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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An EM problem of polarized sphere

Today, I'm doing an EM problem and my question is exactly the same as this site shows: http://www.physicsforums.com/showthread.php?t=133501 To reiterate my confusion, why we can't use that integral ...
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18 views

A conductor passing a magnetic field?

A large thick copper wire(or bar if you like,) passes a magnetic field. What formula is best to calculate the EMF here? $$EMF = BA / Delta(t)$$ or, $$EMF = vBL$$ When the conductor passes the ...
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3answers
45 views

question on dipole moment of water molecule

I read a line today and don't get it: "Molecules with mirror symmetry like oxygen, nitrogen, carbon dioxide, and carbon tetrachloride have no permanent dipole moments." ...
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1answer
41 views

Lorentz force in rotating frame of reference?

This is the common problem of a charged particle moving in a static electric and magnetic field. Say $\textbf{E}=(E_x,0,0)$ and $\textbf{B}=(0,0,B_z)$. In the inertial frame of reference, the ...
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37 views

Is the charge of a current carrying wire real or hypothetical?

Realist753 Theory Relativistic Electrodynamics assert that a current carrying wire being neutral in a laboratory frame of reference appears electrostatically charged to an observer in relative ...
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23 views

Magnetic flux and transformers

I have two linked quesitons, 1. what is magnetic flux linkage's formal definitaiton and 2. when people say the flux flows better thru a soft iron core then the air what do they mean by flux flowing ...
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1answer
70 views

Motional EMF and EMF?

What is the difference between motional EMF = $-vBL$ , and Faraday's law of induction $\displaystyle\mathcal{E} = \left|\frac{d\Phi_B}{dt}\right|$? Aren't they the same? What is the relation of ...
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1answer
78 views

The relation between the movement of electrons and energy

So, I've been enjoying reading a lot of helpful posts, but now, I found myself in the need of asking something. I have a hard time grasping the general concept of electricity / how the relation ...
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1answer
53 views

Current induced in coil when magnet falls through it?

So I know that magnetism can be obtained from the combination of electric fields and special relativity. I am familiar with the way one can derive the magnetic field of a current carrying wire felt by ...
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1answer
55 views

The position of center of mass of electron cloud in an atom

I read Griffiths EM today and it says something very interesting but a little bothering to me. It states for an atom, the position of center of mass of an electron cloud lies in the center of the ...
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2answers
78 views

Magnetism due to relativity?

So I have been reading in some books that magnetism does not have to be assumed a priori, but can be obtained from the electric field + special relativity. And I have seen how this leads to the common ...
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2answers
88 views

How do Permanent Magnet Generators Work?

Why we don't use this in our houses to generate the electricity? Isn't the electricity generated by the magnets strong enough? Why do we have to pay for electricity if we can just create it by pushing ...
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1answer
54 views

How could electromagnetic waves propagate through space although they have no electrons?

How could electric fields in these waves propagate through space although in space there's no electrons for the electric field to be formed? is there another type of charged particles that carry the ...
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1answer
43 views

Equation for magnetic field due to a current in straight wire [closed]

How can I derive the expression for the magnetic field due to a long straight current carrying wire using the Biot-Savart law? The final expression should be: ...
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0answers
15 views

How to compute radiated power from known instantaneous potential distribution

Friend of mine has a device that has an oscillating electric potential and he wants to know how much power it radiates. He can simulate the instantaneous electric potential using a software package, ...
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1answer
90 views

How fast does current flow? And the magnetic field? [closed]

A conductor carries 10 Amps, and generates a magnetic field $B$. How fast does it take for that current to flow from the wire throughout the entire circuit? And how fast does it take for the ...
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2answers
86 views

Electromagnetism duality theorem

Concerning Electromagnetism, textbooks often refer to the Duality Theorem. Sometimes it is presented like this: «Consider the Maxwell's Equations (with phasors) and a known field $\mathbf{E}_1$, ...
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1answer
31 views

How to Extend Relaxation Methods for 2D Laplace Equation given in Jackson E&M to 3D?

