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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Sources of magnetic fields and violation of conservation of energy

We know that current is a source of magnetic fields , for example when there is a current flowing through an infinite long wire there, this current creates a magnetic field through it, now I know that ...
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My confusions in the proof of Ampere's Law

In the proof of Ampere's law we use a dirac delta function to calculate the integral. However, the dirac delta function is not a function. It is a distribution. Do the vector calculus identities still ...
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Induced voltage as a difference of potentials on a ring

If you put a ring (Picture 1) of wire into changing magnetic field, the process called magnetic induction creates induced voltage. Voltage is by definition (Definition 2) the difference in electric ...
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Are induced electric fields parallel or perpendicular to a conductor?

Suppose you have a conductor embedded in a resistor at a 45 degree angle to the (x,y) coordinate axis, and you apply a time-varying, plane-wave magnetic field in the y direction (see Figure 1). As per ...
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How can we tell if a charge is moving in a Magnetic Field?

In the equation $\vec F=q\vec v×\vec B$ ($\vec B$ is the magnetic field), what is $\vec v$ with respect to? If we were to imagine $\vec B$ to be infinitely large. How could we tell if the charge is ...
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Analogous to hall effect, does electric field applied perpendicular to the current direction through a semiconductor bar create magnetic field?

Hall effect says, when a magnetic field is applied perpendicular to current direction in a semiconductor bar, an electric field (and hence Hall voltage) will be created across the side which is ...
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Helmholtz solved on cylindrical coordinates

I am trying to solve for magnetic fields on the axis of a single loop Helmholtz coil using the Biot-Savart Law and a cylindrical coordinate system. I have some point on the axis defined as $$ P=\begin{...
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Electrostatic Shielding due to an Infinite Cylinder

If I place a line charge inside an infinite conducting cylinder which is grounded would the outside be shielded from the electric field inside the cylinder. Also would the potential outside the ...
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$E(x) = 1.5x \frac{N}{C}$ between $2.5\hspace{0.1cm}cm$ and $4.0\hspace{0.1cm} cm$ [closed]

Determine the electrical potential difference for the fields in the x direction: $E(x) = 1.5x \frac{N}{C}$ between $2.5\hspace{0.1cm}cm$ and $4.0\hspace{0.1cm} cm$ \begin{eqnarray*} \Delta V =-\int_{...
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Why does EM destructive interference reflect on an resonnant cavity chamber but not on a piece of paper?

Case 1) Light passing through a single or double slit with show an interference pattern on a screen with dark fringes. The observer eye wont receive light comming from the dark fringes (and therefore ...
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Why is $a = \frac{Qr}{2m} \frac{dB}{dt}$?

In Gerloch's Magnetism and Ligand-Field Analysis, the following derivation is provided (I'm paraphrasing): Let the body $Q$ be a positive charge. The acceleration along a path, $\frac{dv}{dt}$, is ...
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Difference in Hamiltonian when using conjugate momenta vs. Velocity

I was recently reading through David Tong's "Application of Quantum Mechanics" (found here) and in it he talks about the completely classical example of a charged particle in a magnetic ...
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Propagation of EM waves through a dielectric [closed]

Consider an EM wave propagating from a certain region 1 to region 2, at normal incidence. The amplitude of electric field in free space(region 1) is $$ E_1= 1.0 V/m$$ and that of the auxiliary ...
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1answer
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More confused than I should be about the notion of steady current

One of the textbooks I’m reading states that an electric current is said to be steady if $\nabla \cdot \mathbf J = 0$, which is equivalent to $\frac{\partial \rho}{\partial t} = 0$ by the continuity ...
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My python code does not prove equation 13 from “Physical Space as a Quaternion Structure, I Maxwell Equations. A Brief Note by Jack P.M.” [closed]

I am working through "Physical Space as a Quaternion Structure, I Maxwell Equations. A Brief Note by Jack P.M.". I am coding up his quaternion related equations into python. In python my ...
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Electromagnetic wave in a coaxial transmission line

I am currently working on [Introduction to Electrodynamics] of David J.Griffiths, chapter 9.5.3. The book says: In the coaxial transmission line of inner radius a and outer radius b, TEM modes are ...
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Capacitance ratio in potential energy

In reading about quantum dots in nanowires, the potential energy of an electron due to a gate voltage $V_g$ was given as $-eV_g(C_g/C)$ where $C_g$ is the capacitance of the dot with the gate and $C$ ...
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Confusion with Magnetic Susceptibility

Could someone point out the error made in the following two simple calculations? I am trying to understand how to find magnetic and electric susceptibility using thermodynamics. I know something is ...
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28 views

How to peel off grating layer from CD-R?

