# Tag Info

### Why doesn't the number density of electrons increase with increase in temprature?

At first, in most conductors like metals the valence and conduction band overlap together. There is significant band gap difference occur at semiconductor, insulators. Now, on increasing the ...
• 93
1 vote

### Why doesn't the number density of electrons increase with increase in temprature?

No new electrons emerge with increasing temperature - some of them move from conduction to valence band, so the number of conduction electrons increases, whereas the number of valence electrons ...
• 61.9k
1 vote

### Current flow through a path of zero resistance

The 6 $\Omega$ resistor is shorted by the wire and current takes the path of least resistance, that is through the wire. Alternatively, the combined resistance of the 6 $\Omega$ resistor and the wire ...
• 3,886

### Kirchhoff's voltage law, signs?

If you study loop in the same direction of current, then potential at resistance is negative. If you study loop in the opposite direction, then potential at resistance is positive. If you study loop ...

### Current operator for Bloch electrons

Satisfying the current conservation equation is an unsatisfactory way to define the current because it does not provide a unique answer: you can add any identically conserved current (the curl of ...
• 55.3k

### Perfect conductor and magnetic field

The basic equation relating the fields in vacumn H and those in matter B is the relation $B=H+4\pi M$ where M is the magnetic moment per unit volume called magnetization. Outside matter $M=0$ and ...
• 1,430

### Perfect conductor and magnetic field

This is a question of scale. (And an absolutely perfect conductor is something that does not exist). Superconductors have a band gap that allows dissipation free currents for certain frequencies/wave ...
• 11.9k

### How big can a current resonance possibly get and what are the sources of power loss?

In a system like this, you are correct in asserting that a real-life solution cannot be infinite, so what happens instead is this (details in different systems may vary, and your mileage will be lower ...
• 96.7k
1 vote

### How big can a current resonance possibly get and what are the sources of power loss?

Either the coil is not ideal or you will have radiation of the EM field.
• 6,568

### Doesn't counting hole and electron current lead to double-counting of actual current?

It is interesting to know that such a question is asked even now - in modern era. It is a classical question in electrochemistry, where it was solved by Hittorff. According to his solution, positive ...
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### Doesn't counting hole and electron current lead to double-counting of actual current?

There is a problem of naming. Electron current counts the free electrons in conductive band only as they can roam "freely" and sometimes they recombine with a hole or form hole-free ...
• 1,224

### Doesn't counting hole and electron current lead to double-counting of actual current?

A simple way is to think of a moving hole as electrons filling the vacancy. Then we actually have two electrons moving - the one that previously occupied the hole (but is now in the conduction band), ...
• 61.9k

### How to show that the electrons responsible for a current have an energy within $k_BT$ of the Fermi energy?

Thanks a lot for this session! I have a further question: when we use J = e * n * v to calculate current density in a metal of finite resistance, considering Fermi-Dirac distribution, should n be ...
• 11

### Doesn't counting hole and electron current lead to double-counting of actual current?

Your mistake here is the assumption that both electron and hole are counted in the same band. If this assumption were correct, then indeed, we'd have counted the carriers twice. But actually, electron ...
• 29.4k
1 vote

### Heating effect on superconductors

Your question is a bit too vague for us to give the answer you're looking for, I think. If you have a type-I superconductor (or a type-II superconductor in low magnetic field) then you can pass a DC ...
• 70.5k

### Doesn't counting hole and electron current lead to double-counting of actual current?

Current is measured through a surface. If, in one unit time, a free electron passes through the surface and then recombines with a hole, the current is one unit charge per unit time. If, in one unit ...
• 3,121
1 vote

### Doesn't counting hole and electron current lead to double-counting of actual current?

If we were to count both hole current and electron current, one would say that one electron moved from left to right and also that one hole moved from right to left, which leads to actually count two ...
• 105k
Accepted

### Rotating disc and current?

Preamble The first text I posted was wrong. There was a miscalculation and I forgot to include the electric charge at the rim around the disc in my calculations. I apologise for that. A corrected ...
• 980
1 vote

### 3 Aspects of Voltage that contradict each other

Perhaps the best mental picture for voltage is elevation. Taking Gravitational Potential Energy $E_g= mgh$ divided by the mass, gives us Gravitational Potential: $$V_g= \frac {E_g}{m} = gh$$ And since ...
• 10.2k

### 3 Aspects of Voltage that contradict each other

(Electric) potential energy $U$ is associated with a charge $q$ and its location in an electric system. (Electric) potential $V=U/q$ is (electric) potential energy per charge, a measure that makes it ...
• 51.7k

### 3 Aspects of Voltage that contradict each other

1- Voltage is Potential Difference: Voltage is the difference between the energy levels. The potential difference between two points is the work required per unit charge to move the charge between ...
• 75.9k
Accepted

### 3 Aspects of Voltage that contradict each other

Basically the problem here is using the same words for similar and related concepts that are nonetheless not identical. The fundamental concept that is behind "voltage" is the concept of ...
• 2,397

### Are $U^2/R$ and $I^2*R$ the same?

The correct formula is $P=IV$. Here is a simple argument to understand this, based on Newtonian mechanics: The power dissipated by an electron moving with velocity $v$ under action of force $F=eE$ is:...
• 61.9k
Accepted

### Are $U^2/R$ and $I^2*R$ the same?

You made a mistake in using the formula. In that formula, U is not the transmission voltage, but the potential difference across the electric line. Even though we usually transmit at 220kV, that ...
• 428

### Why don't currents due to revolution of electrons add up?

By Biot Savart's for the moving charge q, where B denotes the magnetic field at the centre of the orbit, $B = \frac{\mu_o qv}{4\pi r^2}$ If the revolving charge is equivalent to a current-carrying ...
• 113

### Rotating disc and current?

Statements regarding emf and current like this one, $\dots$ it seems that there is no current even when an emf is generated (however small, a current must be present), are incorrect. For example, a ...
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### Rotating disc and current?

This goes to a more general question: what constitutes presence of an electric current? In cases like this it is often helpful to consider a parallel case. Here is what I propose: Create a setup with ...
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### Can the speed of light inhibit the synchronisation of a power grid?

A related problem is instability. I don't think the current means to control synchronization issues using dynamic reactive compensators (as mentioned above) is going to be good enough when there are a ...

### If light is electromagnetic then can light produce electricity or attract metals?

“if light falls on a metal it should produce current due to its electric nature but it doesn't” What make you think “it doesn’t “‽ “ due to its magnetic nature shouldn't it attract metal object” Not ...
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### Can the speed of light inhibit the synchronisation of a power grid?

You are correct, so what does the industry do? Grid Design and Control: Power grids are engineered to account for phase differences and signal propagation times. They use various devices, such as ...
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### Can the speed of light inhibit the synchronisation of a power grid?

Even worse than that - any piece of wire has its own inductance and capacitance, which are non-negligible when the line is long. Essentially, a power line acts as a waveguide, which sustains its own ...
• 61.9k
1 vote

### Why does a 60 W bulb glow brighter than a 100 W bulb when connected in series?

The tungsten incandescent bulb with the highest resistance will have the greatest voltage drop and be the brightest since it will consume the most power ($RI^2$). This is the $60 \,\text{W}$ bulb as ...
1 vote

### Is there an electric field due to current carrying wire?

The electric field in a CC circuit is the derivative of the potential with respect to the distance. In a copper conductor the resistance is very low, and a voltmeter will show zero volts between any ...
• 16.9k