In the sense I have a system e.g. in an electronically degenerate state. Can I measure some observable quantity (in general do some experiments) that lets me see that the stat is in deed degenerate?
@CaptainBohemian I suppose its compatible with any fields that has corresponding particle/antiparticle "carryers".
This is a bit out of my depth: why is dark matter the most accepted explanation for the galaxy rotation curve problem? Firstly, any speed curve could be explained by some distribution of dark matter, plus you need to theorize a new type of matter that can't be directly observed, plus you need to predict matter that isn't predicted by the standard model. It just feels like a lot of epicycles
user434058
1:02 PM
Why does the Legendre transform work? I mean, to me it seems that we are just applying an arbitrary transformation, just to change our dependent variables, changing the original function completely. How is this any different from just arbitrarily choosing a new function with the desired dependent variables?
Hi guys I'm doing a test. I have the situation like in the photo. I have a battery 9V and bus linked with an amperometer.
I'm following this steps: 1) I take the BUS and I link it with the negative pole. So now BUS is at 0V.
2)I take the same BUS and I link it with the positive pole (at 9V). Now I don't understand why my amperometer not visualize any current. Precision is setted at 60 µA (60 microAmpere).
In my opinion there must be for a few seconds a current that will become 0 fastly because the BUS get 9V.
@JohnRennie: Hello Sir, I request your attention when you come back.
The following are some questions.
1. In almost all the cases we discussed so far, it seems that inside an ideal wire the electric field eventually always becomes zero, even if the current is flowing through it.
Is there a fundamental reason behind it, or it is just an inductive/reccuring coincidence?
2. I read somewhere on this site that Ohm's Law is applicable only when the free charges in the circuit are moving with constant speed, otherwise not. Is it true?
@Sophie if the galaxy rotation curves were the only evidence for dark matter then you're right that it wouldn't be very good evidence. But the existance of dark matter is suggested by several other unrelated observations.
In fact it was first inferred from studying the dynamics of galaxy clusters not rotation curves. That was done by Fritz Zwicky in 1933, well before Rubin's measurements of the rotation curves.
@Sophie The other evidence is more indirect and comes from modelling of the universe. The observations of the cosmic microwave background made by the Planck experiment suggest the existence of dark matter, though admittedly this is model dependent.
@ABC hi :-)
@DevanshMittal hi, I'll have a look later but right now I'm about to have lunch.
@JohnRennie Hi Rennie, when you are free could you give me some advice about my experiment? I'm in no hurry, it's a pure curiosity. I thank you in advance!
1. A battery is not a producer of charge. It is only a transporter of charge from negative to the positive terminal.
2. Even if we consider your circuit, then even if the battery shifts the charge fro one side to other, the charge flow will stop after some time because in the wire which is connected to the 0V side will have lost so much of charge that it will be deficient of charge and it will resist the further flow of charge.
When my metal wire is at 0V it mean that less electron that the side at 9V. When I link this metal wire at the 9V side I have: 1) The side at 9V 2) MY metal wire at 0V
So I need to ave a current for a few sec, if not why?
I know that I no have a current during a long time... but in my opinion for a few sec I need to see electrons in movement
@ABC : You can consider an electrical circuit like a water-pipe-pump analogy. A pump is like a battery it can shift the water from one side to other, but it cannot produce its own water. If the pipelines are not completely connected in a circuit then there will be no flow of water, only a pressure difference.
Similarly, there will be only a potential difference across the wires, but no current flow.
Electrons inside the wires are also like incompressible fluids so a potential difference will be created without charge flow.
Well if the sphere has a radius R, then the potential at the surface of the sphere is the same as it would be for a point charge Q at a distance R, So it is V = Q/(4 π ε R).
@ABC so suppose you have two spheres, and you move a charge Q between them. Then one sphere has a charge -Q and the other has a charge +Q. Like this:
So there is now a potential difference between the spheres of Q/(2 π ε R)
So if we start with our two uncharged spheres and connect a battery with a voltage V between them then the battery will pump electrons from one sphere to the other, and it will only stop when the potential difference due to moving the charge becomes equal and opposite to the battery voltage.
i.e. the battery voltage and the total charge moved between the spheres would be related by:
V = Q/(2 π ε R)
So suppose you had an ammeter in the wire. I'll add this to the diagram:
In this situation the ammeter would read zero because the charge has moved between the spheres until it balanced out the battery voltage so the net potential difference between the spheres is zero and no current flows.
