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Questions tagged [point-particles]

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What is dipolar charge distribution?

Electric dipole is a system of to opposite point charges separated by some distance. $1.$ How can we have a continuous volume charge distribution from such a collection of point charges? $2.$ Here ...
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1answer
68 views

Empty space inside of atoms [duplicate]

Since most of the space between the nucleus and electron is empty space is that space in a vacuum? I’ve not seen any info on this online or in textbooks does anyone have anything on this?
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1answer
91 views

Non-relativistic limit of Hamiltonian for a free particle in general relativity

The Hamiltonian for a particle moving in a gravitational field can be taken as $$\mathcal{H} = \frac12 \sum_{\mu,\nu=0}^3g^{\mu\nu}(x)p_\mu p_\nu\tag{1}$$ as long as the parametrization is affine. ...
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1answer
52 views

Question about point particle vs. wave equation location

Another uncertainty question, this came up in another forum. As I understand it an electron, for example, is a point-like particle. I take this to mean it exhibits dimensionless properties, but ...
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8answers
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How can neutral atoms have exactly zero electric field when there is a difference in the positions of the charges? [duplicate]

It is said that atoms with the same number of electrons as protons are electrically neutral, so they have no net charge or net electric field. A particle with charge cannot exist at the same position ...
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1answer
123 views

Two Questions about Path Integral from “Gauge Fields and Strings” by Polyakov

My questions are about worldline path integrals from the book Gauge Fields and Strings of Polyakov. On page 153, chapter 9, he says Let us begin with the following path integral \begin{align} &...
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2answers
69 views

When would an object not be considered a point-particle in relation to gravity?

When is the Earth, for example, not considered a point in relation to gravity? I am thinking that if one was asking a question below the crust continously to the core, then this would be an example. ...
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Shape of electron [duplicate]

Today, in the BBC Science section, a headline reads that the Imperial College of London has determined that the shape of the electron is completely spherical. In a physics book I'm reading now, the ...
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1answer
62 views

When/Why Did Physics Discard The Point Charge As An Accurate Representation? [closed]

Is there anything about the implications of the "early" quantum theory of Schrodinger equation, wave-particle duality, or the two slit experiment that conflicts with the idea of a point charge? Did ...
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2answers
56 views

Can an electron pass through a gap smaller than an electron?

If an electron (photon) can pass through a gap smaller than an electron (photon) then it is a wave otherwise it it a particle. Is this a correct way of reasoning?
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2answers
68 views

Geodesic equations from action with auxiliary field

A textbook says that the geodesic equations (for both massive and massless) can be derived from the following action: $$ S = -\frac{1}{2} \int d\tau \:\eta \: (\eta^{-2} \dot{x}^\mu \dot{x}^\nu g_{\...
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1answer
61 views

What was Newton's view of theory of matter? [closed]

Did Newton believe in infinitely small particle theory of matter? Because he talks about axis of rotation, which is locus of the centers of the circles of the rotating body and my teacher said ...
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Doesn't the fact that elementary particles are not black holes prove they are not point structures? [duplicate]

De Schwarzschild radius of a mass $m$ is defined as $$r_s=\frac{2mG}{c^2}(m).$$ So if we insert in this formula the mass of an electron (a point particle, according to mainstream physics), which ...
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3answers
55 views

How does relativity dimensional contraction affect point like particles such as the electron and neutrino?

I might be misunderstanding a basic concept here, so forgive me. I know that the faster an object gets, the more it's dimensions will contract according to the following equation: $${1\over D} = 1-{V^...
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1answer
52 views

Relativistic action is a constant?

Say that you want to find the equations of motion of a free relativistic massive point particle by minimizing the action $$S=-m\int\mathrm{d}\tau\,\sqrt{\eta_{\mu\nu}\frac{\mathrm{d}x^\mu}{\mathrm{d}\...
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2answers
113 views

How can 0-dimensional particles or 1-dimensional strings be 3D matter? [closed]

According to the latest information we got String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings....
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1answer
164 views

2 bodies on an inclined plane, we need to find the force one exerts onto the other as they both slide down

So I've been trying to tackle this for the last few hours, but no dice. (exam practice, by the way) We got two point masses $m_1$ and $m_2$ lying tangent to each other on top of an inclined plane ...
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2answers
2k views

Can the center of charge and center of mass of an electron differ in quantum mechanics?

