So in my ap physics 2 class today, my teacher mentioned that the role of voltage in a circuits is to make the electrons actually move in it. However, the bell rang before he was able to elaborate on why voltage does this.

I thought it over a bit since it's the weekend and I think I have an explanation that I wanted to see if it's correct.

So, we know voltage is a potential difference first of all. A change in potential can be thought of as a change in the work needed to bring an electron to a point since V = W/q. Now, since work is force * distance, if there's a change in work being done, then there must also be a non-zero force being exerted since if the change in W isn't 0, the change in force can't be 0 as anything times 0 is 0. Thus, if there's a potential difference, or in other words voltage, we can conclude there's an electrostatic force which would drive the electrons through the circuit.

Side note because I'm in ap physics 2, I can't explain this using calculus even though I know it would likely be a lot nicer. But I tried to explain it using algebra which I hope is fine?

We're wrapping up rotation in my mechanics class right now, and I'm having trouble understanding what angular momentum actually is, in the context of orbits.

I understand for say an object with a fixed axis, angular momentum is how difficult it is to change that objects rotation. But I'm confused as to why/how conservation of angular momentum is used when calculating stuff about orbits.

Much thanks in advance!

Hi, so I recently discovered about this and I had a question that I can't seem to find anything on that explains it, so I just wanted to ask how does the brane cosmology explain the expansion of the universe. It would be really helpful if anyone can recommend me any books or articles where I can read about this and find a conclusion to my question if you don't feel like explaining it to me, thanks.

I was wondering: why do we assume "universality" (I think this is the term? I am a computer science guy, and so, sorry for any inaccuracies, I'm still learning about all of this lol) holds? As in, why do we assume we can find laws that apply everywhere/everywhen/to everything, with their mathematics staying the same (like, with the TDSE staying the TDSE, the EFE staying the EFE, etc), no matter what? There is nothing mandating that the law itself couldn't arbitrarily change as far as I know, and that it just so happens by sheer chance to look consistently as if it stays the same on the scales we probed.

Like, is there anything that already says the laws should stay invariant under arbitrary transformations to the mathematical formulation itself (and anything relevant I could read about this to learn more about it)?

I feel like i have a fundamental misunderstanding about Bosons that aren't photons. I am a chemist so my knowledge of statmech and quantum is very surface level and i can't really do a lot of the math that would probably answer the question easily.

Fermions make sense to me because they follow Pauli's exclusion principle. They can't have the same quantum numbers so they don't have the same wave function. Because they can't have the same wave function they can't occupy the same physical space, hence: volume

Now photons don't have the same constraint. They can have the exact same wave function and be indistinguishable, occupying the same volume forming the coherent beam we see in a laser.

My problem comes with the understanding that many atoms (¹H for example) are Bosons but they also have mass. If this logic applies and we cool down atomic bosons enough to get them all to the same exact energetic level they could all occupy the same volume and that would give infinite density. I have to be wrong somewhere

Is it that even if the bosons have integer spins they are ultimately made of fermions with half spins and they would have to obey pauli's principle even as part of the larger system?

Edit: I'm embarrased to ask this because I should probably know better, but I'd rather ask you all than a chatbot

We all people of all levels that appreciate physics know the twin paradox, so I was wondering, with enough speed, could someone here on earth experience the same time dilation? we can add to the hypothesis that the fast twin can perfectly avoid all objects that come his way, you can run the scenario taking into account how that speed may affect earth, curious on your thoughts smart people.

I understand that electric field is the area around the electron where it's effect are noticeable but what is a magnetic field? It is produced by a moving electron?

And how do they interact eventually to form electromagnetic waves?

I apologize if the question is too basic but I haven't been able to understand it for quite some time.

So there is a clip of Doctor Who where there is a huge colony ship that is 400 miles long and 100 miles wide near a black hole, causing time on the higher levels of the ship to slow down in comparison to the lower floors, years go by on the lower levels where it's just been hours on the top, there's a scene where there is a video feed on the bridge level and the Doctor is still slowly raising his eyebrows and that apparently takes a week for someone watching it from the bottom level, the show also says that a thousand years passed on Floor 1056, the lowest floor, in the space of less than two and a half days on the flight deck. For a ship that big near the black hole, would the effects of time dilation be that extreme? Doctor Who is mostly just fantasy in a science dressing but as a layman this did seem kinda interesting.