But suppose we suddenly swapped the battery round.
Then the charges on the spheres would reverse. The left sphere would change to +Q and the right to -Q so we'd get a transient current shown on the ammeter as this charge flowed.
And this is basically your experiment. We swap the battery round and we should see a transient current. Then we swap it back again and we should see a transient current in the other direction.
@JohnRennie: Hello Sir, I request your attention when you come back.
The following are some questions.
1. In almost all the cases we discussed so far, it seems that inside an ideal wire the electric field eventually always becomes zero, even if the current is flowing through it.
Is there a fundamental reason behind it, or it is just an inductive/reccuring coincidence?
2. I read somewhere on this site that Ohm's Law is applicable only when the free charges in the circuit are moving with constant speed, otherwise not. Is it true?
1. In almost all the cases we discussed so far, it seems that inside an ideal wire the electric field eventually always becomes zero, even if the current is flowing through it.
Is there a fundamental reason behind it, or it is just an inductive/reccuring coincidence?
2. I read somewhere on this site that Ohm's Law is applicable only when the free charges in the circuit are moving with constant speed, otherwise not. Is it true? If it is true then Ohm's Law should not be applicable in Alternating Current circuits.
Now, the mass of an electron is very small, and the drift velocities of electrons in wires is pretty slow, so in most cases we don't have to worry about the kinetic energy of the electrons. We just assume they accelerate infinitely fast so Ohm's law is always obeyed.
This is true for DC and AC circuits.
Even though in an AC circuit the electrons are accelerating, the deviation from Ohm's law that this causes is so small it cannot be measured.
1. In almost all the cases we discussed so far, it seems that inside an ideal wire the electric field eventually always becomes zero, even if the current is flowing through it.
Is there a fundamental reason behind it, or it is just an inductive/reccuring coincidence?
The charge density in an ideal conductor has to be the same everywhere because if it wasn't the charge would just flow from the high density regions to the low density regions until the density was the same everywhere.
And an ideal conductor offers no resistance to this flow, so it would happen instantly.
And if the charge density is the same everywhere the potential is the same everywhere i.e. the field inside the conductor is zero.
I know it was reported on meta already a few days ago so I'm not the only one, but the fact that nobody seems to really mind, the question itself there having only ~200 views in almost a week, is really weirding me out. Am I the only one that heavily relies on it?
@glS Often by memory or by searching for it explicitly. It's a bit inconvenient but mostly I think I used it to find more questions to read when procrastinating ;)
I learned today by searching Google that the mass of an electron is about 1/1836 of a proton. Also I learned that the electron revolves around the proton at a speed of 10^6 m/s. If I shoot a hydrogen atom towards a double slit, how much does the proton bounce around/oscillate?
I mean the speed of the electron orbiting the proton should compensate for the lower mass of the electron. Does this mean that the proton moves as along wave/sine curve path as it progresses towards the slit? The proton can't go in a straight line in my mind.
@ACuriousMind it's certainly facilitates procrastinating, that's a given. Although tbf, I feel like most of the time spent on stackexchange can be labelled as such lol
well if nothing else, I learnt that most people use stackexchange very differently than I do in this regard, which I never thought about and is kind of interesting
@MatsGranvik Both the electron and the proton are quantum objects and have no definite paths or speeds. The electron does not revolve around the proton in a meaningful way.
@JohnRennie I was thinking about what we were talking about today. All clear, but one thing doesn't come back to me. You actually spoke of two spheres from which the pump drew to transfer charges from one sphere to another.
In the case I described today, however, the situation is slightly different: In fact, if you think that the metal cable first touches the part of the battery with 0V and then the one with 9V, in fact we can think that the battery how much charge it absorbs, so much it releases.
@BioPhysicist, It is usual to talk of work done against friction. It is already clear that the work is done by gravity, and we are simply dividing it into parts, as described by Dale. The idea of work being done by friction, whether positive or negative, is bizarre, because friction does not cause movement (only opposes it), so friction cannot do work. — Charles Francis25 mins ago
