# Tag Info

8

Keeping units in ALL steps leading up to your answer is one of the best ways to avoid silly arithmetic mistakes. I teach high school physics, and when my students neglect to write units as they're working, its much easier to make a silly mistake. Using units allows you to double check that you are only adding or subtracting numbers with like units, similar ...

4

Another way to look at spin, complementary to the other ways, which I find helpful is look at an abstract generalisation of the concept of angular momentum and forget about things like classical tops. This generalisation begins in something called Noether's Theorem which you probably haven't met yet. You need some background but the idea is essentially ...

4

One. The inflationary period is thought to have lasted from around $t = 10^{-36}$ seconds to $t = 10^{-33}$ seconds after the Big Bang. So while you're technically correct to say it lasted less than a second that's a bit of an understatement. Two. See my answer to What was the density of the universe when it was only the size of our solar system? for the ...

4

Spin is not defined as the spin of electron around its own axis. Spin is the intrinsic angular momentum of the electron - intrinsic meaning it does not arise from the electron's motion, but is a property of electron itself. The electron in the atom "can" be described as a particle if you are using the Bohr model of the atom. Quantum mechanical picture of ...

4

We can't see in the heads of committee members, but to understand the tradition of "what for what" in Nobel prizes, I think it is instructive to see the chain of Nobel prizes awarded for developments connected to nuclear magnetic resonance (NMR). Here's a list. There were three prizes in Physics during 1943-1952, two prizes in Chemistry during 1991-2002 ...

3

Attempt to answer questions posted here on the Physics SE. Start by choosing a simple question that looks interesting, and if you can't answer it then start Googling for articles and books on the subject and try reading those. If you still can't answer it then wait for someone else to answer then attempt to understand that answer. If you can almost ...

3

Inspite of this, many physics teachers consult me to only includ the units at the end result. They're wrong. It's always correct to include the units at every step. However, if you're doing work only for yourself, or only for people who understand the role of units well enough not to need them explicitly written, you can get away without writing ...

2

Electrons never follow "particle principles", by which you seem to mean the physics of classical point particles. It's only in certain cases that a classical approximation is sufficient for human purposes, i.e. when we don't care about the uncertainty relations that govern quantum mechanical objects. In general it's better to think about elementary ...

1

Quantum Spin of a particle just represents another degree of freedom (e.g $+/- 1/2$ for electrons) and due to its representation as an "angular momentum operator", it is refered to as "spin". However it is not an analog (or actual revolution of a particle around its own axis). At least not in a classical sense. In summary it represents another degree of ...

1

I'd look at it as an energy storage vs loss situation. Take a patch of earth (square slab) and neglect rotation of the earth around its axis (days) so that the patch always faces the sun. At any time it's receiving an incident solar flux (assume constant) and emitting due to its own temperature. The slab also has some thermal mass (capturing the ground, ...

1

Often in physics, Objects are approximated as spherical. However do any perfectly spherical objects actually exist in nature? Yes, with a couple of qualifiers. For example, a perfectly isolated 4He atom in its ground state is a perfect sphere according to the standard model of particle physics. This follows because the nucleus is in a spin-zero state, ...

1

Meaning, why can't this exist: $dI=\frac{dq}{dt}$ Is this a calculus question? Or is this just by the definition of current? It can exist, and I would consider this a calculus question. When Newton and Leibniz originally invented calculus, they conceived of a derivative as a (usually finite) ratio of two infinitesimal numbers. Infinitesimal ...

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You might also try this site for a "physics problem of the week" kind of thing: www.fearofphysics.com, although the problems seem like they've been running for just a couple of months.

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As others pointed out, the statement probably means that if you want to have a nonzero contribution from creation and annihilation operators acting on vacuum, you need to apply the creation operator first, since $\hat{a}|0\rangle = 0$, in other words, you cannot annihilate if you have nothing. In the question you link to, the situation is different, as that ...

1

Griffiths has a quite good book, Introduction to Elementary Particles. The last chapter (I believe only in the revised edition) is all about gauge theories and culminates in the Higgs mechanism. This book can be read with just a bit of E&M, though a good deal of quantum mechanics will make the reading much quicker. Many of the specific examples can be ...

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