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 [magnetism] tag, as questions about the phenomenon & theory of magnetism do not necessarily involve the discussion of fields.

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Hammering to form a magnet

Take the following diagram. I understand that if we hammered the material in a magnetic field, the molecular vibration caused by the shock of the hammer would cause domains to realign, creating a ...
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Energy in permanent magnet? [closed]

In one video on youtube says permanent magnet is like compressed spirng. He says permanent magnets energy get used when in use. He is comparing magnetic circuit to electric circuit,where he is ...
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Does lower temperature increases magnetism of all magnetic material, and to what extent?

Like magnets, reed switch magnetism decreases at higher temperature and increases at lower temperature. This is because high temperatures increase random atomic movement and misalignment of magnetic ...
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Size of the core inside an electromagnet

The equation of magnetic field of a solenoid is as follow: But in this equation the size of the core doesn't take into account ? If so, what is the minimum size that an iron core must have to ...
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What happens if we fire a wooden bullet through a magnetic field?

What happens to the bullet and the field if we fire a non-metallic (such as a wooden) bullet through the field beside a bar magnet in a vacuum? Do the field lines move out of the way? Do the field ...
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Different ways to calculate magnetic field of a segment of current [duplicate]

Given a segment of current $I$ of length $L$, we want to calculate $\vec{B}$ on a plane that is perpendicular bisector of the segment. From the Biot-Savart law, $\vec{B}$ depends on $L$ and $R$ (the ...
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How can magnetic field lines form non closed loops?

I recently came across this paper "Topology of Steady Current Magnetic Fields", Am.J.Phys (1953) The author points out the erroneous implications of representing magnetic field lines as ...
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Relation between Area under $I$-$H$ hysteresis Loop and $B$-$H$ hysteresis Loop [closed]

If the area under the I-H hysteresis loop and B-H hysteresis loop are denoted by $A_1$and $A_2$ (The symbols have usual meaning as set in electromagnetics), then $A_2=\mu_oA_1$ $A_2=A_1$ $A_1=\...
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Why armature is being rotated in DC generator?

I am wondering about why armatures are being rotated instead of being static? The magnetic flux will still cut the armature windings and because of this induced current will be generated. So, why it ...
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How to remember which way the magnetic field goes?

For the case of a current in a wire, we learn the direction of the associated magnetic field by learning the right hand rule. But what about a bar magnet and the names 'north' and 'south' for the ends ...
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Can a magnet repel a ferromagnetic sphere?

I have to repel a sphere using a magnet for an experiment. But apparently the magnets attract the sphere to the north and south poles instead of (as I expected) to one of the poles. Is there a way to ...
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Modeling ferromagnetic materials in a primitive python FEA

I am working on a primitive 2D DIY magnetic "FEA" solver for a class project. It's really just a big NumPy array where each element in the array can be one of the following: Air Current ...
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Can the following system work as a perpetual motion machine? [duplicate]

A friend of mine came with this idea ! In the photo ,,we have an external magnet ,and as can be seen in picture ,it shuold interact with the upper magnets( that are similar in design to a padestal fan ...
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How to identify north pole of a magnet using only a battery, and a copper wire? [closed]

I have a magnet, a copper wire, and a battery. I need to find the north pole, and the south pole of a magnet using only these items. (not a compass). Here's my attempt: create a circuit, battery is ...
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1 answer
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A doubt about the topic of motional emf from Griffiths

Consider a closed rectangular loop of wire, which is moved to the right with a velocity ${\vec v}$ w.r.t a magnet that provides a static, uniform magnetic field $\vec B=-B\hat{z}$ over a region as ...
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Is this possible? [duplicate]

A picture that I found which boggles me . Is this possible? Will the vehicle move? I understand the answer is no cause than the oil companies will go bankrupt. But is there a physics explanation? Can ...
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Which configuration is better for this electric motor?

I have a problem with this exercise, it's an electric motor. an electric motor works because of a magnet, and a wire with a current (i.e electrons moving in it). by turning on and off the current it's ...
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Do Protons and electrons actually have magnetic poles?

