# 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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### Electric field due to a current carrying loop

I want to know how I can calculate the radiated electric field in the far zone of the loop. Knowing that the loop is in center at the origin of a x-y plane Knowing that the loop as a time-harmonic ...
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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 ...
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### Current density defined by the scattering operator

I have a problem with the definition of the current density. In most literature it is defined as $j^\mu=\frac{i}{2}(S^*\frac{\partial S(A)}{\partial A_\mu(x)})$. I understand that normally we use ...
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### Current Density Boundary Conditions and its Implications

According to Ohm's Law, one can say $\overline{J} =\sigma \overline{E}$ if the field is in a conductor, and $\overline{J} =0$ if it's in empty space. Now if we take the surface of a conductor and ...
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### Strength of Magnet in Magnetic Bearings

While reading about magnetic bearings, one reads about passive/active magnetic bearings. By passive, one means no electric current is put into the bearing for attaining magnetic levitation/flux. In ...
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### The anapole moment, derivation from Dirac current density

Basically I am looking for a way to expand the electromagnetic interaction energy $W = A_{\mu}j^{\mu}$ (both $A$ and $j$ obtained from the Dirac equation) similar to the classical expansion in ...
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### sensitive scale & resistance of analog current meter

Does the resistance of an analog current meter increase or decrease when it is set to a more sensitive scale (lower range)?
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### Drift velocity of electrons in a superconducting loop

Do electrons travel at the Fermi velocity in a superconducting loop? For metals the Fermi velocity seems to be around $10^6$ m/s. So would electrons (in a Cooper pair) travel around the loop at this ...
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### current density in 1-d

I have a slight problem with the notion of the current density in one dimension. For example the probability current in 1-d given by: $J(x) = -\frac{1}{m} Im(-i\psi^*\partial_x \psi)$ calculation the ...
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### Relationship between current, voltage, magnetic field in a torus

I'm trying to find the relation between current density $\boldsymbol{j}$, voltage $v$, and magnetic field $\boldsymbol{B}$ for the time-harmonic approximation in a cylindrically-symmetric coil (a ...
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### Current between supeconducting rings

How to calculate the current between two superconducting rings with radius r separated by a distance d? Please note that being unfamiliar to the concept of superconducting rings, I can't approach ...
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### How much current would be generated by polarity reversal of Earth's magnetic field?

Continuing from my previous question Is reversal of magnetic polarity in a planet an instantaneous occurence? A change in magnetic flux is expected to generate an EMF. In the case where the ...
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### How to apply kirchoff's laws to motors in DC and AC settings

DC case: is it ok to treat a motor as a variable resistor, with resistance depending on the load? AC case: suppose I have a motor that runs at 120V AC and is rated to X watts, how can I determine how ...
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### (boundary conditions) Interface between two lossless media

I'm wondering why there's usually no free charges nor free currents in the interface between two lossless media? no free current "I guess" is due to the insulating nature of a lossless media but why ...
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### induced current in loop

I'm re-asking this question that was posted by @zhzh and subsequently closed. If you have a loop that is spun clockwise as a magnet is moved near it, is a current induced in that loop? If the ...
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### Magnetic field outside the solenoid

I found solution to problem 257 from "300 Creative Physics problems" hard to understand. In that problem we have very long solenoid with coil wounded in one layer. Data such as: density of turns, ...
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### Finding the magnetic field of an arbitrary arrangement of current-carrying wires

For an arbitrary arrangement of current-carrying wire, I am trying to show that the Taylor expansion around $\vec{r} =0$ of a magnetic field $\vec{B}(\vec{r})$ is given by \begin{align} ...
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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 ...
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### Is their a visual difference between air-gap sparks of the same voltage but different current?

For example, if this spark gap has a kilovolt of power behind it @ 1A and the other is also at a kilovolt but is @2A will there be a visual difference (i.e. will the one with higher current be ...
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### Noether Current and Feynman Diagrams

My question is simple. Assume that there is no anomaly and we have found from the lagrangian that there is a conserved current. I want to know what this means in terms of feynman diagrams, not in ...
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### Is shaking a phone enough energy to charge a smartphone?

Is shaking a phone enough energy to charge a smartphone? For example I'd like to use some kind of power generator to charge smartphone's battery but I'm wondering if shaking (by hand, or motion when ...
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### Do these symmetrical looking B field problems have the obvious answer?

I am struggling with two questions whose answers both appear to be trivially zero by symmetry arguments, when I was expecting more derivation to be required. Problem A), An infinitely long wire ...
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### How does non-constant conductivity change surface charge on a wire?