I wanted to create a grating based surface plasmon resonance setup, but I don't want to spend to money to get a grating manufactured. I realized that we can extract a grating by peeling off the metal ...
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1answer
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Thermodynamic electric and magnetic susceptibility

Are there differences between how the electric and magnetic susceptibilities are defined in thermodynamics and electrodynamics? My confusion is that it seems the techniques from thermodynamics (from ...
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How should one visualize the *electric field* formed from a point charge moving at constant velocity?

So we all know how the magnetic field looks like ( qv x r ) While trying to visualize the electric field I got a little bit confused. Because we are not in electrostatic anymore I don’t know if I can ...
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Uniqueness of relations connecting EM field values in two inertial frames

In Einstein's 1905 paper, he has derived the relation connecting the EM field in two inertial frames. It can be seen in this link http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/...
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What makes hard magnetic materials more appropriate for use in microphones than soft magnetic materials?

I can't find a reason out there. Surely it'd be a soft magnetic material is best? What makes hard magnetic materials more appropriate for use in microphones than soft magnetic materials?
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Easy explanation why electron emits photon when accelerated in $E$-field, but not in $B$-field?

I know that when an electron gets accelerated (changes direction) in an electric field it emits a photon. An ad-hoc way of explaining this is, that the acceleration of the electron causes a ripple in ...
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How does light interference explain colour mixing? [duplicate]

I've grown curious about the phenomenon of light interference. If two light sources of different wavelengths that correspond to two arbitrary colours, say blue and yellow, constructively join, they ...
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Voltage distribution around a dipole for different wavelengths [closed]

It is well known that the alternating current distribution in a center-fed dipole antenna (black line) is as follows: Despite the different wavelengths, the current distributions depicted above all ...
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What is the classical E&M explanation for the change in field along the axis of a charge's motion (accelerating and non-accelerating cases)?

I have a question about whether I'm thinking about the field of a moving charge along the axis of its motion correctly, in cases of both acceleration and also constant velocity. I'm trying to develop ...
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Manetic field/Gradient of magnetic field simulation

Does anyone have any recommendations on how I can simulate the magnetic field of permanent magnets? Is there any free software that can do the work please can you recommend me. I wanted to have a look ...
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1answer
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Magnetic flux with two concentric cirular loops

Suppose I have two cocentric circular loops, one with radius $R_2$ and one with radius $R_1$ where $R_2$ is the radius of the larger loop. Current is flowing in the outerloop and is changing over time,...
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1answer
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Proof of electric field intensity due to an infinite conducting sheet

I am reading the proof of electric field due to large uniformly charged sheet. It is give as Consider an elemental strip of width $dx$ at a distance $x$ from point $O$. The linear charge density $\...
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Forms of energy in a closed circuit with a coil

I am a bit confused. When i move a magnet through a coil thats in a closed circuit, what does my kinetic energy convert to? I assume I will create a magnetic field, and that magnetic field will create ...
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Is electric potential energy responsible for energy release in nuclear fission and nuclear potential energy in fusion? What if elec. force not exist?

It was my doubt and I thought it may be electric potential energy which is responsible for energy release in nuclear fission and by bombarding neutron we only provide energy to nucleons to cross ...
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Why don't evanescent waves give rise to electromagnetic waves?

I'm reading about evanescent waves for the first time. I understand that even thought no electromagnetic wave is transmitted across the boundary, an electric field is transmitted which decays ...
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1answer
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What does an external test charge feel along the dipole axis of radiating antenna?

