New answers tagged magnetic-fields
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What is the meaning of $\vec H$ with respect to the total field?
Here the problem is that you are using too many notations and you are changing them constantly.
The master equation in electrostatics :
$$ \vec{D}= \epsilon_0 \vec{E} + \vec{P}.$$
(i) $ \vec{D} $ is ...
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How does a magnetic field interact?
What is happening in the case of a magnetic field? It's not like the atoms have a brain. Something must be making them interact with the field.
For the sake of simplicity, I'll just talk about ...
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How does a magnetic field interact?
Gravity at first was explained as an action at a distance. The Newton theory fitted so well that the same idea was used by Coulomb for electrostatic and even by Ampere in its first theory of the force ...
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How does a magnetic field interact?
The key word here is "field": everywhere there is an electric and magnetic field produced by electric and magnetic sources (and actually they are two sides of the same electromagnetic field)....
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When a magnetic ball passes through a copper pipe vertically
In simple terms compare your arrangement to a vertical solenoid with the ends connected together and a magnet dropping through it.
An induced current passes through the solenoid which sets up a ...
- 93.4k
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Where does the energy for magnetizing a metal come from?
As you said, magnetising a material does require energy. It might seem counterintuitive that a magnetised material can magnetise other ferromagnetic materials (objects) without losing its own magnetic ...
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Magnetic Scalar Potential of Infinite Wire
It depends on the physical interpretation of the quantities. In other contexts, $\phi$ would need to be single valued like if it represented temperature for example. In the context of magnetostatics, ...
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Charged Particle in Magnetic Field
There's potential momentum involved. If $\vec{A}$ is the vector potential then $q\vec{A}$ is the potential momentum. In classical mechanics for a single particle
$$E - q\phi = \frac{(\vec{p}-q\vec{A}...
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Charged Particle in Magnetic Field
For a charged particle in circular motion in a magnetic field, it is not instructive to think in terms of energy conservation: the speed of the particle is a constant, so the kinetic energy $(1/2)\,m|\...
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Voltage across rod in time varying magnetic field
When the magnetic field is changing, there is no notion of "voltage" only of EMF. If you were to couple the two ends of your rod to a voltmeter, its reading depends on the time rate of ...
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How current is induced when there is a change in external magnetic field?
If still charges in a wire loop do not respond to a (or have their own) magnetic field,…
Electrons at rest (even in a wire loop) have their own magnetic field. Each electron is also a magnetic dipole....
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Why does earth's magnetic field change its direction?
Earth's magnetic field can only change due to the effect of another magnet in the form of a planet or any heavy object passing by.
Gravitation is nothing but the magnetism. Earth's revolution around ...
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How current is induced when there is a change in external magnetic field?
It is actually not the changing magnetic field
$\mathbf{B}$ directly, which generates the current.
According to Faraday's law of induction
$$\nabla\times\mathbf{E} = -\frac{\partial\mathbf{B}}{\...
- 36.2k
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How current is induced when there is a change in external magnetic field?
I'm not sure how to explain 'how', because Faraday's law of induction is a fundamental law of Physics, and asking 'how' is asking why there is such a law. There just is.
It's not actually current ...
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Explaining Faraday's Law With Lorentz Transformations of $E$ and $B$ Fields
Just a small detail, Faraday's law is about inducing electromotive forces (emf's), not currents. To make the link between current and emf, you'll need to go beyond Faraday's law and make some ...
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Voltage across rod in time varying magnetic field
If the (time-varying) field surrounds the wire equally to the right and left of the wire, symmetry surely dictates that the voltage is zero: neither end of the rod gains a charge.
Try formulating a '...
- 34.7k
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Griffiths Electrodynamics rotating sphere magnetic potential
The problem is that $(\vec e_r,\vec e_\theta,\vec e_\varphi)$ is a local basis, i.e. it depends on the point where it is constructed. In your calculation, you need to compute $|\vec r-\vec r'|$. $\vec ...
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Source of magnetic dipole potential energy
But since magnetic field has non-zero curl, how can such magnetic potential energy be interpreted? And what is the source of that energy since magnetic field does no work?
Magnetic field is not, in ...
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How does $ \vec{B}$ change when flux surface $A$ gets smaller?
I'm afraid that I can't follow your reasoning, so I don't know if you'll be convinced by these not very profound observations ...
Presumably the magnetic field is due to some source (such as a magnet ...
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How does $ \vec{B}$ change when flux surface $A$ gets smaller?
The magnetic field inside a solenoid of surface $A$ and current $I$ is given roughtly by the equation:
$$B=\mu_0 \frac{N I}{h},$$
where $h$ is the height of the solenoid. As of this approximation, it ...
