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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48 views

Physical distribution of salt anions and cations during electrophoresis

If I have a volume of $L$ liters of salt water at a concentration of $\approx N$ mM NaCl and I pour it into an electrophoretic apparatus (like this one: ). Once we turn the apparatus on, and set the ...
42
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7answers
4k views

Cyclist's electrical tingling under power lines

It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
3
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2answers
347 views

Does a current-carrying wire running through the centre of a solenoid experience force?

Imagine looking at a solenoid from above. Current passing is through it in a clockwise direction. The direction of the field lines therefore is towards the bottom of the solenoid. Now pass a straight ...
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1answer
49 views

Microscopic understanding of Brewster angle

The textbook explanation is that, at Brewster's angle, the electric dipoles excited by the incident light can't reradiate to the reflection direction since it's aligned with the dipole moments. Is ...
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2answers
304 views

Circularly polarized light incident at Brewster's Angle

If a perfectly circularly polarized wave of light is incident on a dielectric medium (coming from air) at Brewster's Angle, what will the polarization state of the transmitted wave be? I am aware ...
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3answers
59 views

internal conversion and the electromagnetic force

I have read that the mechanism behind internal conversion, in which a nuclear transition leads to the ejection of an electron in one of the lower atomic orbitals, is related to the fact that the ...
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2answers
58 views

Induction on a circuit with switch

I have the following circuit: It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes. I thought that the magnetic flux would decrease ...
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3answers
1k views

What problems with Electromagnetism led Einstein to the Special Theory of Relativity?

I have often heard it said that several problems in the theory of electromagnetism as described by Maxwell's equations led Einstein to his theory of Special Relativity. What exactly were these ...
4
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1answer
309 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
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4answers
1k views

Is one way glass possible?

I am not talking about mirrors, just a plain window made of glass like material. Would it be possible to allow light pass only in one direction but not the other?
3
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1answer
52 views

Electromagnetic field and continuous and differentiable vector fields

We have notions of derivative for a continuous and differentiable vector fields. The operations like curl,divergence etc. have well defined precise notions for these fields. We know electrostatic and ...
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1answer
38 views

Magnetic field above center of current-carrying loop

A current $ I = 2A $ flows in a circular loop coil of radius $R = 10 cm$. Evaluate the magnitude of the magnetic field (in Tesla) at a point $P$ on the axis of the coil, and a distance $z=5cm$ away ...
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3answers
245 views

magnetic scalar potential far above a magnetic film

The situation I am looking at is a magneto-static problem of a finite magnetic film with magnetization $\bf{M}$. I would like to find the the magnetic field far above the plate. My expectation is that ...
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1answer
1k views

The electric field of a conductive sphere containing a charge - grounded vs not grounded

Let's suppose we have a sphere but unlike theoretical ones it'll have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge $q$ ...
4
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1answer
74 views

If electrons are magnets do they attract each other?

It is said every electron is negatively charged and so they repel each other but if electrons are tiny magnets(which are responsible for atomic attraction and how solid magnet works) does it mean ...
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0answers
53 views

How did Maxwell conclude that there was a relationship between light and electromagnetism [on hold]

What is the formula behind it? can you provide both the formula and substitute to show how he did it?
2
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1answer
29 views

Questions about electromagnetism [on hold]

Are photons units of energy? What is the relation between EM force, EM energy and the photon? Do EM forces carry energy, do they carry the photon (as unit of energy)?
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0answers
30 views

Solenoid pull force & Velocity calculation

So I am building a circular accelerator (like a slightly oversimplified model version of CERN) as a physics project, and I'm at a dead end. I believe I need to know how to work out differential ...
3
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2answers
56 views

Finding the direction of the magnetic force acting on a conducting wire

I have a problem in finding the direction of the force when a conducting wire is placed in a magnetic field. If I use Fleming's Right Hand rule I get a circular magnetic field, so what will be ...
0
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1answer
42 views

What are magnet poles?

You see the poles of a magnet on every magnet picture, and they are said to be in the direction of magnetic field lines, but what does that mean? Is the number of electrons different on one side of ...
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2answers
139 views

Can electromagnetic fields be used to shield electromagnetic radiation?

Can electromagnetic fields be used as shielding for electromagnetic radiation?
4
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1answer
450 views

What is the meaning of this “let there be light” joke?

A middle school teacher across the restaurant is wearing this shirt, and I certainly don't get it.
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1answer
62 views

How are contact forces electromagnetic forces?

How are contact forces electromagnetic? What are they: Coulombic forces between polarized molecules in contact? Or they are the magnetic forces due to the motion of subatomic particles? I also would ...
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2answers
87 views

Why is emf equal to PD when circuit is “open”?

I understand that the set up of a battery consists of very good, but not ideal conductor, and therefore, some internal resistance exists. Also, I get that emf is the PD that would exist if the ...
0
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1answer
46 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
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2answers
141 views

Who foots the (magnetic) energy bill?

Gravitational attraction and electrostatic attraction/repulsion are intrinsic properties of matter, any particle (electron, proton) for some unknown reason can produce KE at a distance. But magnetic ...
3
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2answers
171 views

What is the difference between induced current and eddy current?