From what I’ve read protons and electrons only are attracted to each other because they have opposite electric charges. What I don’t understand, though is that I’ve also read that both protons and ...
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Why is the cross product used in electrodynamics [closed]

For example, $F = I \times B$. Why would the force be perpendicular to both the magnetic field and the current? Why would the resultant force be totally unlike its 2 sources; this could be likened to ...
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Where does the energy to move an electron with a magentic field come from?

I know that the energy comes from the magnetic field, but if the electron is moving, then how does the magnet give the electron energy? If the magnet is moving, then the kinetic energy of the magnet ...
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Is a moving electret considered a current, and would spinning one make a magnet?

If I establish a permanent electric charge on some volume of material (as in an electret) and then move that material through space — is that motion equivalent to a current in a wire? Also, since a ...
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Is magnetic field strength (Teslas) inversely proportional to distance from the magnet squared?

I can't find a solid answer on this, I've read that magnetic force is inversely proportional to distance squared so does that mean it's strength is too?
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Why does Cadmium have a large natural linewidth? And why does this make Zeeman shifting more difficult?

I am reading through this paper and am unable to find information on the following statement: "Since Cd has a large linewidth, high magnetic field gradients are required to shift the Zeeman ...
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Metal/good conductor frame in a changing magnetic field

Imagine that we put a metal/good conductor frame into a changing magnetic field. Due to the fact that this is an electromagnetic induction phenomenon, an induced EMF can be measured between the two ...
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Magnetic field of dipole derivation

How can we derive the following formula: $$\vec{B}(\vec{r})=\frac{\mu_0}{4\pi}\left[ \frac{3(\vec{m}\cdot\vec{r})\vec{r}}{r^5} - \frac{\vec{m}}{r^3}\right]\; ,$$ I want to derive it as a limit of a ...
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Hamiltonian of an electron in a magnetic field

Suppose I have an electron in a magnetic field given by: $$\vec{B}=B\hat{z}$$ The potential energy of this system is given by: $$U=-\vec{\mu} \cdot \vec{B}=\frac{g\mu_B}{\hbar}\vec{S} \cdot \vec{B}$$ ...
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Force on a charged particle due to varying magnetic field [closed]

Magnetic field in a cylindrical region is varying as B = kt/r, ('k' is a positive constant, 't' is time and 'r' is distance from the center). B is directed upwards perpendicular to the cylindrical ...
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Repulsive Force between two cuboidal magnets

I have 2 cuboidal permanent magnets(same grade) which are kept a small distance apart. They are kept such that they will repel each other. How do I calculate the force of repulsion between them? I ...
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Is there an upper boundary to magnetism?

This is gonna take some explaining, and full disclosure: I'm still undergrad, so please, forgive my ignorance here. Though please also hear me out: magnetism, like gravity, falls off with distance, ...
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What is the force an electromagnetic crane exerts on ferromagnetic scrap?

Some electromagnets are used for lifting ferromagnetic materials. A cross-section of such an electromagnet is below - original image from this page; edited by me. Let's say you have an magnetically ...
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2 votes
2 answers
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Problem understanding the London Equation in superconductors

I'm facing quite a lot of issues trying to understand the reasoning, that leads to the London equations in superconductivity. In many places, the logic seems somewhat circular. I've followed the ...
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Will a stronger electromagnet magnetize/demagnetize a weaker permanent magnet

I have this setup wherein an electromagnet and a permanent magnet are kept seperated(same poles aligned) by a small gap which is fixed irrespective of repulsion. Now considering that the permanent ...
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How do I show mathematically that net intrinsic magnetic moment of a substance is $0$?

The following argument is made, keeping in mind the classical theory of diamagnetism. I'm not including the correct quantum mechanical theory in this question. Consider a material in the absence of ...
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How does the Poynting vector know who is the primary and who is the secondary of a transformer?