Traditionally for a free electron, we presume the expectation of its location (place of the center of mass) and the center of charge at the same place. Although this seemed to be reasonable for a ...
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Is continuum mechanics a generalization or an approximation to point particle mechanics?

Newtonian Mechanics is usually presented as a theory of point particles (and forces). My impression of the status of continuum mechanics is that it is mostly taken as an approximate description for ...
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2answers
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Can somebody explain why the action in the picture is true?

I can provide the resource for where this is from. Can somebody explain how to get this expression?
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2answers
67 views

Does the propagation of light through spacetime, with regards to how we describe it, depend on when a photon is a wave function or point particle?

When for example, a photon is emitted from an atom, does that photon propagate through spacetime in all directions away from the atom in the form of a sphere (Wave Function) and then at some point ...
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0answers
47 views

Integrating Out Auxiliary Field of point-particle Polyakov Action

The Polyakov action of a point-particle is $$S[X,e]=\frac{1}{2}\int d\tau\left(\frac{\dot{X}^{2}}{e}-m^{2}e\right)$$ with the $(−,+,+,+)$ Minkowski sign convention. How to perform the path-integral ...
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3answers
211 views

Electric field from disc versus point charge

A uniformly charged disc of radius R and net charge Q with an x-axis through the center of the disc will have an electric field in a point $x_0$ on the x-axis $E=kQ(1-\dfrac{x_0}{\sqrt{x_0^2+R^2}})$ ...
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1answer
67 views

When and why are we allowed to treat a rigid body as a point mass?

When the subject Mechanics first taught, it is common that we explicitly state that the Newton's laws are valid only for point masses, and then we give examples of rigid bodies colliding with each ...
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1answer
65 views

From non-relativistic to relativistic action

There is a derivation of relativistic action that treats space and time symetricaly which is just playing arround with the square of kinetic energy in the non-relativistic action and plugging in speed ...
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1answer
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Point particle with a magnetic dipole?

I have read these questions: Are contravariant basis vectors and basis 1-forms identical? Where John Rennie's answer says that electrons do have an electric dipole moment and we imagine that in math ...
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3answers
86 views

What are point objects?

I can't seem to get the idea of point mass into my head. Why are equations of physics applicable on only point masses and should be altered while dealing with object that has a collection of points? ...
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0answers
75 views

Energy of single particle is equal to energy of multiparticle system it consists of?

The solution to a problem of atom fission led me to this question. In this problem the mass of original atom nucleus, masses of two child atom nuclei as well as zero kinetic energy of first atom ...
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1answer
31 views

Real points vs. Physical points and what replaces them

Quantum field theory and relativity share the need for point particles (besides we have learned how to deal with extended objects with more or less success). Heisenberg uncertainty principle ...
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1answer
66 views

A question about the Lagrangian for a massive or massless point particle

In my lecture I learned that the action that can be applied to the light ray is written like below: \begin{equation*} S[x;e]=(1/2)\int [(1/e)g_{ab}\dot x^a\dot x^b-m^2e]ds \tag{1} \end{equation*} ...
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1answer
220 views

Lagrangian formulation of free massive point particle in special relativity

I wonder if there is a way to reproduce the 4-force generalization for Newton's equation for a free particle i.e. $$ m\frac{d^2x^\mu}{d \tau^2} = 0, \qquad \text{for} \, \mu =0,1,2,3, \tag{1} $$ ...
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1answer
42 views

Spin of 2 oppositely charged fused point charges

If i fused 2 point particles (with opposite charges) together, would it be possible to change the magnitude of this object's spin by applying a torque on it? Also, if they are joined together, wouldn'...
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1answer
172 views

Does Radar detect objects or has point cloud

Many times in literature I read that Radar is used to detect objects. How does this work? Are reflections from objects with Radar not point reflections? And how does Radar know that all reflections ...
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1answer
770 views