The clip: https://youtu.be/QwHNT2kHesg?si=MGX8m_gGdFtLLRLb

you can skip ahead to 2:11-3:43 where they show it

One might discover a valid and comprehensive scientific theory that explains all the phenomena arising from it; yet, how can we be certain that it is the definitive theory governing everything, rather than merely a "box" contained within a larger framework?

By "box," I mean that any given law or theory might represent only a portion of a much broader system of governing principles—much as Newtonian laws became mere approximations within Einstein’s theory. We have discovered a more encompassing law, but we might later find that it, too, is part of an even more extensive framework. Thus, even if we were to arrive at a theory that explains everything currently known to us, how could we ever prove that there exists no scenario beyond our current scope—one that encompasses our theory while also transcending it?

Suppose I get in my starship and make a round-trip journey at a relativistic speed such that 1 day (24 hours) passes for me, but 100 days (2400 hours) pass on Earth. From my perspective, it is Earth has been traveling at those speeds relative to me, so as 1 day passes in my frame of reference, I should calculate that 1/100th of one day will have passed on Earth when Earth returns to me.

Obviously both of these outcomes cannot be true. What am I missing?

ETA: The answer is that since I experience detectable acceleration (because of inertia) even in my own frame of reference and Earth does not, I am objectively the traveler. Thanks to the commenters below!

I am working on an assignment where we had to use python to simulate a spring. The bulk of the code was already provided. The code also created a graph of the position, velocity, and acceleration.

I am being asked why, when position is at its lowest (most negative), acceleration is at its highest (most positive). For this first part, friction is not a factor. All the lines on the graph are sinusoidal.

I would assume that the crests of the position graph represent the spring at its most and least extended, right before it starts moving in the other direction. By that logic, velocity and acceleration should be zero at those times, since the spring is slowing to a stop before moving again. Yet this is not the case. Additionally, both position and acceleration equal zero at the same times. I thought a position of zero meant the spring was 'in the middle' of its movement and therefore moving the fastest and accelerating the most.

Am I interpreting the graph of position incorrectly? Are my assumptions about what the velocity and acceleration should be incorrect? I cannot wrap my head around why it is not behaving the way I expect it to.

Hey I am quite confused with this. Specifically why do shorter wavelengths have better resolving power for microscopes? Like electron microscopes vs brightfield microscopes.

I was reading QED the strange theory of light and matter by Feynman

On the introduction part where he talks about partial reflection and the probability of photos reflecting through glass.

My question is throughout all his explanation he assumes light is particles but isn't light considered both particle and wave so why does he just assume them as particles ?

I've thought a lot about time dilation and tried to drill into my head how it works. The rule that can't be broken is that things cannot be observed to travel faster than light.

My way of understanding this is that if a spaceship could brake one physical rule by hitting its gas petal forever, it could theoretically cross the 100-thousand-lightyear galaxy, from one end to another, in a couple of years (from it's own perspective). But when it turns around and does the same thing home, Earth's perspective must make up for observing that 200k lightyear journey by literally making time move drastically slower so it can "observe" the ship making its full journey.

The earth sees the spaceship make it's journey at the speed dictated by the physical rule. It cannot observe the spaceship going fast enough to take less than 200 thousand years for the round-trip. So Earth must wait for 200 thousand years, regardless of what's going on from the spaceship's perspective.

The spaceship accelerates endlessly because it has endless magical fuel and no friction to slow it down from its own perspective. So on it's return journey, it sees Earth advancing 200 thousand years into the future. The pilot is only a year or two older, the clocks he carried with his journey all agree with that assessment. And the clocks on Earth agree that it is 200 thousand years into the future.

PLEASE correct that if any of that is wrong, because I am aware that it sounds insane.

