# Tagged Questions

A measure of the rate at which electric charge is transported (especially through a circuit), it has units of charge/time.

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### How to calculate resistivity of ring [closed]

Q: In a resistance experiment, a gold ring was connected into a circuit as a resistor. The connections touched on diametrically opposite points on the circular ring. Its diameter is 2cm, and the ...
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### Does voltage always cause current?

I'm wondering if the following statement is fully correct: Voltage causes current through a closed circuit, but through an inductor it is the change in current that causes a voltage. Obviously ...
1answer
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### Definition of electric current

As I am taught in school that electric current is the flow of electrons, but in some places I faced another definitions like transfer of energy between electrons. I thought that it is the transfer of ...
1answer
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### What is wavelength of electric field generated in a wire? [closed]

I read link given below: http://en.wikipedia.org/wiki/Speed_of_electricity Above link says _The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic ...
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### What happens to the resistance, voltage and current when DC passes through saltwater?

I have a circuit of a battery(1.5V or 9V) connected to a multimeter, a connected to 2 metal pieces in water(distilled)(see picture, in place of the LED is a multimeter). With the addition of salt 1. ...
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### Does inductors oppose the same change in current that it itself produces in case of DC Current?

We know that the inductor is a solenoid. In that solenoid, due to self induction it produces its own emf. That causes a change in the initial current. To counter this change the inductor produces ...
1answer
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### Self induction in a circular coil

There's a formula for self inductance : $$L=\frac{n\Phi}{i}$$ where n is the number of loops. But the book also says self inductance is directly proportional to $n^2$ I totally agree with the ...
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### Speed of AC and DC current

If AC current oscillates as it passes through a conductor is it slower than DC current which does not oscillates?
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### Does the current in a resistor depend on the speed of electrons?

If the speed of electrons in resistor is slow, then will the current be slow and if the speed of electrons is fast, then will the current be fast? Does the current in a resistor depend on the speed of ...
1answer
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### Doesn't for a steady-state current, current-density $\bf{J}$ needs to be same everywhere?

For a steady-state current, the relation must hold $\text{div} \mathbf{J}= 0 .$ This implies $$\frac{\partial}{\partial t} \; \rho_\text{charge-density inside the volume}= 0.$$ This further implies ...
1answer
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### What is the current of a single charged particle moving at a constant velocity?

Disregarding quantum and relativistic effects what is the current of a single electrically charged particle with charge $q$ moving at a constant velocity, $\vec{v}$? I am looking into to this to make ...
5answers
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### How does a wire carry alternating current?

Consider a simple network of a bulb whose two terminals are connected to two wires with open ends A and B respectively A o--------💡--------o B Now if a DC ...
1answer
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### If current takes the path of least resistance, why do parallel circuits work? [duplicate]

Why doesn't all the current flow down the path with the weaker resistance instead of dividing and going through both resistors (in a parallel setup with 2 unequal resistors)?
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### Will a bulb glow brighter if voltage is increased but current remains the same?

I understand that increasing voltage increases the energy given to each coulomb in the circuit, so in theory more energy given to a bulb should make a bulb glow brighter. But also if voltage is ...
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### Calculating root mean square velocity using current vs time graph

There was a question in my today's physics exam. Me and my friend had conflicting views about the question. Here is the graph and the question is "Calculate the value of Irms using following graph." ...
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### Is resistance the gradient in a $V/I$ graph?

We have a circuit where there is a variable resistor, and we increase this resistance at a steady rate, while increasing current. Thus we have increasing voltage. The gradient is defined by $dy/dx$. ...
0answers
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### Levi-cevita symbol, force on current by magnetic field of other current

I have the expression for F⃗1, the force on I1 due to the magnetic field of the current I2, using the Levi-Cevita symbol. However, one part of this double integral should equal zero mathematically or ...
3answers
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### How does an ammeter in a circuit work?

Apologies for this simple question, but I'm having trouble grasping the concept of how an ammeter works. Taking the following circuit as an example: When the switch is closed, current flows ...
3answers
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### How does loss of energy happen in current flow?

I don't understand how current electricity loses energy while it travels from station to our home appliances and back to station. Does it keep on losing energy and become useless? Can't we use the ...
0answers
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### A 100 000 km wire in contact with a positively charged sphere

A long copper wire (see the picture), placed somewhere in the outer-space, having the length $L=100000 km$ and the cross section $S=1 cm^2$, comes in contact at one of its ends ($x = 0$) with a $+Q$ ...
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### How to measure transverse current-current correlation at arbitrary wave vector and frequency

Let $\hat J^{\mu}(t,\boldsymbol r) \equiv (c \hat \rho(t,\boldsymbol r), \hat{\boldsymbol J}(t,\boldsymbol r))$ be the density-current operator at spacetime coordinate $(t,\boldsymbol r)$, in the ...
1answer
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### Need to find module of magnetic field

I have a post-lab problem During the experiments I have a U magnet on a scale. Inside the U magnet there is a conductor with a steady current. The conductor is made so that the magnetic field is ...
2answers
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### Magnetic field due to current, I don't get how you get the direction, can someone draw it for me? [closed]

Taken from here, 3rd problem. As you can see, no matter from which point current goes, from a to b or from b to a, in either cases, magnetic field would be down, right?
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### Setting up current directions in multiloop circuits with more than one battery

I'm currently working on this problem: I must find the current in each battery. I have certain doubts when estipulating the currents: How many $i$ exists in this problem? The current going in a ...
1answer
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### Integral Represation of Four Current with proper time

I have a question about the four current in covariant representation. the four current is defined as $$J^{\alpha} = \binom{c\rho}{\vec{j}}$$ and i'm having a point charge, ...
1answer
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### Visualising current in the LCR circuit

I just went through the theory of the serial LCR circuit. There current and voltage are 90 degrees out of phase with each other. Now I was trying to imagine an animation with just dots. And I ...
3answers
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### Current and resistance

Why the current is the same at all of the positions in a series circuit??,although there are different voltages at different positions of the circuit. What i know is that as the electron passes one ...
0answers
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### When a coil makes a complete turn, why is does magnetic flux double with respect to time?

