Questions tagged [magnetic-fields]

For questions that utilize the concept of magnetic fields (commonly denoted by the letter B), or for questions whose answers likely involve magnetic fields. More specific than the [tag:magnetism], as questions about the phenomenon & theory of magnetism do not necessarily involve the discussion of fields.

643 questions with no upvoted or accepted answers
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10
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Average force between two parallel finite wires with AC

Imagine 2 parallel antennas (wires) of equal length (a) with a distance r between them. Both have AC currents with identical sine wave forms (equal frequencies and amplitudes) . They are also in-...
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1answer
777 views

Sequential Stern-Gerlach devices - realizable experiment or teaching aid?

At least one textbook [1] uses sequential Stern-Gerlach devices to introduce to students that the components of angular momentum are incompatible observables. Viz., the $z$-up beam from a SG device ...
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2answers
807 views

Could the Faraday's rotating wire experiment be explained without using Lorentz force?

I read Faraday's rotating wire experiment which resulted in the invention of the homopolar motor. He was clever enough to think it that way and make the wire rotate around one of the poles of the ...
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238 views

What is actually happening when a magnetoresistive magnetometer degausses itself?

I'm trying to understand if anisotropic magnetoresistive devices themselves (not other passive ferromagnetic components) can become magnetized, and if so, how the application of a DC 1.1 gauss field ...
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2answers
1k views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
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1answer
607 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
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1answer
907 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
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1answer
212 views

2 Particles in a Landau level interacting via a Central Potential

I am studying Robert Laughlin's paper about the Franctional Quantum Hall Effect http://gtwlx.jpkc.fudan.edu.cn/reference/FQHE-T.pdf . In it, while he is setting up his motivation for the Laughlin ...
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1answer
745 views

Current geometry and Ampere's law

Under the right circumstances, Ampere's law $\oint \vec H\cdot d\vec \ell=I_{encl}$ can be used to deduce the field $\vec H$ at a point from the current enclosed by the circuit which produces $\vec H$....
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945 views

How to calculate the energy transfer between working coil and working piece and dissipated energy of working piece in induction heating?

For who don't know how does induction heating work, a short description can be found here. However, my question is about physics. Assume I have a coil inductor calculated carefully 50 uH as below: ...
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How to calculate the magnetic dipole moment of a bar magnet?

I want to calculate the force between two magnets. In a previous Phys.SE question, I was told that I needed to use the dipole-dipole interaction, but that equation depends on $m$, the magnetic dipole ...
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1answer
12k views

Force from solenoid

I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
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119 views

Why does type-I superconductor behave differently than type-II as $H$ is increased?

As one increases the external magnetic field $H$ above a critical value $H_c$, a type-I superconductor abruptly changes to a normal conductor. But for type-II, the transition is not sharp but gradual ...
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1answer
110 views

Why a superconductor expel magnetic lines of force from inside completely when cooled below critical temperature?

Suppose a superconductor which is in a normal state (i.e. $T>Tc$) is subjected to a magnetic field. As soon as magnetic field switched from 0 to some value, eddy currents will develop on the ...
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1answer
325 views

Is it possible to create arbitrary shapes of magnetic fields?

I've been reading about Transcranial Magnetic Stimulation (TMS) which can activate neurons using a magnetic field, which got me thinking... what if we can pick and choose exactly which neurons to ...
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1answer
33 views

How to store magnets?

I have two identical powerful disk-like magnets (with hole in the center). They have same size. They are stuck together (attract). I have a bunch of sphere magnets. I left them "collide" and form a ...
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82 views

flux threading through superconducting ring

I have been confused over the wording used in this paper describing RSFQ logic circuits: http://www.mukhanov.com/uploads/LikharevSemenov-RSFQReview_IEEETAS91.pdf From page 6: " If such a single flux ...
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37 views

Why can't we take a current carrying bar to determine the magnetic field in Biot savart law?

Our teacher told us that to determine the magnetic field by Biot savart law, we should always consider a wire and not a bar. I don't get the reason for the same. Is it because then we have to take in ...
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Lighting an Electric Bulb with Earth's Magnetic Field

Yesterday, I was solving some problems of Electromagnetic Induction. Suddenly a thought struck my mind. Earth has its own magnetic Field.If an aircraft of metallic body is flying in the air then ...
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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 ...
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246 views

What causes diamagnetism?

What makes a diamagnetic substance repel a magnet? If one explains it on the basis of cancelation of dipole moments in an atom and Lenz's law, shouldn't the diamagnetic substance be attracted to the ...
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63 views

What are the relations between a ladder algebra and an $SU(2)$ algebra?

I am studying elementary models of the Quantum Hall Effect. I don't have a strong background in Lie algebras so I was hoping someone could elaborate on the following observation: For a free particle ...
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67 views

How to write integral to calculate the area where a field is between two limits?

I would like to express the following quantity in a mathematical form, but cannot think how I would write the integral. "The area on the surface of a cylinder where the magnetic field is between two ...
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86 views

Enhancing strength of magnet, practical question

I feel like I'm shooting a squirrel with a howitzer asking my question on the physics stack exchange, but here goes... I have a long ring-shaped (1cm outer dia x 3cm length) rare earth magnet that I ...
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141 views

How do energy eigenstates go between Landau levels?

In the standard presentation of Landau levels, we find that the energy levels of free electrons in a solid of area $A$ and perpendicular field $B$ are separated in energy by $$\Delta E = \hbar \...
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347 views

What can Maxwell's Equations tell us about permanent magnets/ how are permanent magnets and electromagnets related?

