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Questions tagged [electromagnetism]

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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Electric field inside a cylinder containing a grounded wire?

Say we have a hollow cylinder at 100V. I understand that if this cylinder were empty, you would have no electric field inside because if you drew a Gaussian surface, it would contain no charge. ...
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2answers
458 views

Is electric current relative?

Motion as we know is relative. According to this current which is the flow of charges should be also relative . That means that if someone is moving with the same velocity with repsect to the velocity ...
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25 views

Can the relation $div (B) = 0$ give a relation between invariances of $B$ and its components in space?

for example in the cylindrical base, if I find that for reasons of symmetry, the field depends only on $r$ and $z$, can I say that the tangential component of $B$ is $0$?
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Charged Particle interacting with hollow half sphere magnetic field

I need help finding the potential of a magnetic field in the shape of a hollow, half sphere. I also need to mathematically represent the force on a charged particle as it interacts with the magnetic ...
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1answer
21 views

Symmetry of the permittivity tensor of lossless media

I read in various sources that the electric permittivity tensor of a lossless medium is always symmetric. I am wondering how this can be the case, when: Phase accumulation in the medium could in ...
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1answer
29 views

Capacitance per unit length [on hold]

How to solve the following problem: Find the capacitance per unit length of a two-conductor transmission line, if the two wires are of radius a and separation distance d.
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1answer
26 views

Is any other gauge ever more useful than Lorenz gauge for practical calculations in classical EM?

Any problem in gauge theory can of course in principle be solved in any gauge (or, in the case of classical gauge theory, without using gauge potentials at all), but some gauges are much more useful ...
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2answers
51 views

Is a magnetic field required for electromagnetic energy flow through an electric field?

My (perhaps flawed) understanding: Existence of voltage results in an electric field. Existence of current results in a magnetic field. Electromagnetic energy flows in waves in the electric field. ...
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1answer
20 views

Is Poynting's theorem derived from Faraday & Lenz law?

Is Poynting's theorem dependent on Faraday & Lenz law? It's an eloquent equation that shows the electrical to mechanical conversion(and vice versa), but I assume that it heavily rely's on the ...
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16 views

Liénard-Wiechert potential of a charged infinite sheet uniformly translating parallel to itself

This question is motivated by the model used in The Feynman Lectures on Physics Vol II Section 18-4. In the model there are two plane sheets extending infinitely in both their dimensions. Each has a ...
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27 views

Could a spacecraft be propelled into space via a railgun? [duplicate]

Using very large magnetic coils like a railgun, would it be possible to propel a spacecraft at high speeds akin to a rocket into space relying on pure kinetic energy?
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1answer
62 views

Reason for force in Stern-Gerlach experiment

I'm currently working at an assignment, and I'm having some trouble understanding how the magnetic field deflects the silver atoms passing trough it. From what I understood, the atoms are deflected up ...
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1answer
34 views

Direction of the force on a current-carrying wire, related to the energy of the magnetic field

Suppose I have a current-carrying wire with current $I$ coming out of the page, along the z axis. Its field is superposed on a uniform magnetic field $B$ in the x direction (to the right), which ...
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0answers
8 views

Can radio waves be detected using a coulomb torsion balance?

Radio waves are almost everywhere. shouldn't the coulomb torsion balance be sensitive to any passing through the device?
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0answers
15 views

How far does this bar go before coming to rest, if there is a magnetic field through the circuit?

This is a homework thread, please let me know if there is something wrong with it. The exercise comes from the textbook University Physics volume2, chapter 13, #91 It can be found here: https://cnx....
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13 views

Is This Statement On Transformers Correct? [on hold]

This was a statement taken from Quora on why the secondary side of a step-up transformer has less current, he explained it further rather than just plugging the P=V. I, this was by far the best answer ...
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1answer
36 views

How to generate a rotating electric field? [on hold]

Does it involve the use of any magnets? And if so is there a geometric centre where the resultant field is zero.
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0answers
35 views

Wave pulse in 3D

For a 1D wave equation: $$\frac{\partial^2 \psi}{\partial x^2} = \frac 1{c^2} \frac{\partial^2 \psi}{\partial t^2}$$ We can write a solution of the form: $$Ae^{-b(x-ct)^2}$$ representing a localized ...
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2answers
35 views

Force exerted on an uncharged particle in a uniform magnetic field?