Lagrangian of a relativistic free massive particle

Lagrangian for a relativistic free particle can be written as $$L=-m_0c^2\sqrt{1-\frac{v^2}{c^2}} .\tag{1}$$ It gives correct expression of Hamiltonian which is $$H=\sqrt{p^2 c^2+m_0^2c^4}.\tag{2}$...
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2answers
498 views

Are quarks/electrons different sizes (physically/in terms of mass)? [duplicate]

i.e. is an electron bigger/smaller than a strange quark? Is a strange quark bigger/smaller than an up quark? Or are they all the same size? Leading on, are electrons the same size (as in physical ...
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1answer
58 views

Meaning of 'Particle of Mass'

The following quote is from Section II at http://suppes-corpus.stanford.edu/articles/physics/431.pdf A simple pendulum consists of a particle of mass $m$ hanging from a... My understanding is ...
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1answer
148 views

What is the smallest particle? What is the building block of an electron? [duplicate]

I'm curious. Why when I was 5 years old did I rip things apart in half... Always trying to get another half... is it endless? I took a leaf and ripped it in half, and kept on going... God?
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3answers
188 views

About fermionic particles and bosonic particles in RQFT, ST [closed]

In relativistic quantum field theory, we can observe that the Dirac equation is a square root of Klein-Gordon equation. But, we can get the Dirac equation by defining Dirac spinor as $(1/2,0)\oplus(0,...
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1answer
79 views

What is the induced electromagnetic field of a point charge?

If I move a point charge on some trajectory, then it will produce an electric field as well as a magnetic field. As the charge is moving, and as a point charge can not produce a steady current, then ...
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5answers
655 views

Do point charge really exist?

Do point charge really exist experimentally? Am I right with definition of point charge? According to me point charge is a charge having 0 (zero) mass and have 0 (zero) volume.
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4answers
865 views

How to find a particle's dynamics in general relativity?

About a year ago, I took a course on general relativity. It isn't until now that I realize that, given a metric, I am unsure how to find a particle's dynamics. What I mean by that is, normally I ...
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1answer
264 views

With point particles being smaller than their Schwarzschild radius are they forever cloaked in a black hole?

My understanding of point particles is that they have mass and are dimensionless. If this is so then wouldn't the mass cause the particle to have a Schwarzschild radius which would then make the ...
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2answers
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Coordinates for the Thomson problem

Is there a resource that lists the coordinates for known solutions to the Thomson problem? Wikipedia lists the energies, and I'm having trouble solving for $N=78$. The closest I can get is ...
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5answers
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Direction of the electric field of a negative point charge?

How can the direction of the electric field of a negative point charge be going in towards itself (radially), when the charge itself is generating the electric field? Is it generated at infinity then? ...
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3answers
174 views

Why do we invent non-physical concepts (like e.g. point particles) to study physical phenomenons?

There is nothing exist like point particles in reality then why did we invented the notion of point particles and how does it relate to real world?
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1answer
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Why does it make sense to talk about the first-quantized BRST formulation of a relativistic point particle?

My question is about the BRST quantization of a point particle in Polchinski, Vol.1, Section 4.2. The BRST quantization starts from the effective action for the gauge-fixed path-integral. But for the ...
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1answer
199 views

Finding a 'vector potential' such that $\mathbf E = \nabla\times \mathbf C$ for a point charge

Supposedly, "Any divergence-free vector field can be expressed as the curl of some other divergence-free vector field" over a simply-connected domain. So, what is one such vector potential which ...
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2answers
415 views

How to calculate static electric field produced by multiple point charges at a point (simple…)

[I've seen several related questions but I ask for a confirmation here.] I have a set of point charges to model atoms, say: $q_1$ at $(x_1,y_1,z_1)$ $q_2$ at $(x_2,y_2,z_2)$ ... $q_n$ at $(x_n,...
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0answers
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Motion of a catapulted point mass moving freely inside a spherical bucket

I am trying to study the motion of a point mass inside the bucket of a catapult. The catapult is shooting downward (i.e. describing a rotation of 180° from the horizontal axis) and I would like to ...
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1answer
73 views

Moving point particles in quantum mechanics

Given a quantum system with a hamiltonian $H$, if initial state of a physical object is $|\psi\rangle $, then state after time $dt$ is $e^{iH\cdot dt}|\psi\rangle$. It is easy to see that $|\langle\...