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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650 views

How to derive the expression for the electric field in terms of the potential?

How can I derive that $$\vec{E}=-\vec{\nabla}\phi-\frac{\partial \vec{A}}{\partial t}$$ where $\phi$ is the scalar potential and $\vec{A}$ the vector potential?
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2answers
272 views

What is the meaning of electron's magnetic effect cancelling each other?

I'm trying to understand, why magnets attract certain metals but not objects made out of paper, plastic etc. And the answer I got is "in paper, electrons cancel each other". What does it mean?
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1answer
250 views

What does non magnetic and nonconducting mean in reflection and transmission of waves?

So, we were ask to consider the Fresnel Equations for parallel and perpendicular waves (with index of refractions). Then, we are ask to prove some equations in which "... for nonmagnetic ...
2
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1answer
92 views

Does the 1-D poisson's equation have monotonic potentials if $\rho=\rho(\phi(z))$?

I am solving the 1-D poisson equation: $$\frac{d^2 \phi}{dz^2}=-4\pi\rho(\phi)$$ with the additional requirement that $\rho(\phi(z=0))=0$. If I start by multiplying each side by $\frac{d\phi}{d z}$ ...
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3answers
1k views

Conservation of energy in electromagnetism?

How is energy conserved in electromagnetism? Is it hinted only from Lenz's law or from Maxwell? Also, if a solenoid's flow of current produces a magnetic field, and from the magnetic field forces are ...
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1answer
115 views

Speed of different em radiation in a medium

Does all frequency of light travel with the same speed in a particular material like glass? Since by cauchy's relation refractive index change with wavelength which implies the speed also changes. ...
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4answers
473 views

Temperature and resistance?

Why does resistivity increase with temperature? The explanations I have heard so far are that increasing temperature increases vibrations in the lattice structure resulting in the number of ...
4
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2answers
459 views

Top angular speed of electric motor

I recently came across a question asking the following: If a motor is switched on, it quickly reaches a top speed. Why does it not just go faster and faster and faster? I thought it might be ...
3
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2answers
346 views

Can the $\vec H$-field have non-zero divergence? Does an $\vec H$-field monopole exist?

We know that, $\vec \nabla \cdot \vec B=0$ but $\vec \nabla \cdot \vec H\neq 0$, if $\vec \nabla \cdot \vec M \neq 0$. Does it mean that, in those cases $\vec H$-field has poles although $\vec ...
3
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1answer
280 views

Magnetic field due to electron in Hydrogen?

We can calculate the current density $\mathbf{j}$ of the electron in Hydrogen, and it is given by: $$ j_\phi=-e\frac{\hbar m}{\mu r\sin\theta}\left|\psi_{nlm}\left(r,\theta,\phi\right)\right|^2 $$ ...
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1answer
106 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 ...
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1answer
585 views

Derivation of Ampère's force law

$$\vec{F}_\text{mag} = \frac{\mu_0 I_1 I_2}{4\pi } \oint\oint\frac{\mathrm{d}\vec{l}_1\times(\mathrm{d}\vec{l}_2\times\hat{r})}{r^2}$$ Is there any derivation for Ampère's force law or is it just ...
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1answer
161 views

how electrophoretic display like E ink retains particles on top of capsule without power?

how electronic ink display retains the particles on top of micro capsule when charge on electrode is released or when device is powered off? Doesnt that make particles loose on the dielectric fluid ...
0
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1answer
268 views

Difference between Cotton Mouton effect and inverse Cotton Mouton effect

The Cotton mouton effect is observed when a linearly polarized electromagnetic wave passes through a dielectric medium and a magnetic field is applied perpendicular to the direction of propagation of ...
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1answer
529 views

Electric field in wire's cross section?

 $\Delta V_{ABCDA} = - \int_A^A \vec{E} \dot{}d\vec{l}$ The requirement that the round-trip potential difference be zero means that $E_1$ and $E_2$ have to be equal. Therefore the electric ...
2
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1answer
154 views

How much power would a space craft's magnetic shield require? [closed]

I've read over the decades that a magnetic shield might protect a spacecraft from cosmic radiation. Its a fascinating idea that might only be theory or science fiction at the moment. In regards to ...
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1answer
100 views

electromagnetic induction. Does charge matter?

Just wondering. I know a negative electric charge moving though a coil will induce a voltage in the coil. My question is, would a positive charge, say an ion beam, moving though a coil also induce a ...
15
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1answer
941 views

What is the conclusion from Aharonov-Bohm Effect?

What is the conclusion that we can draw from the Aharonov-Bohm effect? Does it simply suggest that the vector potential has measurable effects? Does it mean that it is a real observable in quantum ...
2
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1answer
424 views

Ferromagnet induced current?

A ferromagnet is attracted to a solenoid's magnetic field. I understand that now the ferromagnet produced a magnetic field, from that process is there an induced $EMF$? Can ferromagnets that are not ...
4
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2answers
1k views

Is the Maxwell Stress Tensor Coordinate Dependent?

I am wondering if the Maxwell stress tensor, defined as $$T_{ij} = \epsilon_0 (E_iE_j-\frac{1}{2}\delta_{ij}E^2) + \frac{1}{\mu_0}(B_iB_j-\frac{1}{2}\delta_{ij}B^2) $$ is coordinate dependent. I ...
5
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1answer
186 views

Is this a valid proof that the four-current is conserved?

