# Tagged Questions

The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.

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### If electric fields can't pass through a grounded shield, how can a magnetic force be generated on moving charges outside a wire? [duplicate]

Say an electron outside a wire is moving with the same velocity as the electrons inside, then due to Lorentz contraction, an electric field would be generated in the outside electron's frame of ...
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### What happens in circuits where the propagation time of the electric field is significant?

Kirchhoff's current law assumes "that whenever current flows into one end of a conductor it immediately flows out the other end". I want to understand what happens when we cannot make this ...
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### If light is affected by gravity, does that mean gravity affects electrical&magnetic fields too?

Since we observed that masses indeed bend the light, and since we consider light to be composed of electrical and magnetic fields, does that mean a single point charge will result with bent electrical ...
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### Problem - Demonstration of electromagnetic energy conservation

I want to solve the following problem from my Classical Electrodynamics' book: Consider a pontual mass body, $m$, with charge $q$, which moves with an uniform velocity $v$ in a region where an ...
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### How does a “permanent” magnet with switch work?

We have several magnetic stands on my university, see here. This stand has a button which makes you able to turn the magnet off and on. It is quite hard to press the button, it seems like something ...
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### Why does a function $\psi(v)$ appear when a Lorentz transformation is applied on Maxwell's equations?

In his 1905 paper, Einstein applies Lorentz transformation on Maxwell's equations, and relates the electric force $(X,Y,Z)$ and magnetic force $(L,M,N)$ in an inertial frame $K$ with spectime ...
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### Eddy currents on a conductors surface, and induced current flow around the loop both coexist?

The title seems confusing, because Eddy currents are induced currents... however, let me explain. Assume the following closed loop passing a uniform magnetic field like so: Now, this loop would ...
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### What force decays an oscillating charge?

Consider this scenario: We have a charge on a spring. As it oscillates, it loses energy to radiation. What force decays the oscillation? As far as I can see, the only forces in the scenario are ...
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### Does charge bend spacetime like mass? [duplicate]

Does charge bend spacetime like mass? I'm not asking if electromagnetic forces can be described geometrically, but if EM fields could correspond to particular curvatures of spacetime, like gravity ...
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### Does magnetic poles exist for a magnetic field generated by straight current carrying conductor?

I read that a straight current carrying conductor generates magnetic field around it in form of concentric circles. If so, where would be the poles of this field.
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### Charged particle under a uniform electric field

Suppose a charge particle $q$ starts to move without initial velocity under the influence of a uniform electric field $E$ pointing in the positive $x$ direction. Express its position vector in terms ...
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### Magnetic Flux Quantization

I remember a few years back in my introductory AC/DC circuits class reading a little bit in the back of the book about magnetism. By this point, I was well aware that magnetic fields are commonly ...
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### Does the induced Electric field develop instantly or lags by $\frac rc$?

Consider a loop of radius $r_0=3 \times 10^8$ cm. A thin bar magnet is passed through its center. This implies that the magnetic flux through the loop will change. Now according to Faraday's law a non ...
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### How does the magnetic field gain energy through the induced non-conservative electric field?

This is a further question derived from this question about storing energy in an inductor. The accepted answer showed that, during the current increasing, it is the induced non-conservative electric ...
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### Do the fields exist without electric charges? [closed]

I read in an old book on electrodynamics by Pauli that theoretically there does not exist any need of charges to be there. Fields can even exist without the charges but still independent fields ...
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### Challenging Magnetostatics Problem - the “blind spot” of a magnetic dipole

I'm reviewing for an electromag exam and I stumbled upon a problem that's really hard to figure out. Here it is: A small magnetic dipole with moment $\vec m = m_o \hat z$ is in a region with uniform ...
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Say we have a non-uniform magnetic field that is static in time. Specifically, let's make it: $\overrightarrow{B}(x,y,z) = x \hat{z}$ Now say we have a metal loop in the xy plane which has a non-...
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### Force exerted by light on a moving mirror

Consider a light with energy density E shining uniformly over a mirror. The mirror has an area A. The mirror is moving at with a velocity β. Calculate the force that the photons exert on the mirror. ...
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### Utility of gauge four-potential $A_{\mu}$ (as opposed to electric and magnetic fields ${\bf E}$ and ${\bf B}$) in E&M?

The action for an electromagnetic field with source charges is given by $$S= \int \left\{ \frac{1}{4\mu_0}F^{\mu\nu}F_{\mu\nu} - J^\mu A_\mu \right\}dx$$ By setting $dS=0$ and taking the Lorenz ...
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### What does “optical conductivity” mean?

Does it just mean "AC electric conductivity"? If so, why have a special name for it, and why mention optical specifically? The wikipedia page on it is very sparse. This (warning, PDF) document just ...
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### Lorentz force, must there be two magnets?

The diagram above demonstrates the Lorentz force, Yet I wonder... must there be two magnets/electromagnets? If the two magnets form a magnetic field of $1$ $Tesla$ acting on the wire, why not use 1 ...
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### force on a moving charge in magnetic field

Need help in understanding the direction of magnetic force in the magnetic field!Totally confused by directions. Why is it that magnetic force is perpendicular to the direction of magnetic field and ...
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### Locations of destructive interference for two spherical waves

I have looked at this, but it did not help with locations. Really this just comes down to mathematical manipulation, which for some reason I fail to see. Here is my paraphrased setup: Consider two ...
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### Why gauge invariance for electromagnetic fields?

What is the physical constraint that gauge invariance is a required condition for electromagnetic fields? What would happen if the electromagnetic fields were not gauge invariant?
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### What is the physical significance of the Dipole Transformation of Maxwell's Equations?

The Question Given Maxwell's equations of the form \begin{align} \bar{\nabla}\times \bar{B} = \dfrac{4\pi}{c} \bar{J} + \partial_0 \bar{E} \\ \bar{\nabla}\times \bar{E} = -\partial_0 \bar{B} \\ \bar{...
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### Definition of Ampere

On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of $2 × 10–7$ newton per metre ...
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### Is the canonical momentum conserved when a particle moves in magnetic field?

Here is a question about the canonical momentum that I had asked some days ago, but I still have one point that I am not understand. Considering a particle moves in a magnetic field with charge $q$ ...
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### Voltage in a circuit?

The voltage of the battery signifies the difference in voltage between the positive and negative terminal What does this mean? The definition of voltage difference I'm familiar with is the amount ...
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### Maximum potential of a van de graaff generator

I know that we can calculate the maximum potential of a van de graaff generator by ( radius* electric field in which corona discharge begin to form in the surrounding gas( according to wikipedia 30 kv/...
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### The signature of the metric and the definition of the electromagnetic tensor

I've read the definition of the electromagnetic field tensor to be F^{\mu\nu}\equiv\begin{pmatrix}0&E_x&E_y&E_z\\-E_x&0&B_z&-B_y\\-E_y&-B_z&0&B_x\\-...
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