67 votes
Accepted

Resolving Conflicting Reports on Fermilab $g-2$ Results

This seems rather incredible that these two seemingly conflicting announcements come on the same day. The pre-print for the Nature paper by the BMW group was placed on arXiv in 2020 around the same ...
ohwilleke's user avatar
  • 3,903
22 votes
Accepted

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
19 votes

Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

The simple answer is that the spins are magnetic dipoles, not monopoles. A monopole will feel a force in any field. But a dipole needs a spatially varying field, because otherwise each pole of the ...
Gilbert's user avatar
  • 11.9k
18 votes

Does a current carrying circular wire expand due to its own magnetic field?

Since current elements moving in parallel attract (not repel!) each other, the electrons inside a wire do in fact feel an attractive magnetic force towards the center of the wire. However, if they ...
Michael Seifert's user avatar
16 votes
Accepted

Could someone explain the muon $g-2$ experiment problem?

The $g$ factor describes the magnetic moment of a spinning particle. The $g$ factor for a classically spinning particle is equal to 1, but in the "basic" (ie, non-interacting) quantum field ...
Andrew's user avatar
  • 48.7k
12 votes
Accepted

How does magnetic moment vector arise from spin 1/2 spinors?

In quantum mechanics, the magnetic moment $\hat{\vec{\mu}}_s$ is an operator, related to the spin operator $\hat{\vec{S}}$ via $\hat{\vec{\mu}}_s=\gamma \hat{\vec{S}}$. What you observe in experiment ...
Rudolf Smorka's user avatar
11 votes

Paradox about the Stern-Gerlach experiment with $B=0$, $\nabla B\ne 0$

Steve's intuition for the measurement operator is in the right direction, but it's not quite the right interaction potential. Getting that right is simple, but it requires the complex task of taking ...
Emilio Pisanty's user avatar
11 votes
Accepted

Has anyone tried to incorporate the electrons magnetic dipole moment into the atomic orbital theory?

To be crystal clear: Has anyone tried to incorporate the electrons magnetic dipole moment into the atomic orbital theory? YES. They've tried and they've succeeded. The electron's spin magnetic ...
Emilio Pisanty's user avatar
11 votes
Accepted

Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

In the Stern-Gerlach experiment you want the atoms to be deflected depending on the direction of their magnetic dipole moment. But you get a net force on the magnetic dipole moment only if the ...
Thomas Fritsch's user avatar
11 votes

Why Is an Inhomogenous Magnetic Field Used in the Stern Gerlach Experiment?

@Thomas Fritsch uploaded a nice picture that provides intuition about the dynamics of the situation. I would just like to add that the force $\textbf{F}$ exerted on this infinitesimal loop pictured (...
Andrew's user avatar
  • 813
10 votes

How neutrons interact if not through an electromagnetic interaction?

The neutron is not an elementary particle. Its overall charge is zero, but depending on the energy available the fact that it is composed out of charged quarks becomes important. Thus both the strong ...
anna v's user avatar
  • 234k
9 votes
Accepted

Why do we assume a neutron has no electromagnetic charge?

It is an experimental fact that neutrons have no net charge. They have a magnetic dipole moment, which points to having charged constituents (quarks), the charges of which algebraically sum to $0$. (...
ZeroTheHero's user avatar
  • 45.5k
9 votes
Accepted

When does $\nabla \times B =0$?

Ampere's law says that $$\nabla \times \boldsymbol{B} = \mu_0 \boldsymbol{J} + \epsilon_0\mu_0 \frac{\partial}{\partial t} \boldsymbol{E}$$ so "far away from sources" means that the current density $\...
J. Murray's user avatar
  • 69.1k
8 votes
Accepted

Why is a current carrying loop considered a dipole?

The current loop creates a magnetic field. This field looks just like that of a magnet, which has two poles (a dipole).
Job Stancil's user avatar
  • 1,079
8 votes
Accepted

Electron to electron interaction

As far as we know, electrons are point particles, so you don't want to try to calculate anything when they are at zero separation. You'd just get infinite forces. Classically, the magnetostatic ...
G. Smith's user avatar
  • 51.5k
8 votes
Accepted

Why do neutrons have magnetic moments with spin?

A neutron is a composite particle consisting of three pointlike quarks. These carry fractional electric charge which adds up to zero, so the neutron has no net charge. But since the quarks do carry ...
niels nielsen's user avatar
8 votes
Accepted

What is the difference between Larmor frequency and cyclotron frequency?

Cyclotron frequency is the frequency of electron rotation in magnetic field, due to the Lorentz force: $$ m\frac{d\mathbf{v}}{dt}=\mathbf{F}=\frac{e}{c}\mathbf{v}\times\mathbf{B}\\\Rightarrow \frac{d\...
Roger V.'s user avatar
  • 58.5k
8 votes
Accepted

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
  • 52.9k
7 votes

What is the magnetic moment, and what does it have to do with the spin of the electron?

An electron acts like a very small magnet. We say "magnetic moment" rather than "magnetic field" because a field is a local property (move further from the electron and the field gets weaker), while ...
Floris's user avatar
  • 119k
7 votes

What is the magnetic dipole moment of a pion?

It is zero. If the magnetic dipole moment $\boldsymbol \mu$ were different from zero, the particle would have a preferred direction in space, which is not possible for a scalar. Recall that, in ...
AccidentalFourierTransform's user avatar
7 votes

Is magnetic force relativistic effect or intrinsic property?

The symmetry of spacetime in special relativity requires that if the laws of physics include the possibility of electric fields, then they must also include the possibility of magnetic fields, and ...
Chiral Anomaly's user avatar
7 votes
Accepted

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

There are two important interactions in magnetic materials: the dipolar interaction and the exchange interaction. The dipolar interaction corresponds to the magnetic fields from the Maxwell ...
Frederic's user avatar
  • 1,139
7 votes

What is $a_\mu$ in particle physics? In terms of a muon; and its magnetic moment? What is, '$a_\mu$ × $10^9$ - 1165900 '?

The spin magnetic moment of a fundamental particle with mass $m$, charge $q$, and spin 1/2 is $$\vec\mu=g\frac{q}{2m}\vec S$$ where $\vec S$ is its spin vector. The "$g$-factor" is a ...
G. Smith's user avatar
  • 51.5k
7 votes
Accepted

Why is tauon not being probed for high accuracy $g-2$ values?

This part-per-million measurement of the muon's anomalous magnetic moment is a part-per-billion measurement of the muons total magnetic moment. Every part-per-billion measurement is hard. This one is, ...
rob's user avatar
  • 89.7k
7 votes

How can the 'spin' of a particle point in the opposite direction of its magnetic moment?

Suppose your "particle" is actually a positively-charged sphere. Spin the sphere about some axis and the moving charges generate a magnetic field. You can use the Biot-Savart law to figure ...
rob's user avatar
  • 89.7k
7 votes
Accepted

Unlike electrons, photons don't have a magnetic moment despite having a spin. Why?

For either charged elementary particles (electron, muon, etc) or uncharged composite (of charged elementary particles) particles (example, neutron) the magnetic moment and spin are proportional to ...
J. Murray's user avatar
  • 69.1k
7 votes
Accepted

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
  • 839
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
6 votes

How does a spinning electron produce a magnetic field?

Lets take the rotating "spherical ball" analogy seriously (actually a torus below). The electron is modeled as a tiny stationnary torus rotating at an angular velocity $\omega$ around the ...
Cham's user avatar
  • 7,411

Only top scored, non community-wiki answers of a minimum length are eligible