Current density is equal to conductivity times electric field. Therefore flux through a cross section of a wire is equal to current over conductivity. Current has to be constant along a wire. This ...
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### Magnetic flux and flow of electrons

What does change in magnetic flux mean? How can you change magnetic flux? How does a change in magnetic flux influence a current of electrons (electricity)?
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### Does the skin effect and the proximity effect cancel each other out?

I'm interested in how the Skin Effect and the Proximity Effect interact with each other. From what I can understand: The Skin Effect is when AC current 'collects' on the skin of conductors due to ...
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### Rapids in a River

Can I expect to have bigger or smaller rapids in a river that has a water level that is higher than average? I am not sure which way this would go. With the lower water, one is closer to the rocks, ...
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### To find the center of charge in current waveform

I have a circuit for which output current waveform has both positive and negative points. Now I want to get a center of the charge for this waveform (means where my net charge is concentrated ?). Is ...
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### Magnetic flux density a small distance off axis from a current loop

I'm currently studying physics at University and struggling with the problem mentioned, it's on a past paper I'm trying to do. I have calculated the B field on axis as (sorry don't know how to format ...
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### Current density in phase space

I have a question which arises from looking at the impact free Boltzmann equation. Let $(\vec{x},\vec{v})$ be a vector in our phase space $\Gamma^N = \mathbb{R}^{6N}$. The dynamics of a state are ...
328 views

### Effect of Current on spring

When Current passes thru a spring , some books mention that it gets compressed. However, I think due to the heating effect of current, molecules will increase kinetic energy and the spring should get ...
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### Force Exerting on a magnetic train

A question is already asked ( How does this “simple” electric train work? ). But I've got a question about it. In order to calculate the force which is being exerted on the system including magnets ...
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### Modeling the working of a transformer by pushing magnet into a coil

Let me go step-by-step. First I will outline a simple transformer. There are two circuits next to each other (not touching). One is an electromagnet set-up (with AC supply) and the other has a ...
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### Measure a current from a charged surface caused by charge separation

I have separated a charge in an electric field like here (http://elektroniktutor.oszkim.de/grundlagen/gr_pict/coulomb5.gif) and I put the metal out of the field. The charge density on the surface of ...
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### Angular current in torus

I'm trying to find the magnetic field due to a torus anywhere in space. I'm approximating it as a circular current loop. What I'm having a hard time with is the current in the theta direction, that ...
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### What were the experiments performed that lead to ohm's Law

How was the relationship of $V=IR$ first developed? What were the experiments that were preformed that lead to Ohm's Law? I understand that they had devices during George Ohm's time to measure current ...
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Can we think that an ideal conductor is a friction less space for electrons since under influence of electric field they create an exactly opposing field and produce shielding effect ?If so then how ...
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### Resistance used to get maximum illumination with proper security

In which way a resistance is to be connected with a bulb labelled 120V-60W when it is connected with 220V DC source so that maximum illumination with proper security is maintained? What is the ...
19 views

### What is the general proof that the torque acting on an irregular current carrying loop in an uniform magnetic field is only dependent on area?

How do you prove that the torque acting on an irregular current carrying loop in an uniform magnetic field in the plane of loop = $I \vec{A} \times \vec{B}$ Where, $\vec{A}$ = Area vector of the ...
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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 ...
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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 ...
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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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### 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 ...
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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?
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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 ...
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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 ...
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### Is this the reason why a circuit with no components (except wire) would discharge the battery faster?

I read that if I simply connect two ends of a battery it would discharge really quick and it would last longer if the circuit contained e.g. a light bulb. I wondered why is this happening and my ...
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### How is ampere defined for alternating current?

Both alternating current and direct current are measured in ampere. But how is ampere defined for alternating current? It cannot be defined from two mutual attraction of two parallel wires carrying ac ...
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### debris removal by electrification

Often during the lifetime of equipment, there is debris or residue accumulation on surface of, for example nozzles. This material accumulation can be quite strong. What kind of forces are they ...
Consider the following diagrams: Diagram(1): A large conductive slab($C$) with resistance ($R$) and region with magnetic field($B$), the conductor moves to the right as indicated with ($v$). ...
The current density $\vec{j}$ can be computed as $\vec{j} = \vec{\vec{\sigma}} \vec{E}$, where $\vec{\vec{\sigma}}$ is the conductivity tensor and $\vec{E}$ is the electric field. This conductivity ...