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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12 views

Why symmetric magnetic field in cylinder cordinates is zero in $\hat \phi$?

I just not understand the intuition and also how to prove it mathematicaly that if I have symmetric magnetic field around z axis, why it would be zero at $\hat \phi$ but none zero at $\hat \rho$?
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Do stationary charges get attracted to a current carrying wire (due to change in electric charge density caused by length contraction)?

I've seen magnetic fields getting explained away by special relativity, for it accounts for the magnetism in the electron's reference frame as an electric field caused by a greater charge density due ...
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31 views

Why don't (or why do) current carrying wires attract a stationary charge placed at a distance?

I've learned that moving charges produce magnetic fields which in turn affect other charges in motion. After seeing explanations that point to special relativity, I am kind of confused. Can ALL ...
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7 views

How symmetric Halbach array can produce asymmetric magnetic field?

I don't understand. If I look at a picture of Halbach array I see it is apparently symmetric. At least both pairs of opposite sides are. Then how can it have "weak" and "strong" opposite sides?
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Electric motor magnetic moment

Suppose I know that an electric motor winding produces 200 Nm torque in 0.2 T magnetic induction field. Is it possible to calculate the rotational magnetic moment produced by that motor winding just ...
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1answer
16 views

Apparent disagreement in curvature drift formulas

Suppose we have some charged particle above the ionosphere. Then in a simplified model, it will experience a curvature and gradient drift. Assume further that a particle is in the equatorial plane. ...
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3answers
51 views

Black Body emission - Coupling of $E$ and $B$ field

I do not understand how a black body emits electromagnetic radiation. I assume that heating a black body creates moving charges which induce a magnetic field. But where is the $E$ field part of the ...
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2answers
26 views

Do diamagnetics attract or repel from magnetic field strength vector direction?

If I place a ferromagnetic body in external magnetic field it will be attracted to field, and hence, body will move in the direction opposite to magnetic field strength vector direction, yes? If the ...
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53 views

Can a moving magnetic field do work on a charged particle?

Imagine a charged particle suspended between 2 horizontal magnetic plates, which create a uniform magnetic field. Now instantaneously, the particle is accelerated to velocity $v$. By my understanding, ...
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43 views

Direction of vector magnetic potential

I was reading the Feynman lecture about vector magnetic potential and I have a doubt about it. For a certain example it is written that: We have, then, a general method for finding the magnetic ...
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16 views

Numerical based on cyclotron [closed]

A particle cyclotron is designed with dees of radius $0.75\,m$ and magnets that can provide a field of $1.5\,T$. What is max. energy of proton that can be obtained?
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How is the vector potential integral derived?

I tried to find an answer to this but haven't yet. Going through Jackson's Electrodynamics, the following steps are used to determine the vector potential as the curl of $\mathbf{B}$. The context is ...
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1answer
48 views

If the magnetic force doesn't do work then what does it? [duplicate]

Let's assume I have a conductive rod with mass $m$ and with steady current $I$. I can make the current flow in such a direction and magnitude that the magnetic force will make it move up, thus ...
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1answer
19 views

What is the force on an iron object due to a magnetic field?

Say there is a solenoid that creates a constant magnetic field in the space around it. An iron ball bearing is placed at one end of the solenoid where the field lines spread out. What would be the ...
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193 views

What determines if magnetic spins align parallel or anti parallel in a material?

As we all know, the magnetic field around a dipole looks like the following: Notice how, directly above and below the magnet, the magnetic fields likes are parallel to the north-south axis. However, ...
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33 views

Electron's velocity speed and angle with time [closed]

An electron follows a helical path in a uniform magnetic field, does its velocity, speed and angle changing with time?
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17 views

Magnetic field of infinite cylinder with angular motion of charge [closed]

I have an infinite cylinder of radius $a$. inside there is a charge with uniform volume charge density $\rho$. the charge is moving inside the cylinder with speed of $\omega(r)=\omega_0\frac{a}{r}$. I ...
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1answer
17 views

Converting units, for the magnetic moment

I found, that magnetic moment is measured by $\rm J/T$, and equals magnetization times volume: $$\vec{m}=\vec{M}V$$ Magnetization, also equals magnetic susceptibility times external magnetic field: $...
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50 views

If mag.field and Ele.field are same,what is the need of studying both?

