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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66 views

Clarifications on Ampere's Law [closed]

I have just learnt Ampere's Law, useful for calculating the magnetic field in situations having a high degree of symmetry. However, I have some conceptual doubts regarding it: Before I begin, I would ...
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1answer
94 views

Is there an easy way to see that $\phi =\frac{\mu_0}{4\pi} \frac{\vec m \cdot \vec r}{r^3}$?

The scalar potential of an electric dipole is given by: $$\phi =\frac{1}{4\pi\epsilon_0} \frac{\vec p \cdot \vec r}{r^3}$$ The derivation of this is quick, however, is there an easy way to see that ...
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2answers
101 views

Why doesn't sunlight ALWAYS get split (into monochromatic) when going through the clouds?

It is my understanding that clouds are largely made up of water, which is known to split white light into its frequency components, and that's why we see rainbows sometimes. My question is, with ...
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2answers
27 views

Should static electricity affect a Crookes radiometer?

I've been playing around with a Crookes radiometer for fun and I happened across something I can't figure out. I know the fundamentals of the radiometer (e.g. partial vacuum, thermodynamics, etc.), ...
2
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1answer
163 views

Confusion in Maxwell's derivation of Ampere's Force Law

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 155" about "Ampere's Force Law". I have some confusion in the following pages: On the top of the page, Maxwell says:...
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39 views

Wave Velocity vs. Phase Velocity

I am trying to understand the difference between 'wave velocity' and 'phase velocity'. I know that generally they are equal, but when is that not the case? I, of course, tried to google it, and didn'...
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49 views

Movement of charges in presence of electric potential difference

This might appear to be a bad question to some but i can't figure out the answer. While reading about an electric circuit, i found this on the web: Over the course of time, one could think of ...
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2answers
236 views

How can we apply Ampère's circuital law in a wire?

How can we apply Ampere's circuital law in a wire to calculate magnetic field around a straight current carrying wire? The length of the wire is not infinite. Using the Biot-Savart law we get $$B = ...
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59 views

Homogenuous Maxwell Equations in the Language of Differential Forms

I understand that if I define electric field to be $E=E_i dx^i$, magnetic field to be $B=B_1 dx^2 \wedge dx^3 + B_2 dx^3 \wedge dx^1 + B_3 dx^1 \wedge dx^2 $, and field strength to be $F= dx^0 \wedge ...
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3answers
193 views

What symmetry gives you charge conservation?

This is a popular question on this site but I haven't found the answer I'm looking for in other questions. It is often stated that charge conservation in electromagnetism is a consequence of local ...
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1answer
159 views

Ampère's law from Biot-Savart law for linear currents with multivariate calculus

My book, W.E. Gettys's Physics, starts from the Biot-Savart law $d\mathbf{B}=\frac{\mu_0}{4\pi}\frac{Id\boldsymbol{\ell}\times\hat{\mathbf{r}}}{r^2}$, i.e.$$\mathbf{B}(\mathbf{x})=\frac{\mu_0}{4\pi}\...
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49 views

Time dependent electric field: Mathematical expansion for local electric field

In many articles and books I see that local electric field is expanded as $$\vec E_0(\vec r(t)) = \vec E_0(\vec R_0) − (\vec a(t) \cdot \nabla) \vec E_0(\vec R_0) \cos(\Omega t) + \ldots $$ For ...
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44 views

Details on the magnetic field of a linearly polarized electric wave

Suppose we are in vacuum and we have an electric field $\vec{E}$ which we assume is simple harmonic wave that propagates through $z$ and is linearly polarized in the $x$-$y$ plane along $x$ i.e. $\vec{...
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1answer
31 views

EM wave generation from different frames of reference

Take 2 observers and a charged particle, Observer S is stationary to the particle and sees the electric field and no magnetic field as he is stationary to the particle. Observer A decides to ...
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2answers
226 views

Relativistic explanation of attraction between two parallel currents

This has been answered a lot of times, and I have been reading about it on many websites, including PSE, but I still don't get it. So please don't mark it as a duplicate unless it really is one. Two ...
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3answers
75 views

How to evaluate energy conservation for magnetization?

A permanent magnet can attract a certain magnetic material mass, giving a certain amount of energy $U_{mag}$. If we first use this magnet to magnetize $n$ ferromagnetic pieces, we would have $n$ ...
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1answer
127 views

Must a universal motor always rotate at the frequency of the AC current?

