Questions tagged [conventions]

A convention is a set of agreed, stipulated, or generally accepted norms. It typically helps common efficiency or understanding but is not required, as opposed to a strict standard or protocol.

Filter by
Sorted by
Tagged with
0
votes
1answer
401 views

Transformation of the spinor indices of Hermitian $2\times 2$ matrices under the Lorentz group

The left-handed Weyl operator is defined by the $2\times 2$ matrix $$p_{\mu}\bar{\sigma}_{\dot{\beta}\alpha}^{\mu} = \begin{pmatrix} p^0 +p^3 & p^1 - i p^2\\ p^1 + ip^2 & p^0 - p^3 \end{...
17
votes
3answers
7k views

If electrons were positive and protons were negative, would life be different? [duplicate]

This was a question on a worksheet during my first week in a class on Electromagnetism. The answer is essentially: No. Life would be no different if electrons were positively charged and protons ...
9
votes
4answers
1k views

Is the number 1 a unit?

In dimensionless analysis, coefficients of quantities which have the same unit for numerator and denominator are said to be dimensionless. I feel the word dimensionless is actually wrong and should be ...
7
votes
2answers
1k views

Is right hand rule arbitrary?

In definition of vector multiplication, the direction of the resulting vector is given by the right hand rule. However I don't know any mathematical requirement to pick right hand instead of left for ...
6
votes
5answers
947 views

Why isn't there a minus sign in Ohm's law, $V = IR$?

Suppose current runs through a resistor from left to right, and we define the left-to-right direction as positive. Then from left to right, the potential decreases. So $V,$ the voltage across the ...
5
votes
1answer
795 views

Index notation Lorentz transfromation matrix

A Lorentz transformation between two different coordinate systems is given by: $$(x')^\mu = {\Lambda^\mu}_\nu~ x^\nu$$ where $\Lambda$ is the Lorentz transformation matrix. I am a bit confused by ...
5
votes
2answers
1k views

Inner Product Spaces

I am trying to reconcile the definition of Inner Product Spaces that I encountered in Mathematics with the one I recently came across in Physics. In particular, if $(,)$ denotes an inner product in ...
1
vote
1answer
7k views

Why does the electric potential for an infinite line charge seem to have to be worked out from negative infinity rather than positive infinity?

The electric field of an infinite line charge in the plane perpendicular to the line charge can be given as: $$E=\frac{1}{2\pi \epsilon r}$$ Where $r$ is the perpendicular distance from the line. ...
1
vote
2answers
454 views

What is the significance of the sign of work done? Does it affect internal energy also?

What does positive and negative work done indicate? What I understand is that positive work done implies that energy is gained by the rigid body and negative work done implies energy is lost by the ...
1
vote
2answers
185 views

Confusion regarding the concepts and derivation of Hydrostatic Equilibrium for a star

First and foremost I am sorry for this; in order to make the questions clear I must first upload the lecture notes from my institution $^\zeta$ for the derivation of Hydrostatic Equilibrium: $^\...
0
votes
4answers
786 views

Question About Potential Energy

I'm confused about how the energy is conserved and the signs of the works are also confusing for me. I have an example on my mind I would like to ask. Imagine a book on the ground. We want to lift ...
0
votes
3answers
270 views

How do I get the negative sign in the expression for Gravitational Potential Energy?

From universal law of gravitation, gravitational force exerted on a body of mass m by another body of mass M is $$ \mathbf F = \frac{GMm}{x^2} $$ where x is the distance between the ...
4
votes
1answer
523 views

Why is electric potential positive?

If there is a positive charge $q$ at the origin of a coordinate system, the electric potential $\phi$ at a distance $r$ from $q$ is (by definition, if we take the point of zero potential at infinity): ...
3
votes
2answers
2k views

How do you derive Fleming's left hand rule?

How do you derive Fleming's left hand rule? What is the theoretical explanation for the directions of the magnetic field, current and the force on the current for being oriented in that way relative ...
3
votes
1answer
644 views

Deep confusion with conventions and signs in geometric optics

This is an equation given in my book. The question is why have they used a negative sign on the LHS? Now, if you try to derive the mirror equation with simple geometry, you get 1/v +1/u =1/f . I ...
3
votes
3answers
560 views

Why are there $1 / 1.602176634 \times 10^{-19}$ electrons in a coulomb?

Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ...
2
votes
1answer
666 views

Why is the electromagnetic four-potential $A_{\mu}$ not an observable?

