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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36
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5answers
6k views

Does a constantly accelerating charged particle emit EM radiation or not?

The Abraham-Lorentz force gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by: $$\mathbf{F_{rad}} = \frac{q^2}{6\pi ...
2
votes
1answer
231 views

How does EM radiation depend on the reference frame?

In special relativity, magnetism is electrostatics in a different reference frame. This is how we explain the magnetic field being produced by moving charges (aka currents). Charges that move produce ...
-1
votes
1answer
65 views

How I can prove refractive index in an environment is this? [closed]

in "Applied Quantum Mechanics" by A.F.J. Levi, is a problem that I couldn't solve it can any solve it: if electrical filed is: $$ \mathbf{E}\left(\mathbf{r}, \omega \right) ...
3
votes
0answers
48 views

Why is diproton unstable? [duplicate]

Diproton is an isotope of helium without any neutrons. It commonly forms in the Sun, where protons are fused constantly. However, it is extremely unstable, and will revert back to two protons almost ...
2
votes
3answers
218 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of size $L$, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and violate ...
0
votes
0answers
17 views

What is the physical meaning of a magnetic conduction current?

In electrodynamics, it is possible to have an electric conduction current, whereby $J=\sigma_e E$, with $J$ being the current, $\sigma_e$ the electrical conductivity and $E$ the electric field (this ...
0
votes
1answer
32 views

What is the interrelationship among PD, T1 and T2 in relation to MRI

Proton Density (PD), Spin - Lattice Relaxation Time (T1), Spin - Spin Relaxation Time (T2) are three major characteristics by which we can distinguish different types of tissues and all three are ...
4
votes
1answer
59 views

Is diamagnetism a static or dynamic effect?

When we put a diamagnetic material in the presence of an external magnetic field $\vec B_0$, the magnetic field inside the material decreases to $$\vec B=(1+\chi_m)\vec B_0,$$ where the magnetic ...
1
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2answers
59 views

Two Black Holes held stationary by EM forces

If two black holes with large enough mass (so that the tidal forces are minimal and the intersection is large) that are held apart by like charges (saddle point stability). Imagine the black holes in ...
1
vote
1answer
300 views

Does the Lorentz force law explain Flemings left hand motor rule and the right hand dynamo rule?

the Lorentz force on a charged particle $F=qv \times B$ can explain Flemings left hand rule (motor rule) and the right hand (dynamo rule) In the left hand rule, the direction of the current gives ...
0
votes
3answers
131 views

Why geologist do not consider molten iron in the magma to be the source of Earth's magnetic field?

This was a question that came in a exam. How come is this possible? Actually, a question on the source of Earth's magnetic field was asked here on physics SE, which matches my opinion. The ...
-1
votes
0answers
32 views

Magnetism shown by the electron [closed]

In a hydrogen atom, can you derive the expression for the magnetic field at the centre of the atom produced by an electron in the nth orbit.?
0
votes
2answers
155 views

Does Faraday's law work both ways, and if so why don't we use d.c for a generator?

Faraday's law in the integral form can be stated as $V = -d\Phi/dt$, where the right-hand side represents the rate of change of the magnetic flux and the left the voltage difference. In other words, a ...
4
votes
3answers
572 views

How to find the direction of an eddy current?

Suppose there is a magnetic field going from left to right. Suppose a thin sheet of metal conductor (e.g. a 1m*1m square) is dropped through the magnetic field such that the plane of the conductor is ...
3
votes
1answer
30 views

Reaching equilibrium in a blackbody and light-matter interaction

Suppose we have a metallic cavity maintained at a fixed temperature. Suppose we start with any distribution of radiation that is not in equilibrium with the container. Gradually, when the equilibrium ...
8
votes
2answers
557 views

Why do we use gauges in Maxwell equation?