I have been reading about how radio dipole antennas radiate "bubbles" of electromagnetic waves, due to the kinking of the field lines from the acceleration of the charges. The bubbles that ...
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2answers
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How can someone mathematically prove that a spherical wave equation solution cannot be a electromagnetic wave? [duplicate]

It's well documented that spherical electromagnetic waves doesn't actually exists. However, most textbooks just brings this concept without any formal demonstration. So, for a spherical wave equation ...
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Why do Diffraction lines in image not match the Spacing on the fringe pattern

In the attached image interference lines spreading out from the slits end up hitting the detection screen with unequal spacing. The spacing gets wider and wider the farther out it goes. On the other ...
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How does you increase the power of an antenna? [closed]

Looking over Maxwells equations some things are immediately obvious. The magnetic flux density and the magnetic field intensity seem to be the reverse of the usual? I am assuming this is to do with ...
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1answer
29 views

Does a square loop carrying a steady current $I$ produce any force?

I have a square loop which carries a steady current as shown below: I was able to find the magnetic field at the centre of the square loop to be: $B=\frac{\sqrt{2}\mu_0I}{\pi R}$ My question is there ...
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Random question about magnetic fields and relative movement [duplicate]

I'm an undergraduate physics student and we're learning about electrodynamics. While studying, a question popped in my head: We know that magnetic fields are generated from moving electric charges, or ...
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2answers
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Can we apply Conservation of angular momentum to a circulating charged particle in a magnetic field?

In the question there is a charged particle undergoing pure circular motion at velocity v and radius r in a magnetic field of field strength B. The strength is instantaneously decreased to B/2, what ...
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1answer
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Magnetic Force exerted by a current carrying wire

I think this is a misunderstanding on my part, but I want to clear this up. If I have a current carrying wire, it exerts a magnetic force of $F_B=IlB\sin(\theta).$ My teacher said that this means that ...
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Please can you help with this practice question? Thank you [closed]

Please can you help me with this practice question? I have used the right-hand grip rule but not sure now. I thought there was one attached but it looks like my question has been closed. It ...
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2answers
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EM radiation fields from a single electron transition

This question is about a description for the EM fields created when an electron decays into a lower energy state spontaneously in a single, isolated atom. The treatment that I recall from grad school ...
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3answers
131 views

Why electric field is scaled by gamma?

Two opposite charges are in a spaceship and are attracted by the electric field $E_s$ But for an observer on earth the Electric force is $$E_e=\gamma E_s$$ Normally the forces are scaled down by $\...
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1answer
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Dielectric constant and conductivity in Maxwell's macroscopic equation

In my experimental physics lecture we are looking at the Maxwell equations in matter (macroscopic Maxwell equations) and there is a point where we jump from $$ \nabla \times \vec B = \mu_0 \biggl( \...
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1answer
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Do electromagnetic fields also have thermal fluctuations?

As far as I know electromagnetic fields have vacuum fluctuations. But can electromagnetic fields also have thermal fluctuations and can they induce current in coils? If the answers are positive. How ...
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Finding out the magnetic field at anypoint outside a permanent magnet (Cylinder,Cube)

I am working in a project where I am using different shaped permanent magnets for levitation of diamagnets. I am facing problem while calculating the magnetic field around these permanent magnets. To ...
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Charge Density for a Rod

In Jackson's Classical Electrodynamics, there's a line charge distribution on a rod of length 2b. Jackson gives the charge density in Eq. (3.132) by $$\rho\left(\mathbf{x}\right)=\frac{Q}{2 b} \frac{1}...
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Experimental suggestions for size and shape of single optical photon (wavepacket)?

Optical photon is an electromagnetic wave produced e.g. during deexcitation of an atom, carrying energy, momentum and angular momentum difference. So how is this electromagnetic energy distributed in ...
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Is the magnetic flux density B constant? Is the magnetic flux constant?

I am working on a project design for Uni and I am stuck. In a magnetic circuit is either the magnetic flux or the magnetic flux density B constant? This magnetic circuit has all different cross ...

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