So in an NPN transistor, when the base-emitter is forward biased, and the base-collector is reversed biased, electrons from the emitter get to the base. That part I understand, electrons are the main carriers, and the junction is forward biased, so they can travel from the emitter to the base. I also understand why there isn't recombination in the base. What I don't get is, when electrons cross from the base to the collector, how is that not the same thing as the reverse saturation current?

Hello reddit,

In a past life I was a researcher in analysis, and I taught many courses in various subject, not only analysis related. Right now I work in industry, but I kept teaching interested students on the side. After a while, a group of us decided to create a mathematics and physics self-study discord, and this is what I want to present to you today.

We are a small and tight community with as common goal to self study mathematics. We have a varied mix of high school students, undergrads in various fields, PhD students and interested amateurs who are currently working and study math or physics on the side. This is what is going on in our discord:

- Regular members get their own category which they can manage completely,

- Members work through a book in a subject they (and me) are interested in. They can ask any questions or clarifications.

- After members did a few problems, I check them and I give feedback on how to improve their proof.

- There is an optional leveling up system available where members get points per problem they solve correctly.

I know this server helped a lot of people in the past, but it is not for everybody, for example

- You must be able to work rather independently. We do not offer lectures or a guided reading.

- You must be willing to do mathematics properly. This means with proofs and from the ground up. The same with physics, it should be calculus based.

We offer the following combination of books. If any of your choosing is not listed, feel free to ask:

- Intro proofs: Bloch "Proofs and fundamentals"

- Calculus: Lang's "First course in calculus" and "Calculus of multiple variables"

- Single variable analysis: Bloch's "Real numbers and real analysis" combined with Spivak's "calculus"

- Analysis in R^n: Hubbard and Hubbard "Vector Calculus, Linear Algebra, and Differential Forms: A Unified Approach", together with Duistermaat and Kolk "Multidimensional real analysis" and Spivak "Calculus on manfolds".

- Abstract analysis: Carothers "Real analysis"

- Topology: Lee's "Introduction to topological manifolds"

- Abstract algebra: Anderson & Feil's "A First course in abstract algebra"

- Linear algebra: Friedberg's linear algebra

- Intro mechanics: Moore's "Six ideas", and Morin's "Problems and solutions in introductory mechanics"

- Classical mechanics: Taylor's "Classical mechanics" and Gregory's "Classical mechanics"

- Special Relativity: Morin's "Special relativity for enthousiastic beginners"

There's a lot other possibilities, but these are the subjects that are currently popular.

If you think you're a good match for our group, send me a private message telling me a bit about yourself, what subject and book you'd like to do, why you want to self-study and what your eventual goals are in math/physics. If we feel you're a good match for us, we'll message you the link!

Have a fabulous day everybody!

I've tried looking online, but surprisingly, nothing has given me a straightforward answer. In a sinusoidal sound graph, what variables go on the graph? I'm given the hertz, decibels, and wavelength. It is a constant noise

I have looked up the correct solution to the correct answer 2/5 MR^2. But I am just curious to see what mistake I made in my working the kept leading me to the wrong answer.

https://preview.redd.it/ykjwyy7balwc1.png?width=796&format=png&auto=webp&s=ea37460b7a3774e829ab14cff4e028a2a044cc8e

If I have any combination of convex/concave/plano- lenses, can I find the resultant power (the equivalent convergence, as I'd like to see it as) by summing-up their individual converging abilities?

In short, is there any scenario where we have a system of two thin lenses kept in contact with each other, but the powers don't necessarily sum up? Thank you. (This is not a HW question really, I just had to set some kind of a flair)

I do not particularly want to take gen chem because everyone I know that has taken it has spoken about it as if it was the worst class ever lmao (Gen chem at my school is coined by the uni as being the hardest intro class they offer) but if it would provide me with the skills I need I can thug it out.

To be honest, how important is GRE physics score fot sm having homors in STEM but not particularly physics and wants to apply mcs oversea in EU?

Since 10th grade in high school I've managed to scrape by by not focusing during class, doing the homework at the last minute, and forgetting all the things I "learned" in the process. This was fine until college, and year after year got more and more difficult, until this semester (finishing up my junior year) when everything crumbled and I'm now going to get a B and a C.

Grades aside, though my GPA is very good, it's state school syndrome, and I really don't know any physics. Sure, I solved a lot of (easy) problems for classes, and could learn the concepts again, but at the moment I have no permanent knowledge and it shows: my intuition sucks, I've become slow and stupid at basic tasks, etc. I've barely learned anything in my classes this semester, mostly through lack of effort. All of this particularly shows in how underprepared I am for the research I'm doing.

I'll try to come back, but I just wondered if anyone else had actually gone so far as I have in the wrong direction and managed to come back.