While solving the Maxwell's equation we often use the Lorenz or Coulomb gauge, but why is that? Are the equations unsolvable if the gauge is not fixed? Or is it just for the simplicity?
0
votes
0answers
11 views

Magnetic field of a current loop on a boundary between materials

How can I find the magnetic field of a current loop on a flat boundary between two materials given their magnetic permeabilities $μ_1$ and $μ_2$ and a field $\overline{H_0}(\overline{r})$ which the ...
2
votes
1answer
391 views

Right hand solenoid example

I am just studying for my physics final exam. I came across this example and am not satisfied with the answer. My answer is that Z is South and the current would flow from Y to X because of the ...
2
votes
1answer
72 views

Electric field due to a time and space varying current density?

Let us say I have a current density: $$\vec J\equiv\vec J(\vec r, t)$$ This will produce both a time varying magnetic field and a net charge density. Both these effects produce an electric field. If ...
2
votes
2answers
151 views

Scalar and Vector Potential

I am a physics undergraduate student currently studying electromagnetics. I have previously studied electrostatics and magnetostatics yet the concept of scalar potential, $V$ and the vector potential, ...
-1
votes
2answers
61 views

How to calculate the maximum value of a 2 dimensional vector from complex $x$ and $y$ phasor components? [closed]

I have a pretty simple goal, but I haven't been able to figure the best way to achieve it. The components of a 2D Electic Field vector are both phasors, meaning they're time-varying functions in the ...
1
vote
1answer
117 views

Nature Of Bulb Glowing In Parallel LR circuit with bulbs

In the given circuit the switch is initially closed and then opened after some considerable time.Then which bulb will stop glowing first? Which bulb will glow more brightly?Why ? .After opening ...
0
votes
1answer
24 views

Net magnetic field in a loop

The Ampere's law states that the net magnetic field in a loop is proportional only to the current inside the loop. So if we calculate the net magnetic field in figure 1 and figure 2 we would obtain ...
5
votes
3answers
880 views

What are the differences between the differential and integral forms of (e.g. Maxwell's) equations?

I would like to understand what has to be differential and integral form of the same function, for example the famous equations of James Clerk Maxwell: How to know where to apply each way? Excuse ...
-3
votes
1answer
20 views

Mechanism by which a capacitor is charged, using surface charges

I want to gain a deeper understanding as to how a capacitor actually gets charged. Consider a situation in which an uncharged parallel plate capacitor is connected to a battery using two wires. I ...
12
votes
3answers
2k views

How come light waves don't get caught and absorbed by the electrons of oxygen atoms in the in air?

Shouldn't air be opaque since instead of coming into our eye, the lightwaves get caught in the electrons? If oxygen does absorb light waves, how come air is not hot and you can see through it? The way ...
1
vote
1answer
53 views

Tensor notation of Maxwell's equations

Tensor notation of Maxwell's equation read So when we explicitly try to find the Maxwell's equation from the above tensor equation we only get gauss law and curl of B. The div.B=0 and curl of E are ...
0
votes
0answers
11 views

Using Desmos to trace function fields [closed]

Using the online tool of creation of graphic desmos, is there any way to plot a graph 3D in function of their electric field and magnetic field? like that: I'm trying to use the following ...
1
vote
2answers
685 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 ...
0
votes
0answers
13 views

How do i calculate the “attenuation effect” that of a perforated aluminum sheet?

I am conducting an experiment test different configuration of perforated aluminum sheets. I am working with radio waves but i am having am hard time finding any equations or properties for this ...
-1
votes
2answers
78 views

Is electromotive force really a force? [duplicate]

As far the definition goes emf of electromotive force is basically potential difference. It even has dimensions of potential. Then why is it called a force?
0
votes
1answer
34 views

surface charge on a resistive wire in dc circuit

I want to understand how does the energy transfered from battery to the resistor in a simple dc circuit . I readed that it is due to the surface charge the battery creats on the wire . So why this ...
5
votes
1answer
915 views

Maxwell equations and symmetry

Do the full inhomogeneous Maxwell equations obey parity (P) and time reversal (T) symmetry separately or only the full CPT symmetry? I believe the homogeneous Maxwell equations obey parity and time ...
-9
votes
0answers
113 views

What was Dirac's clear intuitive understanding of the electron? [closed]

There have been some recent questions concerning de-Broglie Bohm theory, such as this and this. The latter asked if Dirac's argument against classical mechanics stands in contradiction to Bohm's ...
0
votes
2answers
38 views

Zero electric field in empty region of hollow conductor

A hollow conducting shell having a net charge of +Q has a point charge -Q placed at its centre. From the diagram, it looks like Suppose the radius of the inner shell from the origin is a and the ...
2
votes
0answers
85 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: ...
0
votes
2answers
732 views

How does the magnetic field get induced in car's tire?