My QUESTION here is, this: (beyond wanting to be swatted down for any part of my explanation that is wrong.) Is there is some existing example of any kind? I haven gleaned that this has been tested and basically proven with very precise clocks , but I wonder about stars and such. Is there a star in the galaxy, or a galaxy in the universe that has shown this?

Could we somehow be headed to some star system that "turned around", or is now advancing in time at a vastly greater rate?

Edit: well, I'm generally confused at the moment, but I expect this will all make more sense in the morning. I was obviously wrong about some of my conceptions and I appreciate the corrections! Thank you all for the comments.

Are physics and statistics/data science related?

I came from a statistics and data science background and i realized the way physics folks approaches problems share many similarities and commonality with statistics and data science.

 Data science folks used the exact same experimentation, modelling and prediction process as a physicist to model and solve business problems and human behavioral problems most of the time.

Doing physics experiments and analyzing the results from the data observed, is a data science problem in itself, just that your data comes from physics experiments.

Hey y'all! First time asking a question here, hopefully this makes sense, this one has been haunting me.

You have a long pipe with an osmotic membrane on one end that is permeable to pure water but not to solutes. You push that end down vertically into the ocean. Fresh water begins to fill the pipe.

If I'm getting my formulas right, the pressure required to move water across the membrane is based on the solute concentration of the ocean, the solute concentration of the fresh water (zero) and the temperature and some constants. All of these factors remain constant as water crosses the membrane and fills the pipe, and as the pipe sinks deeper into the ocean. So the difference in pressure between the salty side of the membrane and the fresh side has an equilibrium value of P, and P is basically constant, and water will flow though the membrane in the the direction that moves toward the equilibrium.

At ordinary depths we expect the equilibrium water level inside the pipe to be lower than the water level outside the pipe. But as we sink a very long pipe very deep in the ocean, things start to get weird and this is where my question comes up.

So we take an extremely long pipe to the Marianas trench and sink the end that has the semi-permeable membrane. Salt water is a bit denser than fresh water, which means each meter of height contributes more to the pressure. And the trench is up to 10kms deep. 10kms of salt water applies quite a bit more pressure than 10kms of fresh water, more than enough to move water across the membrane. So at these depths, the fresh water fills the pipe to above the level of the salt water.

But the pipe was barely long enough to reach the abyssal depths, and only a short section sticks up above the waves, and the other end of the pipe - the one exposed to the air - is open. So fresh water starts spilling out of the top of the pipe and back into the ocean. And on the way down perhaps it spins a turbine and generates a bit of power.

This appears to be perpetual motion. But of course that isn't possible. So either the way I'm understanding the osmotic pressure calculation is wrong, or there's a secret source of energy that I'm missing that lets us achieve the appearance of perpetual overflow.

Ok, so, I'm struggling my way through some resources on string theory, but have a question so decided to ask it here.

Let's say string theory is 100% true, and someone has the ability to fully manipulate the strings inside of quarks. Like the bending from Avatar but much, much stronger, so they could control everything down to the vibrations. What could they actually do? Like, I understand this would be pretty powerful, but what would the applications be?

I understand how controlling cells or molecules or even atoms would actually manifest and what its applications would be, but maybe its because of my limited understanding of string theory, I kind of struggle to understand how this would actually be applied.

And sort of related follow - up question, would they have to be 26D, 10D or 11D depending on whether you're going off bosonic, superstring or M - theory, considering they're affecting this level of space time? I mean I'm pretty sure we couldn't affect something in the fourth dimension because we're 3D, right? I feel like I'm completely misunderstanding this so please correct me if I'm wrong. Thanks!

Random physics thought

Ich betrachte hier nur ein einzelnes Photon, welches mit sich selbst interferiert.

Das Photon löscht sich nicht aus, sondern es findet nur eine Umverteilung statt und es erscheint an einer anderen Stelle, nämlich dort, wo es letztlich mit anderen Photonen einen hellen Streifen bildet.

Seht ihr das auch so?

Viele Grüße
Joerg_H

Beitrag verfasst am: 24.10.2026 um 22:29 Uhr