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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Is the induced electric field generated due to the separation of charges in a moving wire in a magnetic field non-conservative?

As the title suggests, whenever there is a moving wire in a magnetic field of constant magnitude, an induced voltage is produced. Now, due to the separation of charges, which is a result of the ...
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Why does current not increase on passing through a zero-resistance wire?

So, I tried thinking about currents and circuits from a molecular point of view this morning but I couldn't wrap my head around the following: [See the attached Image] (1) Since the wires have zero ...
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Proof that the vector area is the same for all surfaces sharing the same boundary

In the book, Introduction to Electrodynamics by Griffiths (4th edition) in question 1.62 part c, we are asked to prove that the vector area is the same for all surfaces sharing the same boundary. The ...
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Using a permanent magnet as an electromagnet core

If I had a large cylindrical neodimium magnet or stack of same, with a coil wound around it. In an unpowered state the magnet would have a certain attraction. With one polarity of current, I would ...
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Create a black hole by crushing metal using an electromagnet

Imagine if you have a magnetic material and you fix it into a position between two electomagnets, or perhaps in the center of a sphere of electomagnets. Is it possible that you could apply enough ...
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Can we apply classical electromagnetism to modern physics? [duplicate]

Is it possible to use Coulomb's, Gauss', Ampere's and Faraday's laws freely in relativity, quantum mechanics, especially in particle accelerators? What are their limitations, or they are false like ...
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Can magnetism create kinetic energy?

If a magnet is accelerated in a solenoid using current, the magnet is gaining kinetic energy from the current. How? If not, there will be no effect on solenoid current. Hence electrical energy will ...
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What current would be measured in the following circuit? [closed]

Two very long copper rods floating in space almost parallel to each other. They intersect at a very slight angle and touch at the point of intersection. Across two ends is a dc power source attached ...
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How is momentum conserved in this situation?

Let’s say we have two inactive electromagnets spaced one light-minute apart, with north poles pointing toward each other. One of the electromagnets is turned on for ten seconds, and then turned off ...
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Does a straight, current-carrying wire always repel a permanent magnet? Regardless of the permanent magnet's orientation?

Several people have told me that a current-carrying wire usually (or always) repels a nearby permanent magnet.... Most recently, I saw this Veritasium video on YouTube: 'How Special Relativity Makes ...
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Can both magnetic and electric fields induce current from an EM wave?

I was reviewing a homework problem I completed for class, but I saw different explanation that contradict each other. My teacher says that this position for the waves is optimal for maximum induced ...
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Calculating curve integral of infinit long wire the Mathematical way?

In the script it's easily given $\int B\,\mathrm{ds} \quad$ where $B$ is the magnetic field of an infinite long wire: $$B(r) = \dfrac{\mu_0\,I}{2\,\pi\,r}$$ and $s$ is a concentric circle around the ...
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Absorption spectrum of sodium

We know absorption spectrum of sodium is near 595 nm wavelength which is in the region of yellow colored light. Why doesn't sodium (valence electrons) absorb EM wave of wavelength smaller than the ...
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Can I use the OpenStax online physics textbooks (University Physics Vol. 2) to study for the 'AP Physics C Electricity and Magnetism' exam? [closed]

I have the Resnick and Halliday Volume 2 but that seems a little advanced for an AP exam(it is however, a wonderful textbook). Does this openstax book have a high enough standard for AP Physics C prep?...
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Voltage across inductor in an AC Series LR circuit

If we assume $v(t) = V \sin (\omega t) $, we can find $V = \sqrt{V_R^2 + V_L^2}$ and find $I = V/Z$ where $Z = \sqrt{R^2 + \omega^2L^2}$ From here we find $V_R = IR$ and $V_L = I\omega L$. It seems ...
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Building Lagrangians for Classical Field Theory

I've been studying quantum mechanics and classical field theory for quite a while now. However, I still struggle with the idea of building scalars from vectors and tensors for the Lagrangian density. ...
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Shouldn’t the Lorentz force acting on an electron increase it’s tangential speed?

I know some of you might be tempted to just answer “No, the Lorentz force acting on a particle is in every point perpendicular to the velocity vector” but please help me with my doubt. Let’s say we ...
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Moving charges in magnetic field

With homopolar generators, I understand that a current flows from the centre of the disk to the rim. And then using Fleming's left hand rule this creates a force which opposes the motion of rotation ...
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3answers
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Why do electrons in currents not produce any Electric Field?

Why a current carrying wire in which current i(t) is a function of time does not produces any electric field ? Athough i(t) happens due to flow of free electrons in conductor then why these electrons ...
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Effect of bends on the radiation pattern in monopole and dipole seawater/liquid antennas (Reference Request)

I'm aware of the radiation pattern and the electromagnetic theoretical analysis behind monopole and dipole seawater/liquid antennas (cf.Investigations of Water-Based Liquid Antennas for Wireless ...
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Magnetic force between two parallel straight current wires one of them infinite and the other one finite

This problem is very easy if the magnetic field from the infinite wire is applied over the finite one and the Lorentz force is calculated straightforward. I'm trying to make the calculation in the ...
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1answer
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Doubt in the interaction energy of the dipole in an electric field

Consider a dipole ($\vec{p}$) in an electric field ($\vec E$) making an angle $\theta$ with the field. We can see that $V_1-V_2=Ed\cos\theta$ In books, the derivation for the interaction energy of ...
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Self-sustaining Homopolar generator

I have been having some confusion with homopolar generators. My lecturer saying that these homopolar generators can be electrically self sustaining and I don't understand how. I get that a current ...
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Electromagnetic radiation retarded charged particle [closed]

Assume the scenario that a charge particle let say an electron is given a velocity $v$ and now it is approaching towards an another electron. Then will this electron under retardation emit ...
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Does an equal $D$ (Displacement current) on two surfaces gurantee that there is no $Q$, Or is it the other way round no $Q$ hence no change in $D$?