Given the opacity in power law form κ = κ_0 P^α-1 T^4-4β and hydrostatic equilibrium dP/dr = -ρg = -Gmρ/r^2 , I want to write pressure P in terms of temperature T in this form given here.

I thought of writing dP/dr = dP/dT dT/dr so that I can have the opacity κ in my equation (since dT/dr = 3κρL_r / 16 pi acr^2 T^3 ) but I don't think that will work here and I've ran out of ideas tbh.

been working on this for 4-5 hours, it essentially is a calculator for the period of a bifilar pendulum. It asks for the moment of inertia (it assumes you are using a simple rod). You can input data sets and at the end it will plot a graph. Right now it only does 1/r and T, but I will continue to work on it.

import math import numpy as np import matplotlib.pyplot as plt

Constants

g = 9.81 # Acceleration due to gravity in m/s^2

Function to calculate moment of inertia of a uniform rod

def moment_of_inertia_uniform_rod(mass, length): return (1/12) * mass * length**2

Initialize empty list to store data

data = []

Ask for the mass and half length of the rod to determine moment of inertia

mass_rod = float(input("Enter the mass of the rod (kg): ")) half_length_rod = float(input("Enter half the length of the rod (m): "))

Calculate moment of inertia for the rod

I_rod = moment_of_inertia_uniform_rod(mass_rod, 2 * half_length_rod)

Ask which variables are constant

print("Which variables are constant? Enter the numbers separated by spaces.") print("1. Radius of rotation") print("2. Filar length") print("3. Mass of the device") constant_vars = list(map(int, input().split()))

Get user inputs for constant variables

r, L, m = None, None, None for var in constant_vars: if var == 1: r = float(input("Enter the radius of rotation (m): ")) elif var == 2: L = float(input("Enter the filar length (m): ")) elif var == 3: m = float(input("Enter the mass of the device (kg): ")) else: print("Invalid input. Please enter numbers between 1 and 3.")

Get user inputs for non-constant variables

if 1 not in constant_vars: r = float(input("Enter the radius of rotation (m): "))

Calculate period

T = (2 * math.pi / r) * math.sqrt(I_rod * L / (m * g))

Store data

data.append({ 'radius': round(r, 10), 'filar_length': round(L, 10), 'mass': round(m, 10), 'moment_of_inertia': I_rod, 'period': round(T, 10) })

Output the result

print("Period of the bifilar pendulum: {:.10f} seconds".format(T))

Option to add more data

while True: add_more = input("Do you want to add more data? (yes/no): ").lower() if add_more == 'yes': # Get user inputs for non-constant variables if 1 not in constant_vars: r = float(input("Enter the radius of rotation (m): "))

    # Calculate period
    T = (2 * math.pi / r) * math.sqrt(I_rod * L / (m * g))

    # Store data
    data.append({
        'radius': round(r, 10),
        'filar_length': round(L, 10),
        'mass': round(m, 10),
        'moment_of_inertia': I_rod,
        'period': round(T, 10)
    })

    # Output the result
    print("Period of the bifilar pendulum: {:.10f} seconds".format(T))
else:
    break

Display compiled data

print("\nCompiled Data:") for idx, entry in enumerate(data, 1): print("Entry", idx) print("Radius of rotation:", entry['radius'], "m") print("Filar length:", entry['filar_length'], "m") print("Mass of the device:", entry['mass'], "kg") print("Moment of inertia:", "{:.10f}".format(entry['moment_of_inertia']), "kg*m^2") print("Period:", "{:.10f}".format(entry['period']), "seconds") print() # Add a blank line for separation

Extracting data for plotting

radii = [entry['radius'] for entry in data] periods = [entry['period'] for entry in data]

Plotting inverse of radii vs period to linearize the data

plt.figure(figsize=(8, 6)) inverse_radii = [1 / r for r in radii] plt.scatter(inverse_radii, periods, color='red', label='Data Points')

Fit a linear regression line

m, b = np.polyfit(inverse_radii, periods, 1) plt.plot(inverse_radii, m*np.array(inverse_radii) + b, color='blue', label='Line of Best Fit')

plt.xlabel('1 / Radius of Rotation (m^-1)') plt.ylabel('Period (s)') plt.title('Period vs Inverse Radius of Rotation with Line of Best Fit') plt.legend() plt.grid(True) plt.show()

Question about the measurement of the vacuum state

I study the measurement of the vacuum quadrature observables with balanced homodyne detection.

I would have thought that measuring successively the same observable should give me the same result, because of wave function collapse.

But every paper I read on the subject assumes that successive measurements are independent. How do you explain this?