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How does an Electromagnetic Accelerator work?
This is not the Lorentz force, which describes electric and magnetic forces on electric charges, it is the force of a magnetic field on a magnetic dipole (here's the wiki page about it).
When the coil ...
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Can a magnet stop when falling through a copper tube?
So theoretically if the magnet falls fast enough, can it completely stop for a moment?
No.
The falling magnet induces an emf in the copper because there is relative movement between the magnet and the ...
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Can a magnet stop when falling through a copper tube?
You have just calculated where the induced fields balance the field of the magnet (approximately). I suspect what you really wanted to do was calculate the speed at which the force between the magnet ...
- 128k
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Integrating current density for cylindrical symmetry
If I understood you correctly, your current density is
$$\vec{J}=\begin{cases}
\frac{I}{\pi a^2}\hat{k} &, \text{ for } 0<r<a \\
\vec{0} &, \text{ for } r>a
\end{cases}$$
The mistake ...
- 36.2k
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Mathematical expression for self-inductance constant $L$?
Self inductance is purely magnetostatic property. No need for Faraday’s law. The issue with your approach is that you’ll formally get infinite inductance. Tou can already see the beginning of the ...
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Connection Helmholtz free energy and $H,M,B$ fields
Magnetic work in equation ($1$) is correct. It can be derived from analyzing the work required to vary the magnetic $B$ field. To be precise, because, in general, the fields are not uniform, it should ...
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How does a compass behave on magnetic south
If you hold compass in standard position it's magnetic field lines can't align with Earth magnetic field lines because they are perpendicular to each other. This position is unstable, because $N$ of ...
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How does a compass behave on magnetic south
If you are standing at the magnetic south pole and holding a magnetic compass in the usual 'horizontal' plane, then no direction of the needle points to north. The needle is orthogonal to the 'north' ...
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Is the magnetic field generated by a surface current always a vacuum field?
You misconception is probably that if:
$$
\nabla\cdot B=0\\
\nabla\times B=0
$$
then $B=0$ (assuming finite energy). The domain of the field is important, and the result is valid only for the infinite ...
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What is the intuitive reason why Ampere's law is incorrect?
The other answer already shows that Ampere's law and
charge conservation in their differential forms
$$\vec\nabla\times\vec{B}=\mu_0\vec{J}$$
$$-\frac{\partial \rho}{\partial t}=\vec{\nabla}\cdot\vec{...
- 36.2k
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What is the intuitive reason why Ampere's law is incorrect?
It is a mathematical fact that the divergence of the curl of a vector field is always zero. I'm not sure whether one can give an "intuitive reason" for this or not, but it is in fact true.
...
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Connection Helmholtz free energy and $H,M,B$ fields
The 'real' internal energy is $dU=TdS+HdM$, and hence the Helmholtz free energy is given by $dF=-SdT+HdM$. Therefore, Equation 2 is incorrect [when $F$ is the Helmholtz free energy] and Equation 3 is ...
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Are the $H$ and $M$ fields parallel?
Contrast the vacuum equation
$$
\nabla\times\mathbf B = \frac 1{\mu_0}\left(
\mathbf J + \epsilon_0\frac{\partial\mathbf E}{\partial t}
\right)
$$
against the matter equation
$$
\nabla\times\mathbf H =...
rob♦
- 86.2k
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Are the $H$ and $M$ fields parallel?
The relationship between $\mathbf H,\mathbf M,$ and $\mathbf B$ is
$$\mathbf B = \mu_0(\mathbf H+\mathbf M)$$
Under certain conditions, some systems exhibit linear response, in which $\mathbf M = \chi ...
- 67.4k
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Are the $H$ and $M$ fields parallel?
They are for e.g. diamagnets and paramagnets. However, ferromagnets (and antiferromagnets) maintain their magnetization, even when the magnetic field goes to zero (hysteresis). Therefore, in those ...
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Could a closed looped of wire with current move itself?
There are two fundamental reasons why this doesn't work like that:
Lenz's law:
the direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic ...
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Measuring the magnetic field strength on the surface of a neodymium magnet (without a gaussmeter)?
The Earth is not just like a bar magnet, not really, right?
Aren't there 4 types of magnetism:
permanent, electro, static and geo?
It seems basic, but it's not accurate at all. Like the statement ...
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Electron Orbit at the Null Point (DC sputter coating)
In general you would use magnetic fields to steer an electron beam, not to accelerate it. Typically, sputtering does not need magnets. It uses heat to boil atoms off a source. The atoms fly everywhere ...
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Can the magnetic field do work?
A simple case... Parallel wires carrying currents in the same direction will attract each other and do work on each other if they are allowed to come together against restraining forces. Wire X sits ...
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