So here I have a picture of a motor (I think it is a universal motor but I am not sure). My question is if this motor is connected to an A.C. supply, does it have to rotate at the frequency of the A.C....
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2answers
171 views

Electromagnetic wave properties of light vs radio waves

Both light and radio waves are electromagnetic waves.that means they have almost similar properties. Both are EM waves ,E and H fields are there in both waves.....but My question is "radio waves can ...
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1answer
59 views

Derive probability current density - factors of 2 discrepancy [closed]

To derive the probability current density for a particle in an electromagnetic field, we calculate $\dfrac{\partial \rho}{\partial t} = \dfrac{\partial}{\partial t} (\Psi^* \Psi) = \dfrac{\partial \...
2
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1answer
67 views

EMF produced when a square wire is pulled from the corners [closed]

The shown diagram is of a square conducting frame being pulled from the corners. The velocity of A and B is given as $u$. Each side has length $a$. I was trying to find the induced emf at the instant ...
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1answer
184 views

A working “mechanical” model that explains current in a wire/circuit?

In class we learned about point charges, electrostatic force, voltage, current etc. and discussed circuits along the way. Now problems arise when I try to apply the learned concepts to explain how ...
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66 views

Which photons pass through a circular annulus?

Passing light through a circular sieve: Well, actually, let’s think about radar or microwaves with a wavelength of order a centimeter or two, so you can tailor your aperture, say by etching a silver ...
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1answer
96 views

How large or small can frequency in the EM spectrum get?

The largest frequency range is gamma rays, but does the EM spectrum 'stop' somewhere? Like is there a limit to how large a frequency can get? Or how small frequency can get? Is it one of those things ...
2
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1answer
522 views

Will two opposing magnetic fields cancel out?

When a conductor induces eddy currents that creates a magnetic field opposing the change that created it, would the two fields at some point cancel out? Imagine the change to be so great, it ...
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3answers
142 views

How does the conductor knows which side is outside?

For a electrostatic equilibrium state, we know charges only stay on the outer surface of the conductor. But, how does the conductor know which side is outside? If it's about the curvature, then ...
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85 views

Why are photons bosonic?

I am studying the quantization of the electromagnetic field. My text quantizes by changing amplitudes to ladder operators, by putting in an action and by imposing bosonic commutation relations upon ...
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1answer
193 views

Obtaining quantum Hamiltonian for charged particle from path integral formulation

I was working on Shankar 8.6.4, which is about obtaining in one dimension the Hamiltonian operator of a charged particle from the path integral formulation. First, I get the propagator over a time ...
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1answer
199 views

If magnetic field cannot change velocity, what changes the velocity of a compass needle?

I am really confused after reading the properties of magnetic field. I thought magnets could speed up things when they attract them.
2
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1answer
171 views

Relativistic Lorentz Force with constant, perpendiuclar, and uniform E and B fields

I am trying to solve the following problem from the Fundamentals of Plasma Physics by Bittencourt (Problem 2.7): Analyze the motion of a relativistic charged particle in the presence of crossed ...
2
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1answer
37 views

Is it safe and theorotically correct to heat a bar magnet using an open fire/furnace, provided conditions given below?

For a project, I will have to heat a bar magnet to any temperature above 40 degree celsius. For this, can I place the magnet over an open fire --like one in a furnace-- for 20 minutes or so to achieve ...
2
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1answer
113 views

Can Maxwell's Equations explain electromagnetic radiation emission in an atom?

Can Maxwell's equations be used to explain the process of spontaneous emission when an electron drops from a higher energy level to a lower energy level? According the Maxwell equations, a changing ...
2
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1answer
158 views

Is it true that the self-force prevents a classical particle from falling into a Coulomb potential? What is the physical explanation of this result? [closed]

In 1943 CJ Eliezer published a paper claiming that the self-force prevents a zero angular momentum particle from ever reaching the center of an attractive Coulomb potential (and what's more that it ...
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1answer
52 views

Do changing magnetic fields always produce solenoidal electric fields?

