22 votes
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Why compass needle doesn't precess about but aligns by magnetic field?

A good compass contains a liquid that dampens the motion of the needle. An ideal frictionless needle would constantly oscillate around the direction of the magnetic field and if the needle was free to ...
FlatterMann's user avatar
  • 2,998
21 votes
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Is the magnetic vector potential "real" in classical electromagnetism?

The vector potential is gauge-dependent and unobservable in both classical and quantum mechanics. Only gauge-invariant quantities — including the electric and magnetic fields — are observable. Even in ...
d_b's user avatar
  • 8,154
19 votes

Magnetic field changes induced by vibrating electric guitar string

Here's what's happening: The permanent magnet in the pickup propagates a static magnetic field which extends out from the magnet tip. The ferromagnetic string resides within this field. Because the ...
niels nielsen's user avatar
11 votes

Magnetic field changes induced by vibrating electric guitar string

Although niels nielsen's answer contains valuable information (the last 3 paragraphs), I think user1079505's answer is closer to the truth, and the article (that I don't have access to) hinted at by ...
Zaaikort's user avatar
  • 492
10 votes

Magnetic field changes induced by vibrating electric guitar string

Electric guitar strings are ferromagnetic. In the presence of magnetic field from the magnets in the pickup, the strings become magnetized. As they move (vibrate), the magnetic field they generate ...
user1079505's user avatar
10 votes

Is the magnetic vector potential "real" in classical electromagnetism?

One way of thinking about this issue of what quantities are 'real' is to compare it to the more familiar situation with position. There are quantities in physics with the property that the quantity ...
Nullius in Verba's user avatar
10 votes
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Doubt regarding an assumption made in an example in Griffiths

There are two ways of justifying what Griffiths concludes. The first approach—which is the one he adopts—is to argue that there is no possible magnetic field consistent with the spherical symmetry. ...
Buzz's user avatar
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9 votes

We can see magnetic field lines but how about electric field lines?

You can use, for example, grass seeds in oil. See this video lecture, "Demo-by-Lewin(8.02-02-2): Display electric fields with grass seeds" Update: James Lincoln has some additional methods ...
robphy's user avatar
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9 votes

Is electromagnetism simply a model?

Everything is a model. If you walk outside and feel the sun on your face, you can either say that your skin is absorbing electromagnetic radiation or that it is experiencing a non-local interaction ...
J. Murray's user avatar
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9 votes

Is the magnetic vector potential "real" in classical electromagnetism?

Let's take the definition of a "real field" from the chapter in Feynman's lectures you linked to: What we mean here by a “real” field is this: a real field is a mathematical function we use ...
Andrew's user avatar
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8 votes

How did a spark generate electromagnetic fields that radiate to places?

EM waves are generated by accelerating charges. The transmitter is a circuit that supports an oscillating current (which requires that the charges are accelerating during the oscillations). The ...
BowlOfRed's user avatar
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8 votes
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Magnetic field changes induced by vibrating electric guitar string

Great question. The first part of the answer by niels nielsen is quite inaccurate, talking about "dragging" and "wiggling" of field lines to make the concept more intuitive, but in ...
Sam Gallagher's user avatar
8 votes
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Metals and Magnets

Newton's Third Law tells us that if object A exerts a force on object B then object B will exert an equal and opposite force on object A (this is required by the principle of conservation of momentum)....
gandalf61's user avatar
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8 votes
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How do Electric and magnetic fields generate each other (mathematically)?

Linked differential equations may reinforce one another leading to unbounded exponential growth - but they do not have to. For example, the linked equations $\displaystyle y = \frac {dx}{dt} \\ \...
gandalf61's user avatar
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7 votes

Landau tubes and Fermi sphere

My understanding is that the question is about figure like this one (taken from thread What's the coordinate of the Landau tube?): Magnetic field does not change the number of electronic states or ...
Roger V.'s user avatar
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7 votes
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If you stacked Jupiter and Saturn pole-to-pole (magnetically) on top of each other, would the magnetic force come close to the gravitational one?