It makes sense that Maxwell's equations tell us that there are no monopoles, but can the equations tell us anything else about the magnetic fields of permanent magnets on their own, i.e. without ...
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780 views

Relation between Zeeman effect and Larmor precession

I'm wondering if there's some intrinsic reason that the precessional frequency, $\omega$, of a spin about an external magnetic field matches the frequency of a photon emitted (or absorbed) in ...
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243 views

Ferrofluids and normal-field instability

Could someone explain whether the spikes and uneven surfaces you see in ferrofluids being attracted or repelled away away from (against gravity) the surface of ferrofluids would happen if the fields ...
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1answer
67 views

Why does cutting slots in metal improve magnetic flow?

When machinists mount work pieces for grinding they often use magnetic chucks and fixtures are set on those chucks. The thicker the fixture, the more the magnetism is attenuated. Sometimes that is a ...
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505 views

Confused on how to apply the volume integral form of the Biot-Savart law

In Viscous evolution of point vortex equilibria (Jing, 2011) the author explains that the velocity equation for the Lamb-Oseen vortex is found by applying the 2D Biot-Savart law to the vorticity ...
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143 views

Lifting capacity of superconductors

How does the strength of the magnetic field the superconductor is exposed to affect its ability to levitate/lock heavier objects? Or is there no relationship (for superconductors) between magnetic ...
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1answer
809 views

Graphene - Can it produce a magnetic field?

So it's probably a quick no on this one, but since I haven't majored in physics or chemistry I have no idea if it's possible to make Graphene generate a magnetic field. Assuming power or current is ...
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264 views

Why does “liquid metal” do this?

I just watched this video where a guy uses magnetic "liquid metal" along with a standard magnet to create some pretty cool spiky shapes. The magnetized liquid, when brought near the standard magnet, ...
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Where can I find actual data for the hysteresis of a magnetic material?

When a magnet is exposed to an external magnetic field it can be further magnetized or demagnetized. This amount can be found by looking at the hysteresis curve for that magnet. I do not have the ...
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2answers
461 views

Flux linkage of 2 coils in series

I have a question from magnetic circuits. I am an engineering student, therefore I will neglect some minor errors. There is a simple magnetic circuit with 2 coils ($C_1$ & $C_2$), with $N_1$ &...
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749 views

Induced current created by a magnet falling in a copper tube?

One of the educational demos often shown in schools is to drop a high strength neodymium magnet down a length of copper tube. It takes a considerable time to exit the tube because as it falls it ...
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2k views

Calculating the electro magnetic field strength needed to repel a permanent magnet

I have a design where a permanent magnet is placed on top of an electromagnet. When the electromagnet is powered off, the permanent magnet is attracted to the iron core of the electromagnet. When the ...
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2answers
679 views

Contradicting forces on a circular loop under current in magnetic field?

I have the following general conceptual concern. Think of a thin conducting loop of radius $R$ placed in the $x$-$y$-plane at $z=0$. There is a homogeneous current density $\vec{j}$ running through ...
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657 views

Current to cancel Earth's magnetic field

I'm making a lab report where I've used a Helmholtz coil and a magnet to find Earth's magnetic field intensity in the area by relating the frequency of oscillation of the magnet when a certain current ...
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63 views

What name would you give to the method of approximating an arbitrary magnet with many smaller dipoles?

Let's say I had an arbitrarily shaped permanent magnet, with total magnetic moment $M_{0}$. Ways to calculate the magnetic field of this magnet include an analytic solution (if one exists), as well ...
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+50

Should a Bismuth coin slide slowly down a magnetic ramp?

One way to check for counterfeit silver/gold coins is to "slide" them down a ramp made of neodymium magnets. You can see an example here, or just look up "silver slide". I have seen this work for ...
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Can the magentic flux inside a material increase even if the material is already saturated?

I have the following problem: I simulated a construction of two magnets with an angle of around 30 degrees and Vanadium Permendur between them. Vanadium Permendur should have a Saturation Induction ...
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Being the Earth modelized as a big magnet (or solenoid), which would be the field intensity of such a model inside the solenoid or magnet axis?

This question has arisen from the Carl Friedrich Gauss’ statement that the Earth could be modeled, concerning his calculations about the earth’s magnetic potential, as a big magnet, himself having ...
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60 views

Is the speed of Magnetic field infinite?

My real question is: From Amperes law we know that there is no magnetic field outside of the coaxial cable because the magnetic field generated by the inner wire and outer shell are equal but in ...
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How much energy does it take to magnetize a neodymium magnet?

I read somewhere that when neodymium magnets are made they are subjected to intense magnetic fields in order to magnetize them. So lets say we have a cubic neodymium magnet that is .5 by .5 by .5 ...
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28 views

Magnetic quadrupole and dipole

We can create a quadrupole magnetic field disposing magnetic dipole as in this figure I have two question about this: How can I write the quadrupole moment in terms of the dipole?; Is it correct to ...
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57 views

Reason for repulsion of like charges

Like charges repel and unlike attract. Like magnetic poles repel and unlike attract. Since electricity and magnetism are fundamentally linked , is this attraction/repulsion due to the magnetic fields ...
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1answer
57 views

Can I replace a multipole expansion by a combination of separate dipoles?

If I want to be able to model a magnetic field flux density $\mathbf{B}$ from a magnetic source located at the origin at a position $\mathbf{r}$, it is my understanding that I can represent $\mathbf{B}...
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0answers
56 views

What's the explanation for Kohler's rule?

In a metal, the increase of resistivity ($\Delta\rho$) due to an applied magnetic field follows a functional form of the parameter $\rho_0/B$ (residual resistivity divided by magnetic field) i.e. $$(\...
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1answer
58 views

How to generate a very steep magnetic field gradient

I am a student that's very new to magnetism in general and I've been reading/learning for the past few weeks. I'd like to build magnetic tweezers to use in materials with kPa order stiffness - so I ...