In Jackson (3 ed) chapter 1.13 an outline is given for using relaxation to solve laplace equation in 2D. The general procedure in 3D involves minimizing the quantity $$I[\Psi]=\frac{1}{2}\int_V ...
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2answers
59 views

Visualising the magnetic field [closed]

How can we visualise the magnetic field?How to visualise magnetic field due to current carrying conductor having poles(like north and south for ordinary magnetic). Can we determine the north and the ...
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1answer
57 views

Does the transmission axis matter for sending polarized light through polarized glass?

If I have polarized light and I send through only one polarized glass plane, does the transmission axis matter, or will the intensity be halved no matter what.
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1answer
60 views

Correct formula to express the potential generated by a single layer charge distribution

Assume that the closed surface $S$ encircles a volume $V$, and that a surface charge with density $\sigma$ ("single layer") is distributed over $S$. My question regards the electrostatic potential ...
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1answer
42 views

Biot-Savart Law from linear to volume current distribution

Biot-Savart law for a linear current distribution is: $\displaystyle \vec{B}=\frac{\mu I}{4\pi}\int\frac{\vec{dl}\times \vec{r}}{r^{3}}$. In the book that my professor uses says that if we have ...
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1answer
96 views

Why is $D$ a $2$-form and $E$ a $1$-form?

Usually in electrostatics we start by introducing the vector field $\mathbf{E}$ representing the electric field due to some charge distribution. Later when we study fields in materials we consider the ...
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3answers
124 views

If electromagnetic waves have magnetic fields, why beam of flashlight is not disrupted by a Magnet?

Wikipedia article about Electromagnetic Radiation says "As an electromagnetic wave, it has both electric and magnetic field components". And this discussion also confirms Light is EM wave. Since we ...
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1answer
69 views

Is every electromagnetic radiation considered “light”?

Somebody mentioned on Freenode chatroom for physics that All Electromagnetic Radiation are delivered in form of Photons not just light. Is it true? Does that mean if we get a THF electrical ...
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67 views

What is the intepretation of the electromagnetic tensor?

Let $A$ be the four-potential, then we know that we can form the electromagnetic tensor as $F=dA$. This is usually done as a way to have a better writing of Maxwell's equations. So, to simplify the ...
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14 views

The Magnetic Field around the Proton Beams in the LHC

How would one most appropriately model the magnetic field around each bunch of protons in the LHC at maximum operating speed? Is the assumption of a wire of the same length of the beams (~30cm) with ...
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117 views

Why dimensionality of the Electric Charge varies with the spacetime dimensions?

The point is: We can find via dimensional analysis that the electric charge dimensionality varies with the dimension of space-time. $$[\text{charge}] = eV^{(3-D)/2}$$(You can see below the way I did ...
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2answers
136 views

The gauge covariant derivative and it's substitution

I was wondering wether it would make a difference (in general) if I were to were introduce the gauge covariant derivative $$D_\mu=\partial_\mu+ieA_\mu$$ In the Lagrangian density and then derive the ...
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3answers
133 views

More about the Right Hand Rule?

We started learning about electromagnetism in physics class, and the Right Hand Rule comes in handy as seems easy to use, but I'm curious as to how it actually works. I guess it's more of a math ...
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61 views

Currents and magnetic fields produce forces that affect things they're not touching?

I was doing this past paper and am a little confused by question 5) part c)ii) I correctly calculated that the force acting on the rod due to the magnetic field is 0.016N but I can't see how ...
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40 views

Debye Hückel Theory valid for ions?

I am wondering about the following: Is Debye Hückel Theory only used if you look at how an external "strong" field(like a potential by a sphere that has a charge that is 1000times higher than the ...
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2answers
50 views

Alternative symmetries for the Maxwell Lagrangian?

I'm wondering about how to show that $A_a\rightarrow A_a+\alpha\partial_0A_a$, with $\alpha$ infinitesimal, is an infinitesimal symmetry of $\mathcal L=-\frac14F_{ab}F^{ab}$. \begin{equation} ...
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27 views

EMF & Magnetic force at the same time?

