Questions tagged [point-particles]
The point-particles tag has no usage guidance.
125
questions
12
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How can electric field be defined as force per charge, if the charge makes its own, singular electric field?
The electric field $\bf{E}$ represents how much force would act on a particle at a certain position per unit charge.
However, if we actually place a particle in that position, the electric field will ...
0
votes
0answers
73 views
Do elementary particles have a density?
The SM supposes elementary particles are structureless unless composite objects like hadrons. For bosons, that can occupy the same state, we can define energy or mass density. The same happens but ...
1
vote
1answer
37 views
Electromagnetic field of a point charge seen from a rotating reference frame
Let us consider a point charge sitting in the origin of our coordinate system. If we change to a rotating system, will the field of the point charge still look the same? Intuitively I would say yes, ...
0
votes
0answers
29 views
Why the continuous arrangement of point masses(particles) at infinitesimal separations leads to a extended system?
I am basically talking in terms of Newtonian mechanics.The Newton's laws started with a good and easy assumption of particles as point masses.This assumption clearly reformed physics and a great ...
1
vote
0answers
22 views
Equilibration of a solid body with two points particles
How do I find $\theta$ as it $\theta (m,d,l) and AB=L$? And the cane which holds the two points particles(mass) is massless . In A the point mass is $3M$ and in B the mass is $M$ where there is no ...
2
votes
1answer
56 views
Sizes of Elementary Particles
Present observation shows that elementary particles have no internal structure, and have no real size as they are described by wavefunction.
Something that therefore confuses me is that on a lot of ...
1
vote
1answer
77 views
Do point particles really exist? [closed]
The vast theories of physics (and mathematics) lay on the notion of material point. However, relativity and quantum mechanics cast doubts about the ultimate existence of zero-dimensional points:
...
0
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1answer
75 views
Rotation of a Point Particle
I wonder if there is a meaning of rotation for a point particle.
Does a point particle have angular momentum and does he reply to torque?
3
votes
2answers
141 views
Is it possible to have mass with zero volume?
As we had always studied that matter occupied space and has mass and our universe is made of matter so do that mean that there is no case where mass is present without volume .
1
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0answers
46 views
Point-like particle vs naked singularity? [duplicate]
In general, physics seems to consider elementary particles such as electrons to be point-like. On the other hand, naked singularities seem to cause all sorts of trouble, including closed time-like ...
0
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1answer
45 views
What is dipolar charge distribution?
An electric dipole is a system of two opposite point charges when their separation goes to zero and their charge goes to infinity in a way that the product of the charge and the separation remains ...
1
vote
1answer
76 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?
3
votes
1answer
132 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. ...
0
votes
1answer
53 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 ...
27
votes
8answers
6k views
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 ...
3
votes
1answer
131 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}
&...
0
votes
2answers
79 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.
...
1
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0answers
33 views
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 ...
-3
votes
1answer
63 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 ...
0
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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?
3
votes
2answers
80 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:
$$
S = -\frac{1}{2} \int d\tau \:\eta \: (\eta^{-2} \dot{x}^\mu \dot{x}^\nu g_{\...
0
votes
1answer
61 views
What was Newton's view of theory of matter? [closed]
Did Newton believe in infinitely small particle theory of matter? Because he talks about axis of rotation, which is locus of the centers of the circles of the rotating body
and my teacher said ...
3
votes
2answers
236 views
Doesn't the fact that elementary particles are not black holes prove they are not point structures? [duplicate]
De Schwarzschild radius of a mass $m$ is defined as
$$r_s=\frac{2mG}{c^2}(m).$$
So if we insert in this formula the mass of an electron (a point particle, according to mainstream physics), which ...
1
vote
3answers
62 views
How does relativity dimensional contraction affect point like particles such as the electron and neutrino?
I might be misunderstanding a basic concept here, so forgive me. I know that the faster an object gets, the more it's dimensions will contract according to the following equation:
$${1\over D} = 1-{V^...
3
votes
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}\...
1
vote
2answers
126 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....
1
vote
1answer
223 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 ...
29
votes
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 ...
0
votes
0answers
64 views
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 ...
-3
votes
2answers
74 views
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?
1
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2answers
68 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 ...
1
vote
0answers
50 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 ...
0
votes
3answers
284 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}})$
...
0
votes
1answer
82 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 ...
1
vote
1answer
72 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 ...
1
vote
1answer
113 views
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 ...
1
vote
3answers
92 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? ...
0
votes
0answers
81 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 ...
0
votes
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 ...
2
votes
1answer
70 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*}
...
1
vote
1answer
249 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}
$$
...
0
votes
1answer
44 views
Spin of 2 oppositely charged fused point charges
If i fused 2 point particles (with opposite charges) together, would it be possible to change the magnitude of this object's spin by applying a torque on it?
Also, if they are joined together, wouldn'...
0
votes
1answer
205 views
Does Radar detect objects or has point cloud
Many times in literature I read that Radar is used to detect objects. How does this work? Are reflections from objects with Radar not point reflections? And how does Radar know that all reflections ...
1
vote
1answer
881 views
Lagrangian of a relativistic free massive particle
Lagrangian for a relativistic free particle can be written as
$$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}$...
-1
votes
2answers
536 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 ...
-3
votes
1answer
169 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?
2
votes
3answers
192 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,...
1
vote
1answer
88 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 ...
5
votes
5answers
707 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.