I was reading today about electromagnetic induction and eddy currents. But while reading I was unable to differentiate between induced current and eddy current. What is the difference?
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1answer
218 views

How is the current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...
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1answer
111 views

How do you go from quantum electrodynamics to Maxwell's equations?

I've read and heard that quantum electrodynamics is more fundamental than maxwells equations. How do you go from quantum electrodynamics to Maxwell's equations?
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1answer
136 views

Surely force on shell can't be balanced by field momentum?

Imagine a particle with charge $q$ at rest at the origin. It is surrounded by a concentric spherical insulating shell, also at rest, with charge $Q$ and radius $R$. At time $t=0$ I apply a constant ...
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1answer
125 views

Symmetries of a Uniform Magnetic Field

Simple question. A system with a uniform electric field everywhere in space has translational invariance in the directions perpendicular to the electric field but no translational invariance parallel ...
0
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1answer
20 views

Torque due to a magnetic field

We have two infinite perpendicular wires, each carrying a current "i" and the distance between them is l. If both the wires are free to move, how will they arrange themselves because of the torque ...
4
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2answers
161 views

When can I use $\wedge$ instead of curl?

It seems in some circles the wedge product is used in preference to curl. I have a basic understanding of Green and Stokes' formula, I wish to use the $\wedge$ notation from now on. Can someone tell ...
0
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1answer
56 views

phase difference between incident plane wave incident on a dipole and radiation fields from dipole

i have an incident plane wave and a dipole, consider that plane wave incident on dipole. at this moment what happen for dipole ? we know that after incident of plane wave on dipole, the radiation have ...
0
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1answer
48 views

Circuit breaker Trips during thunderstorm

The circuit breaker at the electrical mains trips at home when there is a thunderstorm outside. Why does this occur?
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4answers
159 views

Sequence of E and B field in radio waves and in single photons

In antenna technology we distinguish between nearfield and widefield. In the nearfield the electric and the magnetic fields are shifted by 90°. If you look closer you can see that there are two ...
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0answers
12 views

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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2answers
63 views

Moving coil contradiction of Faraday Induction

In Faraday's Induction Experiment, the e.m.f. induced in the induction coil becomes zero when the relative velocity of the coil and the magnet becomes zero. But one can also argue from a stationary ...
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3answers
88 views

How do charged particles interact?

You'll have to forgive me if this question is too wrapped up in "classical" thinking. I've read that electrons and protons interact by trading photons, but this only raises more questions. What ...
0
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2answers
35 views

Mass of magnet measured by weighing machine [on hold]

I put a magnet on a weighing machine, suppose it shows its mass. Part-1 of the problem: Now, I put another magnet on above this magnet so that it is repelled by the first magnet on the weighing ...
1
vote
1answer
125 views

Electric field generated by a point charge moving at the speed of light

As you see, this is the electric field generated by a point charge moving at constant speed v. I know that when $v$ -> 0, $E$ is just the Coloumb Law. But how do you interpret $E$ when $v$ -> $c$ ? ...
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1answer
153 views

Force on a coil inside of a voice coil

as a homework exercise we were given the following question: Given the following voice coil: With depth = 20 mm Permanent magnets with B = 1.23 T, Ur = 1.1 The coils has 20 turns, I = 1 Ampere. a) ...
0
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1answer
101 views

Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...
4
votes
6answers
5k views

Does a current carrying wire produce electric field outside?

In the modern texts of electromagnetism in the presence of stationary currents the electric field is assumed conservative $\nabla \times E =0 $. Using this we get $E_{||}^{out}=E_{||}^{in}$ which ...
1
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1answer
72 views

Why there is a time gap between the news presenter and the field reporter?

This one has been in my mind for years, but I simply can not find any reason for this event. The Situation Imagine yourself sitting in the living room while watching television. It's 9 p.m and the ...
0
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2answers
23 views

Inductor Charging and Discharging

Why does the inductor discharges in the same path ( same direction of current ) as that of its charging unlike a capacitor which discharges in the direction opposite to its charging(current direction ...
0
votes
1answer
33 views

Junction conditions in GR including electromagnetism

I have recently learned about the Israel junction conditions in GR (as explained in for example Gravitation by MTW). I then tried to generalize it when including Electromagnetism, ie matching two ...
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1answer
28 views

Poynting vector plain wave

I'm calculating the poynting vector for a plain wave and I have some doubt. $$ \bar S = \frac 1 2 \bar E \times \bar H^* = ... = \frac {| \bar E|^2} {2 \zeta} \hat i_k $$ Now if I consider a ...
3
votes
1answer
72 views

Why would a rotating charged sphere not have time varying electric field?

I have seen Gauss Law being used for a uniformly charged hollow sphere rotating with $\omega$. How is that valid to use Gauss law since it is an electrostatic law and if it is valid, why do we get a ...
2
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0answers
26 views

Momentum operator of a particle in an electromagnetic field

In quantum mechanics, to all observables correspond some self-adjoint operators. In the absence of an electromagnetic field the momentum operator is clearly $\vec{P}:=\frac{\hbar}{i}\vec{\nabla}$. ...