EMF, $\mathcal{E}$, is equal to -d($\phi$)/dt Where $\phi$ is magnetic flux. Why is it that if from $t_1$ to $t_2$ the coil makes a complete turn and during this turn the normal vector's angle with ...
2answers
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### How to find the direction of an eddy current?

Suppose there is a magnetic field going from left to right. Suppose a thin sheet of metal conductor (e.g. a 1m*1m square) is dropped through the magnetic field such that the plane of the conductor is ...
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### Current flow in wire due to TEM wave

Below I've drawn (skillfully) a wire and AC voltage applied at one end. Note the wire goes on indefinitely. I am confused on the direction at which the E field propagates and the direction at which ...
5answers
654 views

### In what case does a Coil with Current flowing through it NOT produce a Magnetic Field?

According to what I've studied, when Current flows through a Coil, a Magnetic field is produced. Recently, I've been told that there is a case in which Current flows through a Coil Without producing ...
1answer
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### Given a large, multiloop circuit with resistors and batteries and I'm not sure how to simplify it. (When NOT to combine resistors?) [closed]

So here's the circuit, and the question is looking for current i: All resistors are 2 ohms and the batteries are 5 volts. I'm trying to apply the resistance combination Kurchov rules to make this ...
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### Time derivative of Noether charge

I understand that the Noether charge can be written as $$Q= \int d^3 x J^0$$ and the time derivative of the Noether charge is zero $$\dot Q=0$$ but how would you explicitly calculate it?
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### Physical intuition behind the fact that current in an inductor lags behind the emf/voltage by $\frac{\pi}{2}$ in an AC circuit

What exactly is the reason that current through an inductor in an AC circuit acquires maximum value after quarter a time period ($\frac{T}{4}$) of the emf getting the max value?
2answers
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### Inductive reactance 0 for dc current?

Inductive reactance: $$X_L=\omega L=2\pi f L$$ is the opposition to the flow of current by an inductor. Frequency $f$ being $0$ for a DC current, the inductive reactance too is $0$. But doesn't the ...
2answers
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### Which expression to use for electrical power?

We know that electric power can be written as $P=VI$, or $P=\frac{V^2}{R}$, or $P=I^2R$. But when to use which one? Sometimes two different formulas give different results! Please explain with ...
1answer
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### Why is $\ell$ in $\mathbf F = I~\ell \times \mathbf B$ taken as shortest distance?

The equation $\mathbf F = I~\ell \times \mathbf B$ works good for a straight wire. But in cases where the wire is bent in the form of a sine curve or in a circle and the magnetic field is assumed ...
0answers
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### Bar magnet moving perpendicular to solenoid

Suppose i have a solenoid of finite length and i take a bar magnet and move the magnet's North pole along the solenoid. The magnet end/pole is essentially against the outside of the solenoid forming a ...
1answer
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### What is general strategy to find the resistance of medium?

What is general strategy to find the resistance of medium? These are some of the examples Two metal balls of the same radius $a$ are located in a homogeneous poorly conducting medium with ...
1answer
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### How can I redraw this circuit where I can apply Kirchoff's Voltage and Current Law to get correct Voltage at V?

I thought I can draw the -5V as a source but it did not turn out to be right, unfortunately.
2answers
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### Why does all the current flow through a short circuit if its voltage drop is considered zero?

Path of least resistance vs. short circuit I know the path of least reisistance has been clarified already However, to derive the equations you need to assume that the voltage of each parallel ...
2answers
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### Why do old tube lights flicker?

When tube lights become considerable old, they start flickering; i.e when the tube lights are switched on, they don't glow immediately, but start flickering. After some time they glow. Sometimes they ...
1answer
588 views

### Why do voltmeters and ammeters have high and low resistance respectively?

I understand why voltmeters are connected in parallel and ammeters are connected in series, but why is it that to measure voltage, you must have high resistance, and to measure current, you must have ...
0answers
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### Is the current density ${\bf j}$ an observable?

The current density ${\bf j}$ is most easily defined from$$I = \int_{\text{cross-section} \atop \text{of wire}} \textbf{j}\cdot d\textbf{S},$$ where $I$ is the current flowing through the wire. From ...
2answers
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### Parallel resistances

How is the net resistance lesser than the least resistance when resistances are connected in parallel? The electrons will have to flow through one of the resistances, right? To me, it seems like the ...
0answers
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### Irodov Problem 3.256

This is what the problem states: "Two long parallel wires of negligible resistance are connected at one end to a resistance R and at the other end to a dc voltage source. The distance between ...
4answers
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### Are insulators non-ohmic conductors?

Can insulators/ dielectrics be considered as non-ohmic conductors ? As they apparently breakdown when sufficiently large potential differences are applied across them.
1answer
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### Is the magnetic force on a current carrying conductor dependent on velocity?

A current carrying conductor is kept stationary in a region of uniform magnetic field, and another conductor, carrying the same amount of current, is moved with a certain velocity in a region of ...
2answers
51 views

### diffusion velocity

i understand that diffusion is the movement of particles from high concentration areas to low concentration, but what is the cause of that movement atomically. And especially in the case of charge ...