Lohrenswald
世界的 bottom ranked physicist
Do anyone have information on the whole gravitational waves news thing?
Science questions not worth a thread I: I'm a moron!
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Electricity puzzles me.
There's speak of negative and positive charges, but in reality there's a fixed array of nuclei with electrons able to move, and positive charges are just the lack of electrons, right? Then there's the principle that in conductor the excess charge goes to the surface. If it's positive that would mean that in reality the electrons would leave the surface. Why would they do so if they repel each others? Wouldn't it be more reasonable that the surplus negative charge goes to the surface, but if there's lack of electrons, they are equally distributed in the whole conductor?
This might be because it's electrostatics. So, the explanation would be that it's not a statical situation?
That explanation also bugs me, since I have no reason to think there are any electrostatic situations. Of course that is irrelevant to the things that assume it, but what if I deal with a real life problem. How do I know that the electrons aren't on the move?
Then also, another question: suppose you have a solid metal ball of radius 1 m that is cocentric with an insulator sphere of radius 1.0001 m. They both have the same charge, equally spread out in the insulator. Is the charge in the conductor on it's surface, or just a little shy of it since the repelling force of the charge in the insulator?
Okay, one more: Suppose you have a charged solid metal ball. The argument for the interior having 0 electric field is that otherwise there would be a force acting on the charges, and the situation wouldn't be static (at least that's what Young and Freedman say). Couldn't there be a force that's radially outward from the center of the ball? You could even imagine it being there because of one single electron. The same force that keeps the charge on the surface (and doesn't allow it to fly away) keeps them on the surface in this scenario.
These were probably a little stupid questions, but I feel like many people have similar difficulties in understanding and in making it explicit too.
There's speak of negative and positive charges, but in reality there's a fixed array of nuclei with electrons able to move, and positive charges are just the lack of electrons, right? Then there's the principle that in conductor the excess charge goes to the surface. If it's positive that would mean that in reality the electrons would leave the surface. Why would they do so if they repel each others? Wouldn't it be more reasonable that the surplus negative charge goes to the surface, but if there's lack of electrons, they are equally distributed in the whole conductor?
This might be because it's electrostatics. So, the explanation would be that it's not a statical situation?
That explanation also bugs me, since I have no reason to think there are any electrostatic situations. Of course that is irrelevant to the things that assume it, but what if I deal with a real life problem. How do I know that the electrons aren't on the move?
Then also, another question: suppose you have a solid metal ball of radius 1 m that is cocentric with an insulator sphere of radius 1.0001 m. They both have the same charge, equally spread out in the insulator. Is the charge in the conductor on it's surface, or just a little shy of it since the repelling force of the charge in the insulator?
Okay, one more: Suppose you have a charged solid metal ball. The argument for the interior having 0 electric field is that otherwise there would be a force acting on the charges, and the situation wouldn't be static (at least that's what Young and Freedman say). Couldn't there be a force that's radially outward from the center of the ball? You could even imagine it being there because of one single electron. The same force that keeps the charge on the surface (and doesn't allow it to fly away) keeps them on the surface in this scenario.
These were probably a little stupid questions, but I feel like many people have similar difficulties in understanding and in making it explicit too.
Meteor Man
Cylindropuntia imbricata
your questions are puzzling me a little
but that's okay. The hardest part of any endeavor is human communication.Yeah, basically.
Electrons do repel each other. Actually, they create an electric field that causes a force on other charged particles nearby. That's why they move towards the surface, because at the outer limit of their motion, the surface, the net force on each particle is zero, and they align equally spaced with electric fields pointing radially outward from the surface perpendicularly.
If you bring another conductor nearby that has a voltage between them, a potential difference, the electrons will jump to the region of lower potential. Why? Because there's a new force acting on them from the presence of the charged conductor (in this case, positively charged, with a lack of electrons). But they only leave the surface of the original conductor enough to equalize the potentials of the two conductors. Voltage1=Voltage2 now and there's no forces acting on the e-.