We learnt magnetic field is electric field but viewed from a different frame of reference. Then why do we need to worry about B. We could very well take a frame where it is E?
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146 views

Is the electron spin $g$-factor value implying the particle is a composite one?

As I understood the highest possible value for a magnetic moment of a point charge having the same amount of charge as an electron and rotating with same electron velocity and confined in the same ...
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1answer
18 views

Magnetic levitation: completely zero friction or just very low and if the latter, what is the source of the friction?

I am thinking of the now easy-to-obtain speakers where a spherical speaker is suspended above a platform. If this was in a vacuum and the sphere was spinning, why would it ever slow down? What is the ...
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31 views

Magnetic field equals 0 [closed]

If I have a coaxial cable of infinite longitude and the magnetic field equals 0. Is it possible electric current exist?
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1answer
25 views

What was Faraday's experiment's of induction?

I'm searching about the experiments, which Faraday has done in order to discover the electromagnetic induction phenomenon. I didn't find what I need on google scholar nor google books. Can you ...
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1answer
37 views

Magnetic force on the proton due to the current [closed]

Figure shows a snapshot of a proton moving at velocity v = (-200 m/s)j toward a long straight wire with current i=350mA so mA At the instant shown the proton's distance from the wire is D 2.89cm In ...
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2answers
23 views

What produces Magnetic Intensity H in a bar magnet?

Here I have added a section from Griffith's Book. It says that H need not be zero when the free current is not zero. I can't wrap my head around this. With the example of an electromagnet, I can ...
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17 views

Torque on a ring with some charge on its rim rolling on a rough horizontal ground in a constant vertical magnetic field [closed]

What is the magnetic moment of a ring of radius R with some charge uniformly distributed over its rim and rolling on a rough ground with a constant angular velocity? My textbook assumed the magnetic ...
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46 views

Magnetic field inside a paraboloid [closed]

I recently came across a question in which one had to calculate the magnetic field due to a current carrying hollow paraboloid having the the equation $y=x^2+z^2$ at a point $(0,r, h) $. I've no idea ...
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30 views

Magnetic monopoles and magnetic field lines

in all my physics courses I have been told that magnetic field lines are absolutely closed lines. It is well explained in this wikipedia page, which states: A magnetic field has no sources or sinks ...
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1answer
22 views

Magnetic islands formed after magnetic reconnection

I often read about magnetic islands forming as a result of magnetic reconnection? To my understanding, those are irregularities of the magnetic field right? If not, could someone help me understand ...
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1answer
42 views

How does electric and magnetic field interchanges in light propagation? [closed]

We know that magnetism is just a illusion created because of special relativistic effects and there is only electric field which is really there in reality. so how can we say that this imaginary ...
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37 views

Why would a ring magnet's flux decrease once distance starts getting small?

I went ahead and use python magpylib to simulate a ring magnet joystick, and then graph flux over distance of the ring magnet. Here's the code: ...
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5 views

Proximity effects at high frequencies

I was trying to understand Skin and Proximity Effects in conductors when I stumbled onto this. I'm using QuickField 6.4 for reference. According to theory, when I place a permeable material, such as ...
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13 views

Computing magnetic field from eddy currents

Suppose a body with arbitrary shape, uniformly magnetized (with a magnetization $\textbf{M}(t)$) and submitted to an external magnetic field $\textbf{H}(t)$. The variable $t$ is the time. Based only ...
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2answers
83 views

Interpreting discontinuity in the Poynting vector

An infinite cylinder of radius R has a uniforme current distribution through its surface $\vec \kappa (t)=\kappa(t)\vec \phi$. Find a) the magnetic field generated by the distribuition, b) the induced ...
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9 views

Interface conditions with two interfaces following each other

Consider three media: A vacuum ($\epsilon_0, \mu_0)$; An isotropic medium ($\epsilon_2, \mu_2)$ to contain the liquid in medium 3; A liquid ($\epsilon_1, \mu_1$). On the first interface (medium 3 to ...
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1answer
24 views

A current carrying loop kept in a uniform magnetic field is rotated about its axis

In a recent examination i encountered a question in which a current carrying coil was rotated clockwise about its own axis in a uniform magnetic field directed along the axis and out of the plane of ...
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2answers
38 views

Why do ferromagnetic materials concentrate flux lines?