I was amazed to see how my car's tire started attracting the nuts and bolts, how in the world did it induce the magnetic field? Never heard of such phenomena!!!
1
vote
1answer
60 views

Induced current for conjoined loops in varying magnetic field

How do we calculate the emf and the current induced in two loops of wire, that have a portion in common between them? The specific example we had solved in class is: The left side loop is a square ...
0
votes
2answers
55 views

Does a rotating electric field vector (in circular light) moves slower than the speed of light?

We can imagine circular light as a combination of two rotating vectors moving through space: the electric field vector that´s rotating together in sync with the magnetic field vector. As the vector ...
0
votes
2answers
3k views

Landau Levels and Magnetic length

I am looking into Landau Levels and keep coming across a magnetic length defined as follows for a 2D system: $l_B=\sqrt{\frac{\hbar c}{e B}}$ I have seen numerous sources say this is: $l_B\approx ...
0
votes
3answers
250 views

What determines the magnitude of force of an electromagnet?

Let's assume that there is an electromagnet with $B = 1 Tesla$ , what is the magnitude of the force it could generate on other dipoles equal to it's surface area? Can the magnitude of force (in ...
9
votes
4answers
20k views

Can the Earth's magnetic field be used to generate electricity?

Since the Earth has a magnetic field, can it, in theory, be run through a conductive metal coil to create electricity?
2
votes
1answer
87 views

Can light be a spinor?

A recent discovery suggests that photons can have half-integer spins. This seems to contradict the well understood notion that photons are vector (1-form) fields What does this mean for the ...
0
votes
1answer
34 views

Can circular light be produced with more than one vector, tracing out multi stranded strand helixes in space? [closed]

Is it possible to construct circular light with more than one vector tracing out a one strand helix in space? For example, circular light with two vectors in opposite directions (on the same line), ...
0
votes
0answers
25 views

The velocity of the charged particle in the magnetic field [duplicate]

Enrico Fermi considered that moving charged particle gains energy in the magnetic field, so its velocity increases. According to the Lorentz Law $\vec F=q\vec v\times\vec B$. Work is $A=\vec ...
-1
votes
1answer
37 views

Maximum electric field of a circular ring

How do you differentiate the equation for electric field of uniform ring $$ E_x = \frac{kxQ}{(x^2+r^2)^{3/2}} $$to get the maximum at a point? My book says $x = \frac{r}{\sqrt2}$. I tried ...
2
votes
2answers
130 views

Reference frequency

In my experiment, I have to obtain a relation between magnetic field of a solenoid and the applied Vp-p. The setup is as follows. http://www.teachspin.com/instruments/faraday/ Does my choice of ...
1
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4answers
783 views

Where are the poles in a magnetic field generated by a current carrying straight conductor?

Consider the magnetic field generated by a straight current carrying conductor. Where are the poles?
-1
votes
1answer
41 views

Manifestly Lorentz invariant quantity [closed]

Doing my reading for electrodynamics I came across an exercise where I am asked to argue that $\partial_\mu\partial^\mu \phi = 4\pi\rho $ is manifestly Lorentz invariant. I know that ...
0
votes
2answers
38 views

Why doesn't the group velocity of a plasma EM wave equal the phase velocity here?

For plasma EM waves we have the dispersion relation $$\omega^2=\omega_p^2+c^2k^2$$ where the plasma frequency $$\omega_p^2=\frac{n_e e^2}{\epsilon_0 m_e}$$ One can show that $v_p v_g=c^2$, i.e., the ...