Would the magnetic force come close to the gravitational one? No. Nowhere near close. Wikipedia gives a different value for the magnetic dipole moment of Jupiter -- $2.8 \times 10^{20} \text{ T} \...
Ghoster's user avatar
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7 votes

Metals and Magnets

To enlarge slightly upon gandalf61's answer, when a piece of ferromagnetic metal is placed near a magnet, it (temporarily) becomes a magnet too, and so the two magnets then attract each other.
niels nielsen's user avatar
7 votes
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Why is there a current in a changing magnetic field?

To cancel an electric field, the charges would have to configure themselves in such a way that they cause the exact same electric field, just with opposite direction. Then the two opposing electric ...
Vercassivelaunos's user avatar
7 votes
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If a loop is not complete, and magnetic field passing through it is changing; will their be emf induced in the loop?

Yes. What will happen is that an emf will induced in a complete loop which consists of a part of the complete loop shown with full lines and another part of the complete loop defined by a dotted line (...
Farcher's user avatar
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7 votes
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Why does $\mathbf{A}(x) = \frac{1}{2}(\mathbf{B}(x) \times \mathbf{x})$ work?

This identity holds only for a uniform magnetic field $\vec B$. Many terms cancel in your formula. It remains $$\eqalign{ \vec\nabla\times\vec A &={1\over 2}\big((\vec\nabla.\vec x)\vec B-(\...
Christophe's user avatar
  • 3,548
7 votes

Can the $E$ field exist without any $B$ or $H$ field?

In a world where the laws of Newton hold (i.e. the "non-relativistic" world), an electric field $E ≠ 0$ and magnetic field $B = 0$ would have the same appearance in all moving frames of ...
NinjaDarth's user avatar
  • 1,954
7 votes
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How can electric wave and magnetic waves have any angle between them?

In 3-dimensional space a monochromatic electromagnetic wave can be characterized by 3 directions. First is the direction of propagation, in other more technical words this is the direction of the ...
Frederic Thomas's user avatar
7 votes
Accepted

Why is there no current in a magnet even though magnetic field is present?

Static magnetism is caused by atomic magnetic moments. Certain atoms, such as iron, have unpaired electron spins$^*$ that persist in materials and can then align to give ferromagnetism. There exists ...
my2cts's user avatar
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6 votes
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How was the Biot-Savart law for magnetic fields derived if the Lorentz force had not been experimentally derived?

The original law deduced by Biot and Savart was less general than what we now call the Biot-Savart law. It amounted to the following statements: the magnetic field a distance $r$ from a current-...
d_b's user avatar
  • 8,154
6 votes
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How can we define Magnetic Potential Energy if it is non-conservative?

There are situations in magneto-statics where scalar magnetical potentials makes perfectly sense. The most well-known example is a permanent magnet and its surroundings of air for instance. There is ...
Frederic Thomas's user avatar
6 votes
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Electromotive force computation and induced current

Yes, there are very fine subtleties. Is this approach used only for the sole purpose of making the infinitesimal element of wire $d\mathbf l$ point vertically and making the integration trivial? It ...
Ján Lalinský's user avatar
6 votes
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Basic question about magnetic circuits

Your second assumption is incorrect. $\Phi$ and $B = \Phi/A$ are determined by the whole magnetic circuit, not just the solenoid. Specifically, $\Phi = F/R$, where $R$ is as you have calculated. ...
Puk's user avatar
  • 13.5k
6 votes

Why is there no current in a magnet even though magnetic field is present?

The magnet is composed of tiny magnetic moments that are essentially current loops. At atomic level, these come from both orbital angular momentum and spin angular momentum. This is the orbital motion ...
JohnA.'s user avatar
  • 1,688
5 votes

Physical meaning of the invariance of the dot product of $\vec{E}$ and $\vec{B}$

You may already know that contractions of tensors produce Lorentz invariants (similar to how dot products are invariant under rotations in non relativistic mechanics). This result is natural from the ...
Mauricio's user avatar
  • 5,436
5 votes

Why compass needle doesn't precess about but aligns by magnetic field?

Precession occurs if the object has angular momentum. The torque then locates the angular momentum vector causing precession. A compass needle has no angular momentum, so there is no precession
Jerrold Franklin's user avatar

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