Why within classical field-theory the electromagnetic four-potential (usually $A_{\mu}$) not an observable? In classical mechanics we don't have problems with energy measurements and in quantum ...
1
vote
7answers
351 views

Is direction of angular velocity just a definition or has a physical significance?

I am a high schooler so I don't know a lot of fancy maths but I do know some of calculus and multiplication of vectors as dot or cross product. I am learning about Angular velocity. And I am confused ...
1
vote
5answers
4k views

Why is charge of the electron negative?

How did scientists figure out that the charge of the electron was indeed negative? I know how the cathode ray tube experiment works, but how did Thompson know that the plate that the cathode ray beam ...
1
vote
1answer
1k views

Transposition of spinors

Suppose we have two 4-components Dirac spinors, that is two non commuting objects, $\psi$ and $\chi$. We know that: $ \bar{\psi} \chi= - \chi \bar{\psi} $ $ \bar{\psi} = \psi^{+} \gamma_0 $ $+=T*$ ...
1
vote
1answer
1k views

Charging a capacitor (terminals)

When a battery is connected to a conductor electrons are repelled from the end of the conductor connected to the negative terminal of the battery and flow to the end of the conductor connected to the ...
1
vote
1answer
1k views

Meaning of a battery being 2.5 V

So this is a question with which I have been stuck for a while and it all revolves around the concept of volt. I have posted a LOT of questions on physics forums while trying to understand the ...
1
vote
4answers
2k views

The sign of the current flowing in a circuit

I was doing the following problem: And I was asked to find Iy. I found Iy to be 2.64 using KCL. However, the right answer was negative 2.64. Is it negative only because there is a dependant ...
0
votes
3answers
1k views

Should these be called “weights” or “masses”? [closed]

Yeah, those circular metal disks. Weights or masses? I call them weights because when I attach them to a spring I'm interested in their weight, but it feels odd saying a "Pick up the 100g weight". "...
0
votes
2answers
370 views

Direction of angular velocity

please help me here! Im confused, is direction of angular velocity perpendicular to the plane of motion, or along the plane of motion?? From hyperphysics - http://hyperphysics.phy-astr.gsu.edu/...
0
votes
2answers
2k views

How to understand whether potential energy increases or decreases?

I am confused by how to deal with the negative sign in the equation $U=-GMm/r^2$ in the following problem: If the distance between two masses is tripled, then the magnitude of the gravitational ...
-2
votes
5answers
198 views

Can the initial time be non-zero?

I am just slightly confused about how initial time is chosen in mechanics. I keep thinking I have understood it but some doubts come up later. Basically when I derived the constant acceleration ...
3
votes
1answer
1k views

$i\epsilon$ in the expression of Feynman Propagator

In Peskin, the Feynman's propagator for a real scalar field is first presented in a form without $i\epsilon$ \begin{equation} D_F(x-y)=\int\frac{dp^3}{(2\pi)^3}\int\frac{dp^0}{2\pi}\frac{ie^{-i(x-y)}}...
18
votes
12answers
5k views

Pulley system. Why do I need to put a minus sign?

I am a mathematics undergraduate who's struggling to understand elementary physics. The following exercise looks particularly obscure to me. A $3.70\,\text{kg}$ mass (let's call it $m_1$) is ...
38
votes
4answers
16k views

Can units be plural? [duplicate]

I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole ...
8
votes
2answers
3k views

Can an angle be defined as a vector?

In Classical Mechanics angular velocity, angular acceleration, torque and angular momentum can be defined as vectors with clear advantages such as the possibility to use vector product to simplify ...
4
votes
2answers
1k views

What is the meaning of the negative sign in $\Delta s^2 = \Delta x^2 + \Delta y^2 + \Delta z^2 - (c\Delta t)^2$?

In the equation of the spacetime interval formula $\Delta s^2 = \Delta x^2 + \Delta y^2 + \Delta z^2 - (c\Delta t)^2$ is there meaning for the minus sign before the $(c\Delta t)^2$ or is it just a ...
7
votes
3answers
3k views

Sign of the totally anti-symmetric Levi-Civita tensor $\varepsilon^{\mu_1 \ldots}$ when raising indices

I am confused with the sign we get when we want to raise or lower all indices of the totally anti-symmetric tensor of any rank. Take the metric to be mostly plus ($-+\ldots+$). Then is it $$\...
4
votes
5answers
2k views

Isn't the inductor equation negative?