Since the curl of E is the time derivative of B, $\nabla \times \vec{E} = -\frac{\partial B}{\partial t}$ Do changing magnetic fields always produce solenoidal electric fields? For instance a ...
2
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1answer
141 views

Difference between electric and magnetic field (relating to EEG & MEG)

I study cognitive neuroscience and I periodically run into physics related questions in the context of neuroimaging technologies. My question specifically refers to electric and magnetic fields that ...
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3answers
65 views

Electromagnets, and how $H$ is the thing we set on the dial

Let's say that I have an electromagnet, consisting of Helmholtz coils with an iron core. David Griffiths, in his "Introduction to Electricity & Magnetism" text claims that $H$ is what we set, by ...
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1answer
347 views

Flux linkage inside of a conductor

Can someone explain to me why the flux linkage inside of a conductor is dependent on the cross sectional area of the conductor? My book says that d$\lambda$ = $(x/r)^2\phi$ where $\phi$ is the ...
2
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1answer
289 views

Problem on Einstein - de Haas experiment

I am a Physics student (4th year) and I'm trying to study the Einstein - de Haas effect in laboratory. That is what I got: a suspended Iron cylinder with about 5 cm height and a radius of 0.8 cm is ...
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1answer
119 views

Time changing potential gives rise to “force”?

Imagine a charged particle inside a Faraday cage (i.e. charge on outside, zero electric field inside, but non-zero electric potential on the inside). Suppose the charge distributed on the outside of ...
2
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1answer
33 views

In a noiseless environment, how accurate do today's transmitters send EM waves?

Suppose that there is no external noise in the environment. How accurate are today's TEM wave transmitters in such a case? So if we want to send $200\cos(1000\pi t)$, can transmitters send exactly ...
2
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1answer
71 views

Why is it possible to define a potential difference in a wire that carries a steady current?

My book introduces $\nabla\times E = 0$ as the condition for an electrostatic field, which is what makes it possible to define a potential difference between two points. As far as my present ...
2
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1answer
419 views

Force between two finite parallel current carrying wires

Remark: This is not a homework question...It is pure out of theoretical interest. I asked this the mathematics-community a couple days ago and got no answer, so I figured I'd try here. Most standard ...
2
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1answer
188 views

How to build a MOKE (magneto-optic Kerr effect) magnetometer at home?

I'd like to build a magnetometer at home. The type does not matter actually that much but it should be doable at home. The MOKE (magneto-optic Kerr effect) magnetometer at least appears to use ...
2
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1answer
44 views

What's the strength of a $B$-field inside a magnet?

I understand that the strength of a magnetic field decreases as you get further away from the source of the field. However, whats the strength of the field within a magnet? Is there a field in a ...
2
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1answer
37 views

Explanation for electrostatic energy expression

My question is about the electrostatic energy $We$ expression for $n$ point charges. I just can't figure out where the factor "$\frac{1}{2}$" came from: $We=\frac{1}{2} \sum_{i=1}^{n} q_iV_i$
2
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1answer
602 views

Does the direction of the magnetic field inside a solenoid depends on the direction which it is turned?

Today trying to explain some magnetic stuff, just came across with a simple (at least seems) question. My question is brief: does the direction of the magnetic field inside a solenoid depends on the ...
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2answers
417 views

Finding net magnetic and electric force on charged particle [closed]

This is from my textbook, it is not an assigned problem, but I want to understand. It says: Consider the situation in the figure, in which there is a uniform electric field in the x direction ...
2
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1answer
61 views

EM wave generated with DC battery and coil?

I did below experiment and wanted to confirm the spark is in deed Electro magnetic wave. 1) Took coil of wire , wound it over a iron rod. 2) connected one end of coil to +ve terminal of battery and ...
2
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1answer
162 views

Why do iron filings become magnetized along their long axis instead of in a random direction?

Iron filings can be used to illustrate the direction of a magnetic field, say due to a bar magnet. This works because when exposed to a magnetic field, the filings are themselves temporarily ...
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2answers
77 views

Intuitive picture of diffraction without fictitious secondary wavelet

This question has been asked in a similar way but no satisfactory answers came up. Huygen's principle relies on the fact that if we consider an aperture, every element dS on the surface of the ...
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2answers
215 views

Can you create a powerful magnetic field out of confined circulating electrons only?

Would it be possible, regardless of how efficient it was to do something like this, or why you would actually do it, to generate a really powerful magnetic field simply by producing a cycling electric ...