The four-current of a particle moving along a worldine $X^\nu(s)$ is defined as $$j^\mu(x^\nu) = ec \int u^\mu(s)\, \delta^4(x^\nu - X^\nu(s)) \, ds$$ So here's my proof that this is conserved: ...
15
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4answers
703 views

The problem of self-force on point charges

Allow me to preface this by stating that I am a high school student interested in physics and self-studying using a variety of resources, both on- and off-line, primarily GSU's HyperPhysics website, ...
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0answers
35 views

electron spin separation

I am having doubt whether the electron's up spin moment and down spin moment can be isolated from one another. If it got separated, will each moment acts as magnetic monopole (stable or unstable). ...
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2answers
188 views

Is a magnetic field affected by the presence of air?

Does air affect a magnetic field's ability to attract ferromagnets? If a magnetic field and a ferromagnet were placed in a vacuum would there be a better performance?
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0answers
440 views

Why do electric generators need a prime mover?

Commonly electric generators need a mechanical torque provided by a prime mover [gas turbine or steam turbine or diesel engine,etc] which is needed for the rotation of the generator rotor. I would ...
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2answers
2k views

Capacitors: why is the energy not stored in a magnetic field?

When a capacitor is charging, the rate of change $dE/dt$ of the electric field between the plates is non-zero, and from the Maxwell-Ampère equation this causes a circulating magnetic field. Now, ...
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2answers
146 views

Cause of Magnetic field

According to maxwell-ampere equation, the curl of magnetic field at that point equals the current density at that point + change of electric field at that point w.r.t. time. Now, the second factor ...
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3answers
219 views

Ferromagnets and magnets tend to align in the center. Why is that?

When you bring a large iron plate and a magnet, the magnet attracts the iron plate and it tends to slide itself to the center. When I place it on the edge, it always aligns at the center why is that?
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0answers
739 views

Electric potential on a uniformly charged tube

A hollow cylinder, with no top or bottom, of radius $R$ and length $L$ is uniformly charged with density $\sigma>0$. I have to find the point on space where a point charge $q>0$ has to be drawn ...
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0answers
55 views

Expanding the Magnetic field?

A magnetic field $B$ can apply a force $X$ on a ferromagnet when there is no gap to separate them, $r$ = 0. When that gap ($r$) is increased to 3 cm, the force drops to zero. How can that be solved? ...
3
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1answer
231 views

A dielectric sphere in an initially uniform electric field and representation theory of SO(3)

I learned recently that the highest order spherical harmonic required to represent the spatial distribution of decay products of a particle can be used to determine its spin, by using arguments ...
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4answers
197 views

Why change in Capacitance?

I do know it that the overall capacitance decreases if two or more capacitors are connected in series,but do not understand why? I do understand it mathematically but don't get the physics behind it.
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1answer
196 views

The workings of the Hall effect?

I want to ask about the workings of the Hall effect. Why do the electrons come to rest on the edge of the wire? The magnetic field pushes them up, and the electric field pushes them forward. Shouldn't ...
5
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1answer
231 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
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2answers
716 views

Surface charge in a wire?

The image above shows the distribution of the surface charge in a current carrying wire. The surface charges distributes themselves to make sure the field inside the wire is always perpendicular to ...
3
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1answer
956 views

Electric field in a wire?

The electric field in a wire is parallel to the wire's surface (it is always pointing "to the front"). If the electric field did not point in this direction, surface charges would build up and ...
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3answers
1k views

Does there exist a single plate capacitor(conductor)?

Does there exist a single plate capacitor(conductor)? if yes How will you define the capacitance and potential(difference) of such conductor?
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1answer
312 views

Question related to Equipotential Surface

How will you show that equipotential surface is always directed perpendicular to electric field?
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5answers
1k views

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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1answer
2k views

induced emf when a wire or coil travel through a magnetic field

I've recently learned that If you move a conductor through a magnetic field, an emf is induced across the ends of the conductor: $E = BLv $ I've also been told that the same is true for a coil but ...
2
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1answer
146 views

What is the Significance of Magetic Field Energy Density?

If we consider electrostatic field energy density for an electrical field then by analysis we find that it is the representation of the total electrostatic potential energy of the considered ...
3
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1answer
165 views

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 ...
3
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0answers
834 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 ...
3
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2answers
635 views

Frequency of rotating coil

Given a coil initially in the x-y plane, rotating at angular frequency $ \omega $ about the x-axis in a magnetic field in the z-direction. This uniform time varying magnetic field is given by $B_z ...
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1answer
140 views

Characteristic quantities in Fiber optics

I'm having trouble finding typical quantities in fiber optic communication. In particular, what kind of powers are generally used (or what is the minimum that fiber optics receivers can detect ...
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4answers
238 views

A question about canonical momentum and arbitrariness for potential in magnetism

The following question confuses me: There exists magnetic field $B_z =- \beta x$ where $x > 0$, and a particle is incident from origin point $(0,0)$ with pisitive charge $q$, mass $m$, and ...
0
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1answer
125 views

Faraday's law question: wire falling over two rails

I'm having trouble with this problem. The picture shows a wire of mass m and length l which slides on two rails, with certain angle with respect to the horizontal.The whole circuit has a resistance R ...
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5answers
4k views

Why does a faraday cage protect you from high currents?

In an electrostatic case it is clear that that in a space enclosed with a conductor (without charge in it) the electric field is zero. This is often demonstrated in physics shows like on the ...
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3answers
239 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 ...
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2answers
1k views

Magnetic field of a moving point charge

I'm having a hard time to grasp one simple idea. Let us say we have a moving point charge, q, with velocity v, in the x direction. Now calculating both the Magnetic and the Electric fields is easy: We ...