I have a question: in electrostatics, when we write the generalised coulombs law, or the generalised potential field causes due to a charge distribution $\rho$ , we are intrinsically speaking about macroscopic charge density right? These densities i suppose are averaged over a small area, and hence gives us the macroscopic density?
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That question above was closed as needing more focus after the question already had an answer. I had to edit the question so that it asks only one q...
Well every moving object has a rest frame. If we use that frame then an object not moving at constant speed would have to spontaneously start moving from rest.
Despite it's reputation for complexity GR provides a really good way to understand this. Newton's first law is equivalent to the statement that an isolated object has a proper acceleration of zero. And it would be strange indeed if an isolated object developed a non-zero proper acceleration.
Recently I've seem here one answer telling that Newton's first law really assures the existence of inertial reference frames. But how is that? I really can't see it. As I know, Newton's first law says:
Every body continues in its state of rest, or of uniform motion in a right line, unless it ...
Our modern idea that physical theories are given as pairs of a mathematical formalism and a dictionary that connects that formalism with the physical world is very much anachronistic here. It might be interesting in some sense to see if this can be applied to pre-modern theories of nature, but we should not claim that that's "what they meant" or that this then is a faithful representation of their beliefs
@RyderRude it's again about precision of language: You're not going "full Aristotle", you're doing something inspired by but wholly distinct from Aristotle's philosophy.
I think at those times it wasn't important that the laws have any significant predictive power. Other things like esthetics perhaps, were more important
@Amit that's not necessarily true: Archimedes came up with predictive theories of buoyancy and levers, for instance!
but there was not this modern, all-encompassing scientific worldview that everything follows such mathematical laws and can in principle be described and predicted in this way
the Greeks could build machines, they understood steam power, it's not as if they didn't know things about physics and used that knowledge in practice
Right, but am I correct that there was a kind of a divide between natural philosophers and mathematicians? The mathematicians like Archimedes, Thales, etc. were often very practical, but then when they came up with very practical results, they didn't afaik associate any model with it did they? Similar to astronomy as Slereah just mentioned: when they calculated things very accurately, they didn't find it necessary to have a model that goes along with it to explain why it is so accurate
it's more that this sort of science or mathematics wasn't seen as central to the task of philosophy
it's just knowing how stuff works: when I melt this rock, iron comes out, when I construct a lever, I can move greater weights, etc. In the pre-scientific world view, this doesn't really have anything to do with what stuff is
Like in that "unreasonable effectiveness of math" paper, Wigner is doubting reductionism
He says biology may be non reducible to physics
@Slereah Once we assume a metric, the metric-compatible connection is the Christoffel symbols instead of a 0-connection like Aristotle vector field. So the metric compatible connection can b a deep reason for Newton's first law, rather than Aristotle's law
Well reformulating Newton in light of Einstein is already not an easy task, but I don't think you can squeeze Aristotle into all this mess :) and if you manage to, it won't bear much resemblance to what he probably had in mind
@Slereah i'm interested in a modified version of this idea, which doesnt favor an origin. Like want a universe where all origins r equivalent. So i need translational invariance to make it interesting
I want this becuz usual physics treats all constant velocity frames as the same. So i want to treat all translated origins as the same or "equally good"
@Amit it is a vector field. It gives u a first order universe becuz u only need the initial position to determine the trajectory. The vector field gives the velocity to the particle
@RyderRude you're just describing the notion of an affine space and many formalizations of elementary mechanics do treat space as affine (off the top of my head for example Arnold)
I hav an idea : the current state is determined by initial positions of everything in the Aristotle universe. The law of physics maps this state to a 0-connection. Then the particles follow that flow. And then we rinse and repeat in small time-steps @Slereah @ACuriousMind
Maybe it's more interesting what other laws this kind of universe will mess up. Why for example QM wouldn't work any other way, and then no stars, no planets, no Physics.SE.... etc
Not necessarily meaningless, it's possible that some laws are explained by deeper laws. for the case of inverse square law for example I think it's quite accepted this is just a consequence of geometry isn't it?
The area decreases as the square of the distance...
