# Tag Info

### Charge conservation in ohmic material - Apparent paradox

As I understand it, the questioner's claim is that the equation $\frac{\partial Q}{\partial t}+\frac{\sigma}{\epsilon_0}Q=0$ is derived only from the standard Maxwell equations. As a result, $Q$ is ...
• 897
1 vote

### Why don't positively charged metal ions (in a wire) move but electrons do?

The question is why the ions are localised and form a lattice, unlike the electrons. The reason is that they are typically 10000-100000 times heavier than electrons. Therefore they have very small, ...
• 25k
1 vote
Accepted

### Why don't positively charged metal ions (in a wire) move but electrons do?

To answer this question, it is necessary to understand the structure of the metal at the atomic scale. A very simple way to see it is that the positively charged nuclei sit at fixed points in a ...
• 1,594

### Why don't positively charged metal ions (in a wire) move but electrons do?

It’s because the ions aren’t delocalized like the electrons are.
• 31
Accepted

### Charge conservation in ohmic material - Apparent paradox

Differential problems are defined in a domain and require boundary conditions, and the solution to be "regular enough" for the differential equations to hold. If you're dealing with a body ...
• 10.1k

### Dipoles as vectors

"why does it make sense that the resultant of those dipoles would give the same time period as of the original question" Your point is valid, it does feel and and seems misleading that the ...

### Dipoles as vectors

Rotational dynamics. The governing equation of the rotational dynamics of a system w.r.t. to its center of mass reads $$\frac{d \boldsymbol{\Gamma}_G}{dt} = \mathbf{M}_G^{ext} \ .$$ Considering only ...
• 10.1k

### Densities at a Point

It is a matter of approximation. Yes charges would not normally be concentrated into a point, but the specifics of charge distribution will normally contribute to second and third order effects. For ...
• 3,391
1 vote

### Why is the electric potential from a positive sphere not negative?

You have a really wonderful doubt. You did the calculations right, except for the dot product. It is a error easy to overlook.(Note: bold means vectors). Dot product E.dr is equal to Edrcos$\theta$. ...
• 418

### Why is the electric potential from a positive sphere not negative?

The potential energy is the work done on the unit charge as it is moved from $\infty$ to $r$. That is, suppose you are pushing the charge inwards, then the change in PE is the work you do. That means ...
• 357k

### Densities at a Point

You are on the right track. Matter and charge are often treated as continuous. It is a good approximation. Air exerts a pressure on the walls. You think of it as a continuous fluid. In reality, the ...
• 40.2k

### Surface charge density within a parallel plate capacitor on different voltages

Changing the potentials on the capacitor plates will not change the inside surface charges as long as the potential difference between the plates is constant. However, for a given potential difference ...
• 16.9k
1 vote

• 97.4k

### Induced charges due to uniform electric field

(1) When you put an isolated finite metallic object into an uniform electric field, there will be an equal amount of positive and negative charge induced. The total induced charge is thus zero. (2) If ...
• 16.9k

### Is voltage a difference between two charged objects or two positions in a field?

"Like, one pole is positively charged, and the other is negatively charged. And if that is so, then I don't understand why we are not considering any object that is electrically neutral to be of ...
• 36k
Accepted

### Is Coulomb's law wrong?

Coulomb's law only holds for static situations. It describes the force between particles that are always at rest with each other. The situation described in the question is not a static, as the ...

### Is Coulomb's law wrong?

You are making a beginner's misunderstanding, even though it confused all the great minds a few centuries back. Coulomb's law is correct, but it is not what you think it is doing, and you are using it ...
1 vote

### Is Coulomb's law wrong?

Electrostatic as well as electromagnetic force is transmitted with photons, so there is no contradiction. Any "immediate" change is immediate in terms of information travel speed. It cannot ...
• 135

### Does QED explain why Moving charge produce magnetic field?

It's actually the other way round. Maxwell's equations explain why a moving charge produces a magnetic field, and we get quantum electrodynamics by quantising Maxwell's equations. The link between ...
• 357k