A conductor is placed in a magnetic field, current starts to flow, and the wire experiences the Lorentz force of $x$ Newtons. However, what confuses me is that $x$ is a value that is calculated from ...
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2answers
50 views

Can electromagnetic fields be used to shield electromagnetic radiation?

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

Why is the Hodge dual so essential?

It seems unnatural to me that it is so often worthwhile to replace physical objects with their Hodge duals. For instance, if the magnetic field is properly thought of as a 2-form and the electric ...
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2answers
64 views

Will the way you make a coil affect magnetic induction?

We had an experiment about magnetic induction. We discoved that as the number of coils you make the induced voltage and current increases. But I'm wondering if I made the coil messy or made the ...
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1answer
35 views

Charging a capacitor

Below is an excerpt from a physics textbook: One common way to charge a capacitor is to connect these two wires to opposite terminals of a battery. Once the charges $Q$ and $-Q$ are established on ...
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46 views

Which properties of electricity and magnetism are responsible for the speed of light being what it is? [duplicate]

I have always been fascinated by the speed of light. I have wondered how it was originally calculated before we had equipment sensitive enough to clock light speed. All I know about it is that the ...
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1answer
65 views

Ampère's Law for an infinitely long straight wire

In order to find the magnetic field generate by an infinitely long straight wire of radius R, I have to use local Ampère's Law : $\boxed{ \oint_L B\cdot dl = \mu_0 \sum_j I_j}$ If $r > R$ then $ ...
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47 views

Special relativity: moving charge and twisting bar magnet

You have a small bar magnet that is able to pivot about its middle (like a compass needle). You fix the middle of the bar magnet to a point on the positive half of the y-axis. Initially the bar ...
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1answer
61 views

Electric Field Change Freezing Temperature of Water?

I assume that the reason water freezes is because as you decrease the temperature, the kinetic energy of the water molecules decreases and the dipole bonding potential eventually over comes the escape ...
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50 views

how do I calculate the B field (strength and direction) on a point charge due to a permanent magnet

I am trying to create a simple 2D Simulation of magnetic fields similar to this; showing the field lines or the effect of permanent magnets. I keep coming across equations like this: ...
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2answers
31 views

Lenz's law and bar magnet falling from some height above

I am reviewing my electromagnetism studies and I became confused with where energy of induced emf due to Faraday/Lenz' law come from. Suppose that there is a curved track that starts with some height ...
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2answers
88 views

Argument for symmetry of potential

Consider the following electrostatic charge configuration of a spherically symmetric, perfect conductor with total charge $Q = 2q$, where $q > 0$. A point charge $q$ is placed at the position ...
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1answer
40 views

Polarization vector and bound charge

Why is it that the bound charge is $Q_b = - \oint_S{\mathbf{P} \cdot d\mathbf{S}}$? In particular, why is there a negative sign? Hayt's book on electromagnetism describes this as the "net increase in ...
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1answer
40 views

Current Carrying Coil - Torque [closed]

i'm stuck on how to answer the last part of this question and would really appreciate some pointers. A rectangular coil (sides $a = 15 \, \mathrm{cm}$ and $b = 20 \, \mathrm{cm}$, $m= 15 \, ...
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0answers
31 views

Magnetic field of a wire with over 1000 amperes? [duplicate]

Does a large thick wire that has over 1000 amperes of current flowing through it generate powerful magnetic fields? What formula is best here to predict $B$?
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1answer
51 views

Maxwell's equations of Electromagnetism in 2+1 spacetime dimensions

What would be different in the theory of electromagnetism if instead of considering the equations of Maxwell in 3+1 spacetime dimensions, one would consider 2+1 spacetime dimensions?
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1answer
49 views

Current through two inductors after a long time

I'm having trouble with visualizing the following problem, which is asking me for the final, steady current in both inductors $L_{1}$ and $L_{2}$. I was thinking that after a long time, essentially ...