If there's only a few electrons on the conductor, they still align themselves equally spaced on the surface, because that's the lowest energy state for them. Zero net force. Same thing with positive charges, "holes," because where's there's holes, between them there's charge in the form of electrons. And electrons will repel to their lowest energy state. Thus the holes do, too.
Maybe I'm misunderstanding your question.
I'm not understanding you completely, but electrons are certainly always on the move. But they also move very quickly (relatively, on human time scales) and thus you can assume that any unchanging system will have unchanging electric fields, zero net force, and zero electron movement. However, the moment you come in contact with the system in any way that changes the exterior electric field, the charges will move.
This is an interesting situation, and I'm not entirely certain, but I suspect the charges would remain on the surface of the conductor, as I can't see them maintaining equipotential if not pressed up against the edge of the conductor.
Oh, I know Young and Freedman well. Not personally, lol. And remember that in a static situation, Gauss's law says that the electric flux out of a surface is equal to the charge enclosed (divided by the permittivity) and thus without charges enclosed, the electric field is zero.
I don't really understand what you're asking. No, there can't be a force because there isn't one. The force that keeps the electrons on the surface is, if you look individually at each electron, directed at them from every other electron on the surface of the sphere (because each e- has its own electric field). It's not a force that is radial from the center.
I hated taking electromagentism lmao But it's not as bad as thermodynamics. That I abhor.
Sorry if I didn't answer your questions, I only have some college physics under my belt.
It is more complicated than that, but for this discussion that model will suffice.
Electrons repel each other, but they are attracted by the positively charged nuclei. So if a metal ball is positively charged and lacks electrons, an electron on the surface will feel a force toward the center, because there are nuclei there. It will not feel an outward force, because there is nothing beyond the surface to attract it. That means it will move towards the center and there is a lack of electrons and thus a positive charge on the surface. The repelling force of the other electrons is more than compensated by the attractive force of the nuclei.
The electrons are always on the move, even at 0 K. But if there is nothing disturbing the system, it is going to be quasi-static. The electrons still move, but the average movement is zero, so the system appears to be static.
The charge will move a bit away from the surface, because it is repelled by the charge of the insulator.
That could happen, but it would be unstable. Once that one electron moves a bit away from the center, the electrons all start to move. And because they're all moving anyway, no unstable situation can survive for very long.
Also, keep in mind, that a charge is usually just a tiny fraction of all electrons and nuclei. So even a charged metallic conductor will have a lot of electrons at the center.
Thanks! Your replies were very helpful! 
Isn't this in general wrong: the flux can be zero even if the electric field is not. For example a cube orthogonally in a constant field, where the in-flux on one side is negated by the out-flux on the opposite side.
It is painful time to time, but in a good way.
Isn't this in general wrong: the flux can be zero even if the electric field is not. For example a cube orthogonally in a constant field, where the in-flux on one side is negated by the out-flux on the opposite side.
It is painful time to time, but in a good way.
If you drop a basketball, it bounces back up again, but not as high it was dropped from. The potential energy transforms into kinetic energy, which in turn transforms to heat of the floor, e.g. the kinetic energy of it's vibrating particles.
So, how does the wind chill work? I understand you can explain it by wind taking away heat from your body, but how does it work on the microscopic level? Shouldn't the air particles make atoms in your body vibrate? Okay, apparently when a molecule hits you, it begins to vibrate more, since it's looser than the molecules in your body, but why should that make the molecules in your body to vibrate less than before? Shouldn't it be at least random whether your body-molecules vibrate less or more after the collision?
Isn't it literally a matter of moving molecules of warm air away from your skin? Normally, the air slightly above your skin is warmed by your body heat, which causes it to have an insulating effect, as the heat difference between you and this air is slighter than between you and the air beyond it. Wind chill physically moves this insulating layer away from you, and is also effectively spraying a thin mist of water onto you, as it has a certain degree of humidity.
That would make sense. 
If you drop a basketball, it bounces back up again, but not as high it was dropped from. The potential energy transforms into kinetic energy, which in turn transforms to heat of the floor, e.g. the kinetic energy of it's vibrating particles.
Heat in the ball, part of the potential is used to stretch the ball on contact and isn't fully recoverable so you dont get the same height on the bounce. That part warms the ball (and the floor when the ball is in contact with it).