There is a question in the first chapter of RF Harrington's book asking to show that the $Q$ (charge density) will be zero if there is no source. I am posting the solution found from the guide. In the ...
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Why the collapse of electron in nucleus considered unstable?

In Rutherford's Atomic structure proposal, when electron loses energy while orbiting the nucleus and collapses, why do we consider this situation as unstable when there is saturation of electric field ...
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How gravity works? i found found a paper [closed]

I was looking for new ideas on gravity in internet and i found this. this paper is strange, contains different diagrams and modals of gravitational force. it looked interesting to me but couldn't get ...
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1answer
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Moving charges emit electromagnetic radiation. So can sound waves in ionized air produce light?

Moving charges emit electromagnetic radiation. Sound wave propagation through air involves the vibration of air molecules. So in principle, can sound waves in ionized air produce light? This post does ...
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Are the EM fields of charges with different velocities subject to independent Wigner rotations under Lorentz boosts?

I will attempt to answer the following sub-questions in addressing the main question which is the title of my post: In the case of one electric charge of constant non-zero velocity in frame $\Sigma'$,...
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On the nature of the light [closed]

How "correct" is our interpretation of the world with the current theories of physics? More specific I wonder how close to truth is the interpretation of the nature of light, for example ...
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1answer
38 views

How does one derive the electric potential from an electric field?

I'm trying to work through the full derivation to find the equation $$ \underline{E}(\underline{r}) = -\underline{\nabla}V $$ where V is the electric potential and E is the electric field strength. So ...
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Faraday's law of induction applied onto a magnetic film

Situation: Consider an infinite magnetically active film with the film normal in the z-direction and thickness $h$. The dynamic magnetization inside the film is $\mathbf{m}=(0,m_y,0)e^{i\omega t}$. ...
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How to write Biot Savart law in the sense of dirac delta function and distribution? [closed]

We can use dirac delta function to depict point charge, and when applying Biot-Savart Law we may encounter the case in which both volume current and line current distribution occur, so can we unite ...
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Faraday's law conditions

Does Faraday's law only apply when there is a changing B field or change in area for a closed loop? Thanks
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How can the Kt of a brushless motor be independent from voltage? [closed]

I found online the formula that relates the Kv[rpm/V] rating of brushless motors to their Torque[Nm], namely: T = (~8,3)I/Kv. I can see that the units add up, and can follow the derivation of this ...
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What is the “shape” of the electric potential of a moving charge? Can the field be derived as its gradient?

This is motivated by Electricity and Magnetism, 3rd Ed., by Purcell and Morin, Section 5.6. Below is my derivation of the Lorentz transformation of the electric field due to a moving charge whose rest ...
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3answers
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Problem in finding the divergence at a point [duplicate]

I am solving a problem given as Divergence of $\frac{\textbf{r}}{r^3}$ is a) zero at the origin b) zero everywhere c) zero everywhere except the origin d) nonzero everywhere The answer is given as (...
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2answers
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What is nature of pole at center of magnet

We know that a bar magnet has two poles 'North' and 'South' but what is there at the center where two poles meet? please explain me in simple language I am just a physics enthusiast.
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Gauge invariance of a Lagrangian

How do I check whether or not the Lagrangian is a gauge invariant? A Lagrangian is $$ \mathcal{L} = -\frac{1}{4} F_{\mu \nu} F^{\mu \nu} + \frac{1}{2}m^2A_\mu A^\mu $$
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wondering light polarization, enjoying three dimensional cinema

I am very fascinated by physics, but I'm not much into it beacuse I studied it as an amateur many years ago. I'm trying to understand things about optics and polarization of light, and in particular ...
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How to prove Ampere's Circuital Law strictly with the help of dirac delta function

Proving Ampere's Circuital Law when reading this proof I have some difficulties in understanding the usage of dirac delta function,I have learned that the dirac delta function is a distribution ...
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Is the amplitude of the RF pulse affects the NMR signal?

What i knew is that the NMR signal depends on the static magnetic field strength $ B_0 $ (by Boltzmann equation) and its gyromagnetic factor. While the NMR occurs when the RF pulse frequency match the ...
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Em wave with encoded timer [duplicate]

I was wondering if we could transmit an encoded timer with an Em wave modulation which when travelling from transmitter to receiver, calculate the time it existed
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25 views

Magnetic Field due to finite wire

While deriving formula for magnetic field due to a finite current carrying wire, we have used Biot -Savart law, but here in this wire there is accumulation of charge at ends and thus it is non-static ...
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2answers
21 views

Inductance and the current through the inductor

I am a high school student studying inductance, there is some point I can not wrap my head around, my teacher said that if we closed an LR circuit , at t= 0 the current in the circuit is zero and this ...
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1answer
30 views

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 infinitely long wire, this current creates a magnetic field, now I know that this magnetic ...
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My confusions in the proof of Ampere's Law [closed]

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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4answers
53 views

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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14 views

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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63 views

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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