r/PhysicsHelp

I've been doing a small experiment with eddy current braking and I'm confused about the theory.

The setup is pretty simple: an aluminum disk is spun using a rotary motion sensor, and I hold a stationary rectangular neodymium magnet above the disk. The only thing I change is the vertical distance between the bottom of the magnet and the top of the disk (2 cm to 4 cm). I then measure the angular deceleration over the same angular velocity interval each time (20–25 rad/s).

The average decelerations I got were roughly:

• 2.0 cm → 20.24 rad/s^(2)
• 2.5 cm → 12.63 rad/s^(2)
• 3.0 cm → 5.99 rad/s^(2)
• 3.5 cm → 3.09 rad/s^(2)
• 4.0 cm → 1.92 rad/s^(2)

The weird thing is that an exponential trendline fits almost perfectly (R^(2) ≈ 0.99).

I understand the basic mechanism:

• changing flux induces eddy currents,
• the eddy currents create a magnetic field opposing the change,
• the interaction with the permanent magnet creates the braking torque.

I've also seen derivations that the braking force is proportional to vB^(2), so the magnetic field is clearly the important quantity.

What I don't understand is where an exponential dependence on separation would come from physically.

Most explanations I find assume the magnet behaves like a dipole and use something like B**∝**1/r^(3), but my magnet is a rectangular neodymium magnet and the distances are only 2–4 cm from the disk, so I'm not sure that approximation is even valid.

Is there a better expression for the magnetic field of a permanent magnet in this regime? Or is it more likely that the exponential fit is just approximating the actual field over a small range of distances?

I'm mainly looking for the physics behind it rather than curve-fitting advice. If anyone knows a derivation or a good reference on eddy-current brakes with permanent magnets, I'd really appreciate it.

**CONTEXT (skip if you want lol):** Hey all! Been a lurker on here and other similar subs for a while. I’m in the process of writing a (very) hard sci fi story with the aim of having it be akin to the space epics of Dune, Star Wars and Mass Effect while still retaining a firm grounding in real-life chemistry and physics. I think the idea of a vast space epic whose events are governed and deeply impacted by the laws of real physics as we currently know them is an endlessly fascinating idea. What I mean by this is that many of the sci fi conventions we take for granted (like interstellar travel or harnessing the power of stars through Dyson-style arrays) and the engineering challenges that come with these elements is a core part of the story and the worldbuilding.

A big part of my story has to do with people transferring their consciousnesses from their brains into a physical medium like cortical stacks in Altered Carbon (except I don’t want it to be space magic like it is in that story). For my story, this idea shows itself as a semi circle “plate” that slots into a cradle between, and thus interacts with, the two hemispheres of the brain.

The solution I’ve come up with with what I currently know about physics as an amateur enthusiast and bonafide nerd, at its core, goes like this: Cool the brain down to an extremely cold temperature not too dissimilar to a Bose-Einstein condensate, and then while all the particles are moving as little as they can (as far as the Heisenberg Uncertainty Principle allows), you would then somehow command the subatomic particles (in this context: protons, neutrons and electrons) to move in wave form into the “plate” and then let them settle back into particle form there.

**So what I’m asking is what would, from a material and engineering perspective, be needed to execute this? (My story takes place 100 years or so in the future so I’m willing to take some light liberties with computational power and energy requirements naturally)**

Also if I’ve made some false assumptions in the very nature of my question, PLEASE CORRECT ME! I’ve been working on this project for about a year and I’m very passionate about it. I’m excited to discuss all these cool things with you all :)

Hi all,

It’s been a while since I took statics, and I’ll hoping to get some clarity on this question I’m exploring. Assume zero friction.

I’ve got a large planter that I’ve “secured” to the outside of my catio (enclosed patio for a cat) wall. Viewed from the side, this planter is square, and supported in only three areas:

1.) by the wall behind it (wall on the left)

2.) by a wire in tension at a 45 angle, running from the top right corner of the square to the wall above the planter

3.) by a peg underneath the bottom left corner of the square (immediately next to the wall).

Given gravity force mg downward at the square’s center, how can I characterize the distribution of pressure between the planter and the wall along the surface of contact?

