# Tag Info

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The neutron is in no way "composed" of a proton and an electron. It can decay to a proton, electron, and an antineutrino. But that doesn't mean that these three particles literally co-exist inside the neutron at the beginning. Instead, the decay involves some real transmutation of elementary particles. The only thing that one can say because of the decay is ...

6

An early bit of evidence for the neutron as an uncharged constituent of the nucleus, with approximately the mass of the proton, actually comes from the exclusion principle, and the low-temperature heat capacities and excitation spectra of atomic gases. The argument is a little bit subtle, so you'll have to bear with me. First, we have the exclusion ...

5

The two lenses in modern 3D glasses are designed to select the two circular polarizations. The left lens only transmits left-circularly polarized light and the right lens only transmits right-circularly polarized light (or vice versa). The problem is that there is no material which acts as a circular polarization filter on its own. The way in which they ...

3

There is a nice pedagogical review of the quantum Hall effect that can be found here. They explain (sketchily) how to derive an effective action describing the bulk of a quantum Hall fluid, which is a topological quantum field theory: Chern-Simons (CS) theory. The main physics which can be gleaned from this is that any defects in the bulk of the fluid could ...

3

Here is a nice late-undergrad or early-grad-school lab report on the determination of spin states in nuclear decays. The references to that paper (from 1940 and 1950) are relatively accessible, too. As your pullquote says, you get $J^{PC}$ from measuring angular correlations. If a particle at rest decays into two daughters, the angular correlation is ...

3

The basic electric motor you described (see here and here for more complete descriptions than that provided in the question) is interesting and quite subtle to treat quantitatively, and I have not been able to find a good explanation of it on the Internet. I will provide one below. Even if the OP has moved beyond this problem, I hope others might find my ...

3

How can I learn to become an experimental physicist? Bold mine. To start with, one has to become a physicist, and that is the goal of undergraduate physics majors, supply the basic understanding of physics up to the time of study, the experimental results and the theoretical models within which the results make sense. This is a serious job and it is ...

3

The lower mass limits of neutrinos is not 0eV. They have to have a mass, since we can observe neutrino oscillations. This is something the standard model did not get right. Looking at PDG (2014) the boundaries seem to be 0 < m < 2ev.

3

You talk about time, but usually when I think "physical constant" I think of something more like $c$ or $\hbar$. Since we're increasingly using those kinds of constants to construct our units, it's a bit difficult to say how well they're measured--$c$ isn't measured, it's defined, for instance. For that reason, I think we ought to pick a dimensionless ...

2

Gravitational force is really weak compared to the other fundamental forces, so it's very difficult to measure the gravitational constant. This is how Cavendish did it without knowing the Earth's mass: He put two lead balls on either end of a long bar. He hung the bar at its center from a long twisted wire with known torque. Then, he placed two really ...

1

The reason we see an interference pattern on a screen is because of diffuse reflection. This is because in diffuse reflection, the incident light can be considered to be absorbed and uniformly emitted out in all directions. This results in a brightness at a point proportional to the brightness of the incident light. A mirror, however, simply reflects ...

1

Topological Quantum Field Theory (TQFT) is the low energy effective theory for the topological ordered states in real world, such as FQH states. In fact, the name "topological order" was motived from the term "Topological Quantum Field Theory". [See Topological Orders in Rigid States, Xiao-Gang Wen, Int. J. Mod. Phys. B4, 239 (1990) ...

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You dont know the mass of earth, but you know the force earth apply to you. This is F=mg, where g is 9.8 m/seg^2 Buut this is equal to the gravitation force, F=GmM/r^2. G is the gravitation constant, M the mass of earth, m your mass and r the radio of earth. Cavendish was who messure the earth mass. In that time, Cavendish knew the radio of earth and G (he ...

1

From a trivial point of view and in line with what you say $x \propto y \iff x=k_{1}y$ for some constant $k_{1}$. On the other hand $x \propto \frac{1}{y} \iff x=\frac{k_{2}}{y}$ for some constant $k_{2}$. If none of these relationships holds, then there may well be an error in the assumptions in proportionality.

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then shouldn't the variable resistor be connected to the negative terminal of the battery? It makes no difference at all where the variable resistor is connected. The current will be given by Ohms law. viz i = V/R. Regardless of where the resistor is located in the circuit shown the electrons pass through it in the same direction, and the same voltage ...

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