Yes. U r right. These r not meaningless questions. It is possible that some answers may b found in future. But even if there r no answers, it is ok @Amit
@Amit yea. This reason was found for inverse squared law
What do you all like most about physics, the discipline?
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Is it any of philosophical answers, technological advances, math of physics, deeper laws etc, or something else @ACuriousMind @Amit @Slereah @user223626865 @naturallyInconsistent
I can answer this only in a roundabout way: as a kid when I realized you can write computer programs, I was most interested in learning how to program in assembly language because that's the closest one to the machine level as far as I understood, lol
Yes, I was also quite disappointed when I realized it ain't gonna be as easy as learning machine language... but the mystery is also part of the fun... as Einstein put more eloquently
I think you also know you need to separate the complication of the calculations from those of the concepts, it's more the former we have trouble with right?
Yeah, assembly / machine are corresponding. ASM is just giving nice names to the strings of bits that the CPU can perform
@RyderRude I don't think I "like" physics like one likes a hobby. I just cannot imagine a version of myself that would be disinterested in physics and still be me: the desire to comprehend what is going on in the world I find myself in is fundamental to my identity.
Thermodynamics / Stat mech is a bit different in that regard, it is a bit higher level than the fundamental laws (in relation to spontaneous processes in general @RyderRude)
Disinterest can have many causes, but if you know that in general you like a subject but find yourself disinterested, it may be due to no focused goals for example. I think we need some constant feedback that gives us a sense of achievement to stay interested. Like "I understood this chapter of that book" or "I solved this equation for the first time" or "Now I understand from the model why I can't see stars during the day time" (stole that last one from Feynman, lol)
Yes. But even there, I bet that if you see how beautifully stat mech can be used to predict some actual systems, especially if you do it yourself, you will feel a bit differently (or not, just guessing)
@RyderRude Sorry, but I don't think we are alike: You seem extremely focused on "fundamental laws" and naturalness and such things. I don't really care about any of these. I don't want science to explain "why" the world is the way it is, I want science to provide me a framework for making sense of the world around me. To not have to label the phenomena around me as "magic idk" but understand how they are related to each other, how technology works, how my body works.
lmao... weren't you fascinated by reflections as a kid? I remember if I happened to have something shiny and a source of light I could play with that for a long time. like the old fashioned way of playing with a laser pointer today
I have a similar constitution as ACuriousMind. I do not have to know everything, but I must know enough of the basic workings of the universe, at least enough to understand the stuff for modern life.
@RyderRude I don't think you quite understood me: I don't need to know how a modern semiconductor fab works, but it is important to me that I at least in principle understand what semiconduction is. I don't need to know the details of the statics that keep a high-rise from collapsing, but it is important to me that I have some idea of the mechanical principles at work and how someone would go about solving this problem.
I don't need to know every chemical reaction but it is important to me that I know the essential ones involved in keeping me alive.
Not all phenomena. Most phenomena, definitely, but not every minor detail. General principles to understand most things at once is more interesting to me than random trivia.
Maybe there is also something religious about the attraction to Physics. Religion I think in many cases was a quest to connect the personal with the universal. Physics is a way to create a connection to laws which we always try to extend as universally as possible
I mean "something religious" as a sort of a religious impulse which we may have inherited genetically, or are prone to naturally by the nature of human existence
Oh yeah, while I am very interested in GR, quantum gravity, origins of universe and so forth, and like the look of astronomical pictures, I am not interested in endless pretty pictures of galaxies. I want to know the evolution of stars and basic cosmology, but not the details of inflation.
@RyderRude it is abstract enough to be simpler (in the sense that at least in textbook physics things are consequences of very few principles with absolutely 0 counterexamples) than chemistry or biology, but it is concrete enough to be directly tied to reality (as opposed to math qua math)
this is the regime my brain can function in best i think heh
although an alternate explanation is that perhaps what i really enjoy is particular types of mathematics. and physics allows me to dabble and justify learning and using said mathematics :P
Hey everyone! I'm trying to understand why wires that are carrying current in the same direction attract each other.