When a ferromagnetic material is magnetized (by application of an external magnetic field), the domains are aligned in the direction of the field, and the ferromagnetic material itself acts as magnet ...
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1answer
46 views

Get force field from given magnetic moment and magnetic inductance field numerically

Abstract Some time ago, someone gave me the next solution: $$U=-\int \vec{m}\small{(x)}\times \vec{B}(x)dV$$ $$\vec{F}=-\nabla U$$ In other stack site I’ve asked, how to solve it numerically, and ...
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1answer
30 views

Understanding magnetic hysteresis curves

Suppose I have a ferromagnetic material. It is known that the magnetization $\textbf{M}$ can be determined from a magnetic field $\textbf{H}$ using hysteresis curves as exemplified in the figure ...
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4answers
92 views

Why aren’t the electric and magnetic components of an EM wave complementary? [duplicate]

Every visualization of an electromagnetic wave is essentially some variation of this picture: In every one of these graphs, both the electric and magnetic components are shown as being sine waves ...
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25 views

Magnetic field energy in a coaxial cable

I came up with a conceptual problem when I was doing an exercise which involved calculating magnetic field energy in a coaxial cable. The cable consisted of one inner solid conducting cylinder ...
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41 views

Can we define torque on a current carrying loop in a magnetic field without the use magnetic dipole moment?

In my textbook, the torque is defined as $$ \vec{\tau} = \vec{m} \times \vec{B}.$$ Now, how would I write this from first principles, as in from $\vec{\tau} = \vec{r} \times \vec{F}$? I am trying to ...
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13 views

How to derive current density from magnetic dipole moment density?

In the case of Magneto-statics (Only time independent B field), we may solve the B(r) field from current density j(r) using ampere's law. Alternatively, we may also solve the B field if I know the ...
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24 views

How can I find the potential energy and force between a steel sphere and a magnet?

I'm fine with certain simplifying assumptions being made, I currently only know high-school level physics (the simpler the better). Also, if anyone can explain the intuition behind formulae, I would ...
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1answer
30 views

Electromagnetic Induction experiment with solenoid

The experiment is as follows A solenoid is connected to a galvanometer and a magnetised iron rod is passed through it. (the apparatus is not ideal, and there is no zero error in the galvanometer) I ...
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1answer
79 views

Understanding $\textbf{M} = \chi \textbf{H}$

Suppose I have a linear homogeneous isotropic magnetic material, such that $$\textbf{M} = \chi \textbf{H}\tag{1},$$ where $\chi$ is the magnetic susceptibility, $\textbf{M}$ the magnetization, and $\...
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1answer
43 views

Similarity between electromagnetic waves $E/B=c$ compared with the equality between Lorentz force and electric force

I am very tired and in this moment I have not think very well. Using the vectorial expressions of the magnetic and electric field if an electromagnetic wave propagates in a certain direction the ...
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34 views

Quantum Entanglement in Avian Magnetoreception

I had read on several sites that Avian Magneto reception involves Quantum Entanglement. However, I couldn't understand how Entanglement really helps in magnetoreception. I know that when photons ...
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1answer
29 views

Can a magnetic field affect other magnetic field?

If I have square loop wire with current flowing , can the magnetic field of one segment of a wire mess with other's magnetic field? If I were to take an observation point perpendicular to a segment ...
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2answers
95 views

Where does Formula of Force on a Moving Charge due to Magnetic Field come from?

We have been taught that the force on a moving charge due to magnetic field is: $$\vec{F} = q(\vec{v} \times \vec{B})$$ When I asked my professor about its source, he said that it was derived from ...

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