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1500 questions
51
votes
3 answers
Why doesn't this perpetual motion machine using the buoyant force work?
I realize this isn't possible, but I can't see why not, especially if you change the model a little bit so that the balls simply travel through a tube of water on the way up, rather than exactly this model.
Please be clear and detailed. I've heard…
Martin
- 521
51
votes
9 answers
Why doesn't the nuclear fusion in a star make it explode?
I have a rather naive question. In stars such as the Sun, what prevents the whole thing exploding at once? Why is the nuclear fusion happening slowly? I can only assume that something about the fusion is fighting the gravity and slowing the fusion…
sku
- 746
51
votes
6 answers
Isn't the detector always measuring, and thus always collapsing the state?
I have a radioactive particle in a box, prepared so as to initially be in a pure state
$\psi_0 =1\ \theta_U+ 0\ \theta_D$
(U is Undecayed, D is Decayed).
I put a Geiger counter in the box.
Over time (t), the theory says that the state should evolve…
Juan Perez
- 2,949
51
votes
14 answers
What *exactly* is electrical current, voltage, and resistance?
I am taking AP Physics right now (I'm a high school student) and we are learning about circuits, current, resistance, voltage, Ohm's Law, etc. I am looking for exact definitions of what current, voltage, and resistance are.
My teacher, as I'm sure…
Addison
- 625
51
votes
3 answers
Why don't electron-positron collisions release infinite energy?
Questions of the form:
An electron and a positron collide with E MeV of energy, what is the frequency of the photons released.
quite often come up in my A Level course (for often fairly arbitrary E). But this got me thinking. There is energy…
DoublyNegative
- 972
51
votes
3 answers
Why do we use Root Mean Square (RMS) values when talking about AC voltage
What makes it a good idea to use RMS rather than peak values of current and voltage when we talk about or compute with AC signals.
anilkumar
- 623
51
votes
3 answers
Why aren't 100% UV blocked sunglasses safe to view an eclipse with?
I am not planning on staring into the sun during an eclipse or any other time.
I have been reading about how no variety of regular sunglasses are safe enough to view the eclipse with. I'm not talking about being able to see things clearly, but just…
Jeff
- 483
51
votes
3 answers
What are quantum fields mathematically?
I'm confused as to how quantum fields are defined mathematically, and I've seen from questions on this site and Wikipedia articles that classical fields are just functions that output a field value for a given point in space input.
Is this the same…
Oliver Gregory
- 1,022
51
votes
4 answers
What's the rigorous definition of phase and phase transition?
I always feel unsure about the definitions of phase and phase transition. First, let's discuss in Laudau's paradigm. For example, some people say that phase is classified by symmetry. Some people say that phase is classified by order parameter and…
346699
- 5,941
51
votes
3 answers
What is a Lorentz boost and how to calculate it?
I know very little about special relativity. I never learnt it properly, but every time I read someone saying
If you boost in the $x$-direction, you get such and such
my mind goes blank! I tried understanding it but always get stuck with articles…
user09876
- 807
51
votes
5 answers
Derivation of Maxwell's equations from field tensor lagrangian
I've started reading Peskin and Schroeder on my own time, and I'm a bit confused about how to obtain Maxwell's equations from the (source-free) lagrangian density $L = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu}$ (where $F^{\mu\nu} = \partial^\mu A^\nu -…
amc
51
votes
3 answers
Why do excited states decay if they are eigenstates of Hamiltonian and should not change in time?
Quantum mechanics says that if a system is in an eigenstate of the Hamiltonian, then the state ket representing the system will not evolve with time. So if the electron is in, say, the first excited state then why does it change its state and relax…
Abhay Srivastav
- 663
51
votes
6 answers
Why do gases have weight?
I know that a gas is made of atoms or molecules moving freely in space.
When these particles hit the walls of where they're kept in they cause something called pressure.
But these particles never pile up on each other and push a surface down by…
Amir Moeini
- 647
51
votes
3 answers
Do pear-shaped nuclei really have anything to do with time travel?
Recently (in the last week or two), various articles about pear shaped nuclei have appeared, such as this one from Science Alert and this from the BBC
The Science Alert article includes the quote
We've found these nuclei literally point towards a…
Matt
- 1,758
51
votes
6 answers
How did physicists know that there are two kind of charges?
Problems
The question I am about to make is either too stupid or hasn't bothered anyone because its obvious because I can't really find the answer anywhere.
I am currently studying electricity and magnetism and my book starts by telling that matter…
George Smyridis
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