Heat transfer works both ways, but on a macroscopic scale it moves from the warm to the cold body. If the air around your body is warmer you'd get more warmth from it than the other way around for example.
Wind chill feels colder because it moves away the warm air around you, providing new cold molecules to suck heat from your skin.
With the ball, you also have a bit of energy going into sound and air resistance. You might also have a little friction on the floor (at least if you have spin on it, like when you bounce a basketball) and a small horizontal component to account for your not dropping it perfectly vertically from a perfectly still launch.
Think of your skin like a radiator. Because, you know, that is one of its functions. A radiator's efficiency is increased or decreased by a) temperature differential, b) density of the medium, c) conductivity of the medium, d) volume of flow past the medium. Cold air at high speeds (which is why wind chill is modified by expected average wind speeds) increases a and d. Possibly b as well, to a small extent, as cold air is denser than warm.
I do understand the idea that a cold medium takes away heat faster than warmer one, but had difficulties on the mircolevel: how can a molecule suck heat, shouldn't it work like a basketball that hits the floor and cause the molecules in my body to vibrate more?
Based on what's said this far, I'd suppose that the air may heat your body by hitting it, but the effect of it carrying the cold air away is bigger.
I am far from an expert, but the way I think about it is this: The molecules of your body are constantly colliding with molecules of the air. If the air molecules are moving slower (the air is colder) then with each collision energy will be transferred from your bodies molecule to the air. Over the 10 ^ 30 something collisions that are happening all the time this translates to you feeling cold.
If the air is still then the molecules next to the skin will have already collided with the molecules of your body, so will be moving quicker and not remove so much energy. If there is a wind then the molecules next to your body will be being replaced with ones moving slower, so more energy will be removed and you will feel colder than in still air of the same temperature.
Pretty much, yes. Both the molecule and your body have a temperature, when the molecule hits your body there is an exchange of energy but the net flow is to the warmer particle. Which in most cases is from your body to the air, put your hand into the stream from a hair drier and it wont be a cool feeling anymore.
At the microlevel (well, nanolevel), you cannot describe it as an air molecule hitting your body like a basketball hits the floor. Instead you need to think of it as an air molecule hitting a molecule of your body. And although your body is stationary, the molecules are not: They move, vibrate and rotate. They just do not get very far, because they immediately hit the other molecules. But for a very short time, they are very fast and also tend to be more massive than air molecules. So the better analogy would be a basketball that hits a moving bowling ball in mid-air, which would usually result in the basketball moving faster after the collision.
BenitoChavez
Elderly Tortoise
So I've been doing some googling about how celestial bodies maintain their magnetic field. Essentially its from an electrically conductive liquid, liquid iron in Earth's case, rotating inside the core due to thermal and density differences and also due to the Coriolis Effect. But how does a rotating conductive liquid generate a magnetic field?
I know that moving charged particles generate magnetic fields but a moving conductor isn't necessarily carrying a net amount of moving charge. For example, take a solid chunk of iron. There are charged particles, protons and electrons, within the iron atoms but there should be the same number of each particle rendering it as a whole electrically neutral. So you melt the iron and stir it around. The protons and electrons should now generate opposing magnetic fields effectively canceling each other out. But the scientists say this does generate a magnetic field. Why? What am I missing?
I know that moving charged particles generate magnetic fields but a moving conductor isn't necessarily carrying a net amount of moving charge. For example, take a solid chunk of iron. There are charged particles, protons and electrons, within the iron atoms but there should be the same number of each particle rendering it as a whole electrically neutral. So you melt the iron and stir it around. The protons and electrons should now generate opposing magnetic fields effectively canceling each other out. But the scientists say this does generate a magnetic field. Why? What am I missing?
I think the idea is that once the magnetic field is there, you get an induced current when the conducting material gets moved through it in the same way a current is induced in a wire that is moved through a magnetic field. This current then creates its own magnetic field, which then induced more current. So the field amplifies itself until a steady state is reached as long as the material is moving. At one point in time you need a seed field to get the process running, but then it sustains itself by inducing the current that generates it.
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