I've read in several places that the Poynting vector is directed from the primary to the secondary of a transformer (we assume here that the primary is the winding that provide the AC energy, while ...
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Inductance of a toroidal electromagnet with a gap

Suppose we have a toroidal electromagnet of length $l$ and section $A<<d^2$, with an air gap of width $h<<d$ with a wire turned $N$ times around it. Suppose we also know that for a large ...
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Gyromagnetic ratio: units problem

I tried to calculate the gyromagnetic ratio of the proton with this formula: $$g_p = \frac{2.m_p.c^2}{e.\hbar.c^2}.\Delta E \approx 5,28$$ with $m_pc^2 \approx 938 \ \mathrm{MeV}$, $e \approx 1,67.10^{...
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Vector potential due to a spinning spherical shell with a non-uniform surface charge distribution

In Griffith's Introduction to Electrodynamics 4th edition example 5.11 the solution for the vector potential of a uniformly charged spinning spherical shell is given. Now let's assume that the ...
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2 votes
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Spherical cavity inside a uniformly magnetized material VS uniformly magnetized sphere

In Jackson 4.4, boundary value problems in dielectrics page 159, a simple relationship is shown between the electric potentials and hence between the electric fields of a spherical dielectric and ...
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How to determine what strength of electromagnet is needed

I have a project where I want to put an electromagnet on a rail system that can move on a plane. Above the electromagnet I will have a plywood board with some metal circles on top of it. I want to be ...
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Coefficient of friction between magnets

Imagine two axially-magnetized ring-shaped permanent magnets in a repulsive configuration where one is levitating on top of the other (and somehow radially constrained so the PMs remain ~concentric). ...
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Electrons in magnetic field

There is a question which is "A beam of electrons enters a uniform magnetic field of $1.2 \ \text{T}$. What is the energy difference between the electrons parallel and anti-parallel to the field....
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How does one determine the direction of torque?

Torque is given by $$\vec{\tau}=\vec{r}\times\vec{F}$$ and it is a vector quantity. This means the torque has to be perpendicular to both the force and the radius. However, this doesn't seem to be the ...
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Maxwell Ampere law derivation

I have seen a proof on the internet regarding the derivation of the maxwell ampere law in this link: Deriving the Ampère-Maxwell law and even though I am pretty much satisfied in the way he derives ...
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Is this an accurate equation for describing the magnetic force exerted by a solenoid on a ferromagnetic material?

I saw this question, which contained an equation describing the strength of a solenoid: $$F = (NI)^2\mu_0\frac{\text{A}}{2g^2}$$ where: $$F$$ is the force the solenoid exerts on a ferromagnetic ...
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Solution of the Schrödinger equation for a uniform magnetic field

I would like an indication of a bibliography, journal or site that presents the solution, not taking into account the spin and the angular momentum of the particle, of the Schrödinger equation for an ...
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How porous can a ferromagnet before turning paramagnetic?

As there is an inherent size limit to magnetic domains, as the porosity of a ferromagnetic material increase, there will be a point in which a static magnetic field will not be possible. There are a ...
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Radio Waves and the Lorentz Force?

Is the Lorentz Force involved when the EM wave intersects the receiving antenna? I.e. induces EMF on electrons in the antenna conductor? I use the RHR with index finger $\vec v$ pointing in the ...
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Faraday Induction Experiment question

I’m trying to understand the basic physical laws applying to Faraday’s induction experiment as pictured below. The Wikipedia site doesn’t go into detail. When the smaller diameter inductor is inserted ...
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Anisotropic screening for charged particles in a magnetic field

Charged particles in a plasma attract other opposite charged particles and repel those with the same charge, thereby creating a cloud of net opposite charge around itself. This cloud shields the ...
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Is $\vec{B}=0$ in an infinite cylinder with localized magnetization? [duplicate]

Say we have an infinite cylinder, with constant magnetization $M(z)\hat{z}$ along its axis. This vanishes for large $|z|$, say like $e^{-z^2}$. It seems to me that the solution to the equations $$\vec{...
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