# 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.

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### Doubt in negative sign of relative velocities of two objects in same directions and opposite directions

Consider the following examples. case 1)Two objects traveling in same direction one A in the velocity say 50 mph and other B in the velocity 60 mph. If we calculate velocity of A with reference to B ...
1 vote
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### In Peskin & Schroeder's QFT book page 704, the definition of electric charge is opposite?

In Peskin's book Chapter 4, the QED Lagrangian in Eq.(4.3) which contains the interaction term $$\mathcal{L}_{\mathrm{int}}=-q\bar{\psi}\gamma^\mu\psi A_{\mu}\tag{4.3}.$$ From this Lagrangian we can ...
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### Confusion in definition of Potential Energy

Potential energy is defined like this. $ΔP.E=-W_{AB}$. This means that the potential energy at point A minus potential energy at point B should equal the negative of the work done by a conservative ...
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### Why is there no negative sign in the linear form calculation of voltage but there is a negative in the integral form? [closed]

The formula for voltage given $\vec{E}$ and $d$ is $$V = |\vec{E}| d.\tag{1}$$ However, the formula is also $$V = - \int \vec{E} \cdot dr. \tag{2}$$ Why does one of these formulas have a negative and ...
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1 vote
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### Voltage Difference Across Capacitor Plates

If a parallel plate capacitor is connected to a 9V battery, what is the electric potential difference between the two plates? Is it 18V since the positive plate will have an electric potential of 9V ...
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### Sign convention of work

I am studying about reversible steady-flow work, and the author of the book states that Taking the positive direction of work to be from the system (work output) the energy balance of a steady-flow ...
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### Teacher told us we're not allowed to write negative vectors, is this correct or not?

The question is a bit vague, so I apologize for that. To properly explain what I mean, I'll use an example. Let's say we've got two forces going left and right. $F_1$ is $20 \, \text{N}$, and will be ...
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### The equipartion theorem and degree of freedom in case of vibration

I have been taught in chemistry that, the energy of a vibrational freedom is $RT$ (ie, twice that of rotational/translational) The degree of freedom which I found in chemistry, for the vibrational ...
1 vote
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### Confusion related to creation and annihilation operators

I'm studying QFT from Peskin and from the book by Ashok Das, and there seems to be a disagreement between the creation and annihilation operators, for the scalar Klein Gordon field theory. In Das, we ...
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### Factor in Gaussian filter

Why do Gaussian filter kernels commonly used in turbulence studies often have a factor of 6? $$G_{\ell}(|x|) = \left( \frac{6}{\pi \ell^2} \right)^{n/2} \exp \left( -\frac{6}{\ell^2} |x|^2 \right)$$ ...
1 vote
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### A contradiction in the derivation of electrostatic potential energy? [closed]

I am a 12th grade high school student with no knowledge of vector calculus and more advanced physics.. so if possible, please try to refrain from using advanced concepts which I do not understand. ...
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### ${}$ Projectile motion

Basically we take the value of acceleration due to gravity as -ve ($-g$),when an object is thrown in the opposite direction of $g$. But in projectile motion the object first opposed the g and travels ...
1 vote
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### Use of sign convention two times in ray optics

In this particular derivation of refraction happening at a spherical surface in terms of its radius of curvature , image and object distance and refractive index is done by my book as shown When we ...
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### Generators in the fundamental representation of $SU(2)$ are $\sigma_i/2$. Why? [duplicate]
In physics, the generators of the fundamental representation of $SU(2)$ are taken to be $\sigma_i/2$ instead of $\sigma_i$ where $\sigma_i$ denotes the $i$th Pauli matrix. This is because, as I ...