Hello everyone,

I have a assignment question given by my teacher,

To calculate the temperature below which the element He helium cannot be treated classically

I want to know what approach should i use when solving this question.

Sitting at my desk, disappointment washed over me as I stared at the failing grade on my physics exam. Doubts flooded my mind – was I destined to be a physics failure? Fear gripped me, overshadowing my dreams of becoming a physicist.

But amidst the uncertainty, a glimmer of determination emerged. I refused to let one setback define me. With renewed resolve, I sought guidance, redoubled my efforts, and embraced each stumble as an opportunity for growth. Slowly, my confidence began to rebuild, and I realized that failure was simply a part of the journey to success.

Armed with resilience and a newfound sense of purpose, I pressed forward, knowing that my dreams were still within reach. Failure was not the end – it was just a stepping stone on the path to achieving my goals.

Hello! I have been reading on Fourier Optics and I have also done some simulations with Fresnell and Fraunhofer diffraction approximations. I have seen other people's code on PSF simulations where the input field is just planar wave, i.e. the intensity is 1 for the whole input field (all pixels) and no phase. I have understood that point spread function (PSF) is the image of a point, how then can the input field be a planar field and not a point that is propagated to the apperture/lens?

My exam period is about to start at university, but all of my exams are at 9am, and I am the worst morning person ever. I’m pretty sure my average grade for morning exams is a grade lower than for my afternoon exams :/

This didn’t use to be a problem, at school taking A Levels the adrenaline would always get me through it fine, but now I’ve sat so many exams that they don’t really stress me out in the same way.

Anyway, how do you guys make sure your brain is in ‘physics mode’ for doing an exam. I really feel like it can be the difference between an A and a B.

I'm looking for one or more fairly in-depth and clear/educational books on condensed matter physics that cover at least some of these topics: group theory and symmetries in condensed matter physics, introduction to topological matter, theory of phases and phase transitions, superfluidity and superconductivity. But with precedence to group theory and symmetries in condensed matter physics.

I'm trying to rewrite the Schwarzschild stability criterion for convection Γ>Γ1 into n + 1 < N(r) + 1, where γ ≡ (n + 1)/n is the adiabatic index and N(r) is the local polytropic index defined by N + 1 ≡ T dP /P dT, by using the ideal gas law. But somehow I've an extra 1/n in the mix that I don't know how to get rid of. Any ideas on how to fix this?

Yes I should’ve studied/been more on task with my homework but I was not and I just need to accept it.

I have so many resources from my prof it’s just I feel like an overload on my brain idk what to do.

I have access to past final exams and 70% of the questions will be similar/exact question.

Multiple choice will be worth 30 % of the exam I believe.

I guess my two questions are: is it worth it going through the past exam multiple choices? Might honestly only find one that is the same question.

Should I go through my assignments or rely on the past exams ? It will most likely be the basis of our exam but 1% of me isn’t sure idk any tips would help rn

The thing is, I don't know how to start the exercise.

If I only had 2 strings, I would take the angles and divide by the weight, but in this case how do I solve it?

*In the drawing I made the angles with colors

"When necessary, Peter Parker dons his superhero suit and, as Spider-Man, engages in the eternal struggle of good against evil. If the mass of this superhero is 75.5 kilograms and he hangs motionless supported by three strands of his own spider web, as shown in the drawing, and knowing that the strand on the right forms an angle of 30.0 degrees with the vertical wall, calculate:
a) The tension in the vertical strand. (1 point)
b) The tension in the horizontal strand. (1 point)
c) The tension in the strand attached to the wall. (1 point)"

Results:
a) 740 N
b) 427 N
c) 845 N

https://preview.redd.it/tvczphp6ebwc1.png?width=243&format=png&auto=webp&s=9ff9e118edbe63006aeac63dada7e047d45695ac

​

As the title clarifies, this is indeed not homework, and just something that sparked a bit of curiosity. Also, please do not flame me for this, I agree it is stupid and is just something I need to be able to know to finish some code I want to write.

Recently for a solo project I have been doing some research into a bifilar pendulum as a means to measure the moment of inertia of a body. I found that the period of a bifilar pendulum can be given by the following equation.

T=(2π/r)√IL/mg

However, with some mathematical manipulation (that I have attached to this post) I reduced the formula to:

T=(2π/)√L/12/g

The reason L is divided by 12 is because I used the moment of inertia of a simple rod which is given by (mr^2)/12. However, I have witnessed this relationship to be false, in testing, reducing the distance between the pivot point and the centre of mass of the rod increases the period, I have witnessed this in real life testing so I am certain the first relationship is true.

What the hell am I doing wrong? I think I lack some fundamental understanding of the moment of inertia, does it not change even though the distance between pivot point and centre of mass changes? Please let me know.

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