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15

There is a sort of analog called gravitomagnetism (or gravitoelectromagnetism), but it is not discussed that often because it applies only in a special case. It is an approximation of general relativity (i.e. the Einstein Field Equations) in the case where: The weak field limit applies. The correct reference frame is chosen (it's not entirely clear to me ...


7

The magnetic moment of the electron is a magnetic moment, so the right magnetic field around it is $$ \mathbf{B}({\mathbf{r}})=\nabla\times{\mathbf{A}}=\frac{\mu_{0}}{4\pi}\left(\frac{3\mathbf{r}(\mathbf{\mu}\cdot\mathbf{r})}{r^{5}}-\frac{{\mathbf{\mu}}}{r^{3}}\right). $$ The world is quantum mechanical – and so is any viable description of the spin – so we ...


4

TL,DR: Magnetic coupling results in lower transmission of sound energy than physical contact Controlling what surfaces vibrate gives more control over sound generation The same benefit could be achieved with other forms of isolation (e.g. foam) but it wouldn't look as cool. It is bunk, mostly. A magnetically levitating speaker maintains a ...


4

There is a gravitational analogue of the magnetic field. See gravitoelectromagnetism and frame dragging on Wikipedia.


3

This advertising strategy is basically using pseudoscience to get naive people to buy a product. The efficiency problem in speaker design has nothing to do with momentum transfer from the speaker to the air. That's trivial, since the mass of air a speaker moves is typically orders of magnitude less than the mass of the speaker itself. Instead, the (low ...


3

Yes, it is already a reality. Permanent magnets from rare earth alloys https://en.wikipedia.org/wiki/Rare-earth_magnet can exceed 1.4 tesla while ferrite and ceramic ones only have 0.5-1.0 tesla. Making the material as big as a meter or anything you want is just a matter of accumulating a larger amount of the material (or adding smaller magnets). At ...


2

You're halfway there. The quickest way that I can see is via explicit Cartesian components and coordinates. You start by assuming that $\mathbf B$ is uniform, so that you can pull it out of the integral (which you've already used implicitly for dealing with $Y$. That means that you're interested in \begin{align} \frac{\boldsymbol \tau}{I} =\oint_L ...


2

Increasing the number of turns increases the magnetic field if the current remains constant. In your situation, you are postulating (implicitly) that the applied voltage is constant, and that the current is reduced. The product $N\cdot I$ is therefore unchanged, and the magnetic field does not increase when you increase the number of turns of a resistive ...


2

I think a ferrite rod antenna in a radio receiver is an example where the magnetic component of an EM-field is picked up.


2

Nope! The first two large magnets in 1) have nonzero dipole moments, while in 2), the pairs of magnets have the magnetic dipole charge canceled. The right way to divide the large magnet with "S" at the top and "N" at the bottom is clearly to have two magnets with "S" at the top and "N" at the bottom! It doesn't matter whether these two half-magnets mutually ...


2

I think that the problem is that you are considering an electric current at distance $r=0$ with the Biot-Savart formula. It's like when you have a wire with a current and you want to find magnetic field on the wire, or electric field on a point-like charge. In your problem current $J$ is a linear function of distance, but in Biot-Savart you have something ...


1

Well, the details are important. From the abstract in your link of the paper we see that: 1) it is a publication from 1978 2) it calculates rates for positronium annihilation in the very high magnetic fields found in astrophysical situations, 10^12 Gauss It explicitly states that the momentum contribution comes from the magnetic field. In the relevant ...


1

First consider Faraday's law, which states that $$ \nabla \times \textbf{E} = -\frac{\partial \textbf{B}}{\partial t}. $$ We can interpret this as follows: whenever we are generating a magnetic field that changes with time, there is an associated electric field, and vice-versa. An equivalent interpretation is that a changing magnetic field causes a ...


1

I believe that for an ordinary conductor, there is nothing special for a static magnetic field. But this relates only to the electrical properties of that conductor - many material have some magnetic properties as week, and those would of course modify the B field. But for the purpose of this question the B field follows the dipole field "in free space" - no ...


1

There is an electric field due to the changing magnetic field, which has direction left -> right. Due to this field, there is an immediate charge seperation caused in the metal rod (-ve near M and +ve near N) which creates another electric field of equal magnitude and opposite direction to perfectly cancel the external electric field. Thus the net electric ...


1

I doubt my description can be improved other than with a picture. Now the interesting thing here is that I do not know which way the current is flowing, because I do not know which way the compass needles are colour-coded. I guess that the red represents the north pole. I do know that if the current flow is reversed then the needles will swap direction. ...


1

yes, it is the cross sectional area of the core, plane b in your drawing. See http://info.ee.surrey.ac.uk/Workshop/advice/coils/force.html#nfringe for more information.


1

The field from a current in a wire is purely magnetic for a static current. When the current varies with time, there will be radiation. What you are missing is that a radio antenna doesn't operate with DC. You can perhaps understand it like this: the magnetic field from a current loop depends on its magnetic moment, which is current times area. When the ...


1

Hopefully this should dispel some of your confusion. In general, the fields $\textbf{B}$ and $\textbf{H}$ are related by $$ \textbf{H} \equiv \frac{1}{\mu_0}(\textbf{B} - \textbf{M}) $$ This is always true, regardless of the materials involved. We define linear media as materials whose fields satisfy $$ \textbf{B} = \frac{1}{\chi}\textbf{M}. $$ In this ...


1

What explains this "channeling" behavior of ferromagnetic materials? In other words, is this explainable using the normal methods for magnetic field calculations such as Biot-Savart and treating the ferromagnet as consisting of infinitesimal dipoles Ferromagnetism is a quantum phenomenon, but yes, it can be treated classically as a volume of dipoles ...


1

See Appendix B on page 47 and further of this article: Note that the failure of the “rest mass” m to be constant resolves a paradox concerning what one is taught in elementary physics courses: On one hand, one is (correctly) taught that an external magnetic field can “do no work” on a body, so a body moving in an external magnetic field cannot gain ...


1

Using the principle of superposition, it is possible to generate magnetic fields as high as 3-4 T, in small air gaps, using structures built from Nd-Fe-B-based permanent magnets. I was involved in the design and build of structures that could generate highly uniform fields of 2 T+, across an air gap that was 15 cm tall (between the pole faces) and 25 cm ...



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