the electron in the hydrogen atom in the $1s$ orbital does not stay fixed at one position in space. Still, the $1s$ orbital does not carry any electric current. Instead, a $2p_1$ state carries a well definite electric current experimentally measurable through the resulting (orbital) magnetic moment.

Of course, even at the quantum level, there is no current if there is no charge at all. But classical and quantum examples show that the relationship between current and charge densities is looser than usually thought.

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In the absence of a changing magnetic flux, a current flow requires a separation of charges. If you connect a long looping uniform conductor to the terminals of a battery, a variable charge density is require to maintain a uniform field and current density in the conductor. There will be a positive charge density near the positive terminal of the battery and a negative charge density near the negative terminal. Within the conductor, the field driving the current will be proportional to the gradient of the charge density. (Toward the positive end, excess flux will leave through the sides of the conductor.)

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