The inductor "resists" change in current. So say you measure the voltage across the inductor from point A to point B - the current is flowing in from A towards B. Now say the current is increasing. ...
3
votes
2answers
1k views

Sign of Feynman rules with derivative couplings

Feynman rules for derivative couplings always make me confused. For example, the derivative in $gV^\mu\phi^+\partial_\mu\phi^-$ will give you $\pm ip_{-\mu}$, where $\pm$ depends on whether the ...
3
votes
1answer
4k views

Levi Civita Symbol and contravariance vs covariance

I have a question regarding the Levi-Civita symbol and contravariance vs covariance. Some of this was asked in a previous post, but I think I need more clarification. Consider the magnetic field: \...
10
votes
2answers
1k views

Normalising Generators of a Lie Algebra

Ok, so I'm asking this in physics because I'm currently working through part of Srednicki's text on QFT, even though it's really a maths question. In Srednicki's chapter on non-Abelian gauge theory, ...
8
votes
3answers
8k views

What is the difference between UT0, UT1 and GMT time?

Every reference I find says that they are "essentially" the same, which we all know really means that they are not the same, but different only by a some small amount that someone else other than me ...
7
votes
5answers
4k views

How can length be a vector?

Length and current both are not vectors. Then how can we assign the vector $l$ to the length of a wire carrying current while calculating for a current carrying conductor in a magnetic field. Also why ...
6
votes
4answers
35k views

Direction of magnetic field in a solenoid

Using the right hand rule I struggle to visualise/work out how to tell which is the north and south pole. It's all so confusing that the right hand rule refers to conventional current not electron so ...
5
votes
1answer
1k views

Is it okay to Wick rotate to give the negative of the Euclidean metric? Also, could we make the space-like coordinates imaginary instead?

There are 2 parts to my question: 1) Say we choose the metric signature to be (-+++), as in the Wikipedia page. Then the invariant interval in Minkowski space is written: $ds^{2} = -(dt^{2}) + dx^{2}...
4
votes
1answer
644 views

Why isn't the quark charge taken as primitive?

Why are electrons taken implicitly to be the elementary charge? It would save a lot of fractions in particle physics problems.
4
votes
1answer
460 views

Notations for statistical / systematic / numeric errors?

I constantly see the notation $$ 5.143(13) $$ for specifying that a value was measures / calculated to be 5.143 with an estimated error of 0.013. I have come to wonder though, just how commonly ...
3
votes
3answers
908 views

Sign of induced EMF and other elements in AC circuit

I am having problems to determine the direction of the induced EMF in AC circuits. For example, we have an inductor of inductance $L$. The induced EMF is given by: $$ \epsilon = - L \frac{di}{dt} $$ ...
2
votes
3answers
34k views

Direction of current and direction of flow of electrons

My textbook says that the flow of current is from the positive to negative and my notebook say that the flow of electrons is from negative to positive. Why aren't they agreeing on one direction? Who ...
2
votes
1answer
109 views

Confusion even over such a simple application of the Right-hand rule to determine direction of magnetic field

After reading this question on this site I learned that the direction of the magnetic field is given by $\boldsymbol{B}=\frac{1}{\omega}\boldsymbol{k}\times \boldsymbol{E}$ The left diagram below is a ...
0
votes
1answer
2k views

How can energy be negative in a finite square well?

Say if the potential $V(x) < 0$ in the well but the sides or the scattered states its zero potential, anyways How is that the energy in the well is less than zero? Is it because the potential is ...
5
votes
3answers
2k views

Sign convention for the Minkowski metric $\eta_{\mu\nu}$ [duplicate]

In special relativity, the proper time is defined as $$d\tau^2 = c^2t^2-(x^2+y^2+z^2).$$ One usually introduces a matrix $\eta$ to represent it. I have seen two sign conventions. One has three ...
5
votes
1answer
666 views

What are phase conventions in angular momentum and rotation calculations?

I work with complicated angular momentum calculations related to atomic physics; nevertheless, I never need to use anything related to a phase convention (apparently because it's taken care of in a ...
5
votes
2answers
289 views

Does the quadratic mass term $\frac{1}{2}m^2\phi^2$ belong to the free Lagrangian or is it an interaction term?

$$L = -\frac{1}{2}\partial_\mu\phi\partial^\mu\phi - \frac{1}{2}m^2\phi^2.$$ Why is the $\phi^2$ term in the scalar Lagrangian not considered a self-interaction?