I am an Electronics Enthusiast. I wanted to start electronics from the basics. Understand the concepts from the very base. Thus, I have started reading this book (The Art of Electronics) today itself, and while reading, I stumbled upon this very definition of voltage from the book. Which goes like:

>Officially, the voltage between two points is the cost in energy (work done) required to move a unit of positive charge from the more negative point (lower potential) to the more positive point (higher potential).

Which seems like a really good definition. But isn't voltage moving from a higher potential to a lower, and not the other way around? I mean, it's a very holy book for electronics enthusiasts, but then I really wanted to understand this part. If any smart brain could help me understand this. I would really be grateful. Perhaps you could help me build a very correct base for my journey in electronics.

Thanking you in Anticipation!

I'm feeling defeated by the following circuit where I'm asked to find the total resistance. I can't make any sense of the correct answer, fear that I'm misunderstanding the basics about which parts are in series/parallel - would someone please calculate this and briefly explain the steps involved to get there?

https://preview.redd.it/5mipzemcf8ah1.png?width=332&format=png&auto=webp&s=44f53592b1e99459ba0b07e0b2d4c6cdf61801fc

I'm trying to understand centrifugal pumps and I'm stuck on what seems like a contradiction.

In a centrifugal pump, the impeller accelerates the fluid — especially giving it a high circumferential velocity as it moves from the center toward the outer diameter. The fluid also gains kinetic energy.

At the same time, the law of conservation of mass (continuity) says that the mass flow rate must be constant: what enters the pump must exit (Qin = Qout for steady flow), as well as through the pump.

How is it possible that the fluid speeds up while the flow rate stays the same?

Hi everyone. I’m learning high school physics through khan academy and I’m confused about something. Let’s say you’re lifting a dumbbell and the dumbbell is moving at constant velocity right after you begin lifting the dumbbell. Khan Academy stated that the net force acting on the dumbbell is zero. How can this be when the dumbbell is being lifted upwards? If the dumbbell is still moving upwards wouldn’t there have to be a force lifting it upwards? I’m confused.

Hello Community, i am hobby mathematician and want Presentation my Atomformel or Elementarformula 😊 At first everything Not-Kommercial but we discuss about that definetly 🥳

Can you help me to discuss that and lead a good Conversation 👍

Atomformel(Elementary):

E = e^(e√e÷e) ≈ 5,200...

Atomkonstanten:

atom() = 0,7160305500615831612

elektron() = 0,3580152750307915806

proton() = 0,1790076375153957902

neutron() = 0,0895038187576978951

And this could be a Equation for Kernstapltung(in Progress)

Kernspaltung: 0π0 = 0km * π * 0km

Elektron: √(e^(e√e÷e) = 2,2804222777349727375

Proton:√√(e^(e√e÷e) =1,5101067107111910114

Neutron: √√√(e^(e√e÷e) =1,2288639919499598599

Please be friendly and not shy to Question me ;) Kommunikation ist everything :)

So lets start the Conversation

https://preview.redd.it/h8v4utzqpw9h1.png?width=554&format=png&auto=webp&s=ceb04b5ee12fc615c3e19641e6ee22ac13d8d43b

https://preview.redd.it/z18liiaupw9h1.png?width=580&format=png&auto=webp&s=32fb409d630e9b5528cee1512995a6871a3f9b15

IM NEWLY IN THIS PHYSICS STUFF, SELF STUDYING SO CAN ANYONE HELP ME WHAT DOES IT MEAN?

Greetings. I'm fourteen and I want to start learning physics like more? I've been interested in it since a longer while and I really really want to study it in the future, participate in competitions and overally ''know'' it. I am currently learning mechanics. But I feel like my self-studying isn't structured because I have absolutely no clue what to do sometimes. I'm just learning a topic from yt or some pdf I find and then doing practical questions so I'm a little lost, I don't have anyone to explain it to me or overally help with it but it isn't the thing.
If you were to learn physics from scratch what you would do? What resources would you use? And how much time spend on it daily? (I'm almost sure my 3 hours daily is a little too much ).
I'd really apperciate each response and forgive potential English mistakes, I'm not a native.