I'm struggling to understand how to calculate the force exerted on an uncharged particle in a magnetic field. Consider the following setup: a ball of ferromagnetic iron weighing 1kg is placed in a ...
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0answers
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Can matter retain an anapolar magnetic field like a bipolar magnetic field?

This question occurred to me when I came across a study on exciting a radiationless anapole mode in matter using radially polarized light. Here's the study. Now, considering that the anapolar field ...
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0answers
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Why do paramagnetic materials require large fields to achieve magnetic saturation?

I'm performing an experiment on a paramagnetic sample, and the field strengths we are applying range from -7T to 7T, which are impressively large field magnitudes. However, we still are not observing ...
2
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4answers
87 views

Why Voltage Increases in a step up Transformer rather than the Current?

In a step up transformer, we all know voltage increases and current decreases, the answer to more voltage is because it has more windings, but why does more windings induce more voltage, can't the ...
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1answer
42 views

Is a light pulse equivalent to a burst of photons

Reading about photons you get all sorts of weird statements like "time is frozen for a photon", "the photon dies the instant it is born" and "the photon is everywhere and nowhere", etc. Probably these ...
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0answers
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What is a Conduction & Displacement Currrent in a Transformer [on hold]

I understand what a conduction current is, which is esentially a normal current that flows through the windings, what I don't understand is this Displacement Current, is it the magnetic fields flowing ...
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0answers
34 views

Basic algebra problem for a magnetic field

After deriving the basic formula for a magnet field, $$ \textbf{B} = \frac{\mu_0}{2\pi} \cdot \frac{Md}{\left(d^2 - l^2\right)^2} $$ where $\textbf{B}$ is the magnetic field strength, $\mu_0$ is the ...
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Is quantum mechanics misleading [on hold]

Mechanics implies dynamics. Yet all of the qm I have seen is empirical curve fitting of a static system. Maybe they should change the name? For example it seems the point of qm is to do an experiment,...
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By how much does space-time curvature modify the density of an object?

Let's imagine a transparent object with a density $\rho_0$ [$kg/m^3$] (A sphere of glass as example). Now I send through this object a long flat impulse of electromagnetic plane wave with frequency $\...
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0answers
16 views

Solenoid and circular loop, mutual inductance

I have to solve this physics problem: Consider a solenoid with $N$ turns per unit of length and a circular loop, both of radius a. These, are placed one after the other, in a coaxial form, separated ...
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1answer
94 views

Physics mystery : why does it rotate?

In the beginning this thing was floating still and the candle was out. Then someone lit the candle and the flame somehow Made the object spin on the water. See the picture. I know a little bit about ...
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2answers
42 views

How to mathematically express permittivity as a function of frequency

I am a amateur physics student. I am modelling the wave propagation of an EM wave at different frequencies through water placed between two acrylic rectangular materials using COMSOL. But since the ...
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0answers
16 views

Trigonometric relation in Purcell's derivation for potential due to column of polarized material

This may be trivial, but I do not see how Purcell gets the relation $dz \cos{\theta} = -dr$ from Figure 10.15 on page 484 (Third edition of Electricity and Magnetism with Morin, picture attached). I ...
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2answers
32 views

Magnetic moment of a radially symmetric current

In my latest assignment I'm tasked with finding a magnetic moment $\mu$ of a hydrogen atom, whose current distribution $\mathbf{j}(\mathbf{r})$ looks like $$\mathbf{j}(\mathbf{r})=\frac{e\hbar}{3^8 \...
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Magnetic Potential and force on plasma in Zpinch engine.