I have one particular question. Suppose I have two wires that are far away from each other, and they're carrying a certain amount of current. The magnetic field surrounding those wires contains a certain amount of energy. If I bring those wires closer together, does that amount of energy increase or decrease?
@CassieSwett I think that the potential energy must be greater when they are far apart. Since as you bring them together, you can see that some energy is converted to motion and they either attract or repel each other (depending on the direction of the current)
But I'm trying to figure out how all this is compatible with the fact that if we have two parallel wires next to each other, it's "easy" to induce currents through them in opposite directions, and "hard" to induce currents through them in the same direction.
If my understanding were correct, we could make a perpetual motion machine out of this. But we can't make a perpetual motion machine out of this, so my understanding must be wrong.
Here's how the machine would work. Take two long parallel wires, separated by a great distance, and charge them up (so to speak) so that they're both carrying 1 A; this requires some amount of energy E. Now bring the wires close together; they attract each other, so you get some amount of energy from this action.
Now that the wires are together, they have more inductance than they did when they were apart, so when you discharge them back to 0 A, the amount of energy you get back is more than the initial charging energy E.
Or, I'm not sure about the "more inductance" thing, but now you effectively have one inductor carrying 2 A, so when you discharge it, you get 4 times as much energy as the amount you put into each of the two wires to begin with.
But obviously I'm wrong about something.
I suspect that where I'm going wrong is in assuming that it's possible to maintain that current of 1 A through each wire without supplying additional energy.
This is black magic to me I'm not an electronics person lol. But first, I think you need to realize that actually the fact that the wires moved means you already lost some energy to motion. And secondly, inductance only occurs when there is a change in the magnetic field
I'm not sure what you mean by "lost some energy to motion." If I have two objects that are attracting each other, and I allow them to move closer together, I've taken energy from the objects; I haven't given energy to them.
And yes, steps 1 and 3 of my "perpetual motion machine" involve changing the current, and thereby changing the magnetic field, and so inductance comes into play.
Now it's true that the magnetic field will change slightly when the wires move, as the field is dragged along with the wires, but I don't see how that helps you. In a stable current case, they will move some fixed distance apart (or together) and stay there
Yeah, my assumption is that I have some kind of handle or something that I can manipulate in order to move the wires closer or farther from each other.
Energy conservation for better or worse works... perpetual motion machines are actually quite terrifying. If someone built one and forgot to shut it down, it could create a black hole
Heck, let's assume that the handle is oriented vertically, so that when the wires attract each other, it lifts the handle. Obviously the handle now has more potential energy, since it's higher up.
Even the Beatles codified that in a song: "and in the end, the love you take is equal to the love you make" -- replace love with energy and it's the same principle
add to that, we always make some noise, heat, and unavoidably useless stuff additionally to what we really wanted to do. probably also a similar thing happens in love, lol
gee, I now realize that "Maxwell's silver hammer" is on the same album. Were the Beatles sending us secret messages about Electromagnetism?? lol
All right, I think I've figured it all out. I know exactly what the paradox is, as well as the resolution to the paradox.
Suppose I have two loops of wire, far away from each other, both carrying current in the same direction. Then I bring them together. Does the energy in the magnetic field decrease? I had two answers to this question which contradicted each other.
Answer 1: the loops attract each other, so the energy they carry must decrease as they are moved together. Answer 2: the total inductance of the loops increases as they are moved together, and E = 1/2 * L * I^2, and since L is increasing, the energy must increase.
It depends on what's held constant. If no voltage is applied to the wires, then answer 1 is correct; the formula E = 1/2 * L * I^2 holds true, but the current must decrease. If sufficient voltage is applied to the wires to maintain a constant current, then answer 2 is correct, and that voltage supplies the additional energy.
need a sanity check. the Wiki page for the electromagnetic tensor writes the Faraday 2-form as $$F=(E_{x}/c)\ dx\wedge dt+(E_{y}/c)\ dy\wedge dt+(E_{z}/c)\ dz\wedge dt+B_{x}\ dy\wedge dz+B_{y}\ dz\wedge dx+B_{z}\ dx\wedge dy$$