Physics Weekly Newsletter
Physics Weekly Newsletter

Top new questions this week:

How small does sand have to be to get wet?

I think of sand as a lot of very small rocks. Suppose I have a pile of rocks, each about 1cm in size, and the pile is a meter tall. If I pour a bucket of water on the rocks, most of the water will ...

scaling wetting  
asked by Mark Eichenlaub 44 votes
answered by Johannes 27 votes

If the earth is a good conductor of electricity, why don't people get electrocuted every time they touch the earth?

Since the earth is a good conductor of electricity, is it safe to assume that any charge that flows down to the earth must be redistributed into the earth in and along all directions? Does this also ...

current earth  
asked by Swami 25 votes
answered by zeldredge 38 votes

Could I break the sound barrier using circular motion? (And potentially create a sonic boom?)

Ok, Lets say I get out my household vaccum cleaner, the typical RPM for a dyson vaccum cleaner reachers 104K RPM, Or 1.733K RPS. In theory, this disc would be travelling with a time period of ...

newtonian-mechanics acoustics  
asked by FastestManAlive 24 votes
answered by tpg2114 32 votes

In an electron-positron annihilation, in what direction are the photons released?

I read that, in an electron-positron annihilation, at least 2 photons are produced, because of the law of conservation of momentum. my question is: in what direction are those photons released? and ...

kinematics photons electrons quantum-electrodynamics collision  
asked by Moozda 13 votes
answered by Ali Moh 19 votes

Why doesn't light affect a compass?

In our daily life a lot of photons of visible light, infrared and radio etc move around us. We know that light is an electromagnetic radiation. So why doesn't that electromagnetic radiation affect a ...

particle-physics visible-light electromagnetic-radiation photons magnetic-fields  
asked by Bhavesh 12 votes
answered by Floris 19 votes

Why do lines in atomic spectra have thickness? (Bohr's Model)

Consider the atomic spectrum (absorption) of hydrogen. The Bohr's model postulates that there are only certain fixed orbits allowed in the atom. An atom will only be excited to a higher orbit, if ...

quantum-mechanics atomic-physics spectroscopy doppler-effect absorption  
asked by Gerard 12 votes
answered by m0nhawk 6 votes

Why collide a moving particle with a particle at rest, rather than two moving particles?

I was just reading some lecture notes about relativistic and quantum mechanics, and in the later part of this page the author demonstrates that any relativistic particle collision in the "lab" ...

special-relativity kinematics reference-frames collision particle-accelerators  
asked by Ixrec 11 votes
answered by PhotonicBoom 13 votes

Greatest hits from previous weeks:

Can someone please explain magnetic vs electric fields?

I've looked through about 20 different explanations, from the most basic to the most complex, and yet I still dont understand this basic concept. Perhaps someone can help me. I dont understand the ...

electromagnetism  
asked by user1299028 12 votes
answered by anna v 11 votes

Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...

black-holes quantum-gravity hawking-radiation event-horizon  
asked by Devesh Saini 95 votes
answered by user29727 103 votes

Can you answer these?

In the Leidenfrost effect why dropplets of water can travel uphill?

The Leidenfrost effect is a phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid's boiling point, produces an insulating vapor layer keeping that liquid from ...

thermodynamics statistical-mechanics water material-science  
asked by Christos Antoniou 1 vote

Using the boltzmann equation to find neutrino decoupling temperature

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...

homework-and-exercises cosmology units neutrinos  
asked by Indiana 2 votes

What does it mean "Hawking radiation is in a pure state"?

I'm trying to understand black hole paradox but I'm not sure if I understand what does it mean "Hawking radiation is in a pure state". Does it mean if Hawking radiation is in a mixed state then ...

quantum-mechanics black-holes hilbert-space hawking-radiation unitarity  
asked by Robert Grezan 2 votes
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