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Physics : Recent Questions and Answers (Page 568)

Find answers to your questions about Physics or help others by answering their Physics questions.

Defining the inverse of a tensor via the adjugate tensor

My professor definied the adjugate of a tensor $mathbf{t}in T^{1}_{1}(E)$ (E is just a vector space of dimension n) by defining its components as $adj(mathbf{t})^{a}_{b}=frac{1}{(n-1)!}varepsilon_{bi_{2}...i_{n}}t^{i_{2}}_{j_{2}}...t^{i_{n}}_{j_{n}}varepsilon^{aj_{2}...j_{n}}$Then he...

Asked on 02/13/2021

0 answer

In vector algebra, why does the cross product of unit vectors like ${bf i}times{bf j}={bf k}$ become negative when the order is reversed?

Why is it ${bf i} times {bf j} ={bf k}$, but if the order is interchanged it becomes ${bf j} times {bf i} =-{bf k}$? I did...

Asked on 02/13/2021 by KRATOS0990

3 answer

Scalar field energy density

Considering a classical scalar field theory, I can find the canonical energy momentum tensor and if I calculate the $00$ component I get: $$T^{00}= frac{1}{2} dot phi^2 +...

Asked on 02/13/2021

1 answer

What causes the bucket to launch in the air?

In this video potassium is dropped into the bucket and it reacts with the water present in the bucket causing an explosion within the bucket itself. I don't...

Asked on 02/13/2021 by Glowingbluejuicebox

6 answer

What does the definition of boiling actually mean?

Boiling point is defined by wikipedia as:The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding...

Asked on 02/13/2021

3 answer

Differences between "cold" and "collisionless" plasma

I'm just stepping for the first time into plasma physics and I'm studying on Chen's "Introduction to plasma physics" and on the notes of professor Richard Fitzpatrick. I just seem...

Asked on 02/13/2021 by Lyil

2 answer

Why electric potential definition demand the positive sign of test charge?

Why does electric potential definition need to distinct a positive test charge since the result will be the same even with a negative test charge? We could define electric potential...

Asked on 02/13/2021 by nonlinearly

1 answer

How do you relate the number densities of neutrinos and photons soon after the positron–electron annihilation?

I found this equation: $n_nu = frac{3}{4} frac{g_nu T_nu^3}{g_gamma T_gamma^3} n_gamma$ Where does it come from?...

Asked on 02/13/2021

0 answer

Why does jumping off a merry go round in the radial direction cause no change in rotation speed?

Suppose a man is standing on a merry-go-round which is rotating with some speed, if he jumps out in the radial direction, then the rotation speed of the merry-go-round is...

Asked on 02/13/2021

5 answer

$U(1)$ Local Gauge Invariance: What do $q$ and $alpha(x)$ mean?

When deriving the existence of the photon, we start with the free Lagrangian $mathcal L_{text{free}} = bar{psi}left( igamma^{mu}partial_{mu}-mright)psi$ and require $U(1)$ local gauge invariance: $psi(x)rightarrow psi^{'}(x) =e^{iqalphaleft(xright)}psi(x)$,...

Asked on 02/13/2021

1 answer

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