Here is a link to the article I am referring to https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120002875.pdf The article details a concept for a zpinch engine. I have a question about page 4....
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1answer
37 views

Energy in an electric field under a gauge transformation

Consider a (test) charge located at $\vec{r}(t)$ in a static electric field with potential $V(\vec{r})$. The energy for this system is given by $$E = \frac{1}{2} m \dot{\vec{r}}(t)^2 + q V(\vec{r}(t))...
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1answer
31 views

How does a compass work?

When using magnets, How does a compass determine the quantitative value of the magnitude and the direction of the magnetic field?
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2answers
52 views

Ampères law: Getting $\nabla \times \vec H = \vec J_{free} + \frac{\partial \vec D}{\partial t}$ by taking the cross product

I've seen two different versions of Ampère's law and I'm having trouble connecting them: $$\nabla \times \vec B = \mu_0 \vec J + \mu_0 \epsilon_0 \frac{\partial \vec E}{\partial t}$$ $$\nabla \...
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1answer
33 views

How to show monopole term in magnetic vector potential is 0?

Assume we are in a magnetostatic and electrostatic situation. In the multipole expansion of the magnetic vector potential, we end up with $\vec{A}(\vec{r})=\sum_{n=0}^\infty \vec{A}_n(\vec{r})$ ...
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1answer
8 views

Relation between the magnetization and the magnetic field in permeable media

I am trying to re-write the constitutive equation of linear magnetic media in a way that is more convenient to numerical implementation. From most books I have seen that the constitutive relation is ...
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0answers
56 views

How is the voltage across an inductor well defined?

TL;DR: It seems like there are cases where the E-field outside an inductor is non-conservative (like for an infinite solenoid), so the voltage across the inductor isn't well defined. From Feynman's ...
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30 views

What force is responsible for the movement of electrons in a current caused by an electrochemical source?

From what I have read, batteries provide an electrochemical potential that causes electrons to move from the negative terminal through a conductor to the positive terminal. I have read numerous ...
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1answer
17 views

Magnetic force on a wire

Where does the force of magnetic field that is created by current is exerted ? in the positive charge atoms of the wire ?
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1answer
21 views

Potential difference across the ends of a conducting rod kept in a varying magnetic field, Why a time varying magnetic field develops induced emf [on hold]

Why a time varying magnetic field induces emf in a rod? This can be explained with the help of a question: A conducting rod of length 'l' is kept in a time varying magnetic field as shown in the ...
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1answer
31 views

Measuring the earth's electric field

My question stems from a question on the Dutch VWO-level highschool state exam (from 1993) concerning a device to measure the earth's electric field. It consists of two plates positioned above one ...
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2answers
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Regarding the $E x B$ drift in the Earth's magnetic field

So I have a burning question: The only reason that the E x B drift doesn't generate an electric current is because both the electrons and the positive ions move towards the same direction (towards ...
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0answers
36 views

Electric field through planes?? [on hold]

Can anybody walk me through the steps on how to solve this problem, it's for homework, im really bad at math but I'm willing to learn, please help. Thank you
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2answers
4k views

Please help identify this physics apparatus!

This was my grandfather’s and have no idea what it is only that it is some piece of physics equipment! The main black cylinder doesn’t seem like it wants to rotate but not sure if it should?
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27 views

Compass and magnetic field

How can a compass be used for the determination of the quantitative value direction and magnitude of a magnetic field at a point?
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1answer
58 views

What does electromagnetic wave literally look like? [closed]

What does electromagnetic wave literally look like? Some people on the Internet called electromagnetic wave as something that has literally only two dimensions, I think it is because of this over-...
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2answers
45 views

Why charged particles moving in an electric field deflect less at higher velocity?

According to coloumb's law, particles of the same charge should experience the same force, however, when moving at higher velocities, they deflect less. Can this be explained in terms of classical ...
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0answers
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Why does the aluminum ball first jump, then hover in this demo (two AC solenoidal magnetic fields)?

Once in a while google suggest an intriguing video: Professor Eric Laithwaite: The Circle of Magnetism - 1968 There is AC 50Hz driving two concentric, very short solenoids, the outer is longer than ...