Variable Definition and 1000 Threads

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

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  1. C

    High voltage voltage controlled linear variable resistor

    I have some high voltage (300v+) analog circuits that I want to control digitally which requires the use of a voltage controlled linear resistor that can withstand high voltages. I don't expect the current levels to be that high. I originally settled on LDR optocouplers but it turns out they...
  2. BenR

    Field of variable charge distribution over all space

    Homework Statement A charge distribution has uniform density in the x-y directions and varies with z according to: ρ(z) = ρ0e−|z|/t where ρ0 and t are constants. (a) Find the potential V (z) and the electric field E(z) (b) Sketch them clearly showing their behaviors in the regions |z| ≪ t...
  3. Titan97

    Thermal expansion of a rod with variable alpha

    Homework Statement The coefficient of linear expansion of a rod of length 1 meter (at 300K) varies with temperature as ##\alpha=\frac{1}{T}##, where T is the temperature. Find the increment in length when the rod is heated from 300K to 600K Homework Equations $$\Delta L=L\alpha \Delta T$$ The...
  4. J

    Equations with dependent variable missing

    Information from the book: Use 1. v = y' 2. v' = y'' 3. dy/dt = v to solve the differential equation. Question: 2t2y'' + (y')3 = 2ty' I'm stuck at finding the integrating factor (which my book tells me is v-3 in the solutions.) Using the information above: 2t2v' + (v^3) - 2tv = 0 M(t,v) = v3...
  5. Z

    Separate Variable Homework: Solutions & Equations

    Homework Statement m1v1=m1v1'cosa+m1/2v2'cosB 0=m1v1'sina-(m1v2'sinB/2) m1v1^2=m1v1'^2+(m1v2'^2)/4 Homework Equations The solution in my book is v2'=2v1sqrt(3) The Attempt at a Solution I thought to separate v1' at the firts and put it at the second, but I don't know how to change sin and cos then.
  6. Konte

    Variable separation - Schrödinger equation

    Hello everybody, My question is about variable separation applied in the solution of general time-independent Schrodinger equation, expressed with spherical coordinates as: \hat{H} \psi (r,\theta,\phi) = E \psi (r,\theta,\phi) Is it always possible (theoretically) to seek a solution such as...
  7. T

    How long does it take for a rocket to lift off the ground?

    Homework Statement A rocket with an initial mass of 60,000kg ignites its engines and burns fuel at a rate of 300 kg/s with an exhaust velocity of 2220 m/s. How long after the engines start does the rocket lift off the ground? Homework Equations From Newton's second law F = Ma this equation can...
  8. dubya_80

    Variable speed control of multiple dc motors

    I'm building a hybrid car for my kids. I'm up to speed control and hitting a few hurdles. The set up is 4 x 12VDC motors, one to each wheel through a reduction gearbox. I've wired them to a single bus, and have used 4 standard relays to deliver forward/reverse. I have tried varying the speed...
  9. F

    Transforming Double Integral with Change of Variables

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  10. T

    Proving an equality using induction proof not working

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  11. REVIANNA

    Variable angular acceleration and velocity

    Homework Statement This is a general query not a specific problem. In several questions in rotational dynamics they ask the trend in angular acceleration and angular velocity.In one of them they released a rod from horizontal position and asked if the angular velocity and angular acceleration...
  12. I

    Can I undo a derivative to solve for a variable?

    If I have an equation where there is a derivative surrounding the variable, how do I undo the derivative and solve for the variable? Example would be- A= dx/dy when x=m*v*λ-2 and y=y Solving for v. I am a beginner so please explain thoroughly.
  13. Y

    MHB What is the correct probability for P(3<X<4|X>1)?

    Hello, I have this question, which I think I solve correctly, but I am getting the wrong answer. X represent the point that the computer chooses on a scale of 2 to 5 (continuous scale) in a non-uniform way using the density: f(x)=C*(1+x) what is the probability P(3<X<4|X>1) ? I solved the...
  14. Y

    MHB Joint distribution of a discrete random variable

    Hello all I have this question I am trying to solve. In an urn there are 6 balls, numbered: 1,2,3,4,5,6. We take 4 balls outs, without replacement. X - the minimal number we see Y - the maximal number we see I need to joint distribution. I understand that X is getting the values 1,2,3 while...
  15. M

    Serial controlled variable speed motor

    Hello all, I am trying to create a circuit that allows me to use serial data to control the speed of a 12v dc motor I removed from an electric drill. So far I have successfully made the circuit in diagram (a) using a 100k pot to alter the speed. So the next problem to solve is how I can instead...
  16. O

    Pressure calculation for compressible gas and variable 'g'

    I have what seems like a straight forward question, but am unable to find a formula. The question originates from my previous topic which was trolled: https://www.physicsforums.com/threads/blowing-up-a-balloon-using-centrifugal-force.848892/ Basically I have a 1.5m hollow length of 10mm tube...
  17. K

    Expected value of bernoulli random variable.

    "Let X be a Bernoulli random variable. That is, P(X = 1) = p and P(X = 0) = 1 − p. Then E(X) = 1 × p + 0 × (1 − p) = p. Why does this definition make sense? By the law of large numbers, in n independent Bernoulli trials where n is very large, the fraction of 1’s is very close to p, and the...
  18. G

    Finding Solutions to a PDE with Polar Change of Variable

    Homework Statement Find all ##{\cal C}^1(\mathbb{R}_+^\star \times \mathbb{R},\mathbb{R}) ## solutions to the pde ##x\frac{\partial f}{\partial y} - y \frac{\partial f}{\partial x} = cf##, where ##c## is a constant. Use a polar change of variable. Homework Equations Trying to bring the...
  19. W

    Find stationary points of a two variable function involving

    Homework Statement Find all stationary points of the function G(x, y) = (x^3)*e^(−x^2−y^2) Homework Equations fx=0 and fy=0 The Attempt at a Solution Gx = 3x^2*e^(-x^2-y^2) +x^3(-2x)e^(-x^2-y^2) = e^(-x^2-y^2)(3x^2-2x^4) Gx = 0 implies 3x^2-2x^4=0 x^2(3-2x^2)=0 hence x =0 ,+or-...
  20. P

    Power and Change of Kinetic Energy in Variable Mass System

    I am thinking about something that is getting me quite confused. To illustrate what is confusing me let's have a cart moving in the horizontal direction on a track without friction, and a motor which adjusts to keep the kart at a constant speed V in the positive direction. Now, it's snowing so...
  21. M

    The limit of random variable is not defined

    Let ##X_i## are i.i.d. and take -1 and +1 with probability 1/2 each. How to prove ##\lim_{n\rightarrow\infty}{\sum_{i=1}^{n}{X_i} }##does not exsits (even infinite limit) almost surely. My work: I use cauchy sequence to prove it does not converge to a real number. But I do not how to prove it...
  22. M

    Understanding sensitivity of a variable using derivatives

    Hi, Please see the attached file. Homework Statement I am trying to figure out K’s sensitivity to x and y. I wish to know which variable K is more sensitive to. Both x and y are positive decimals less than 1. I want to be able to say a 1% increase in y results in a p% change in K. As the...
  23. T

    Absolute extrema 2 variable function

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  24. B

    Algebra help isolating a variable

    This is such a basic question that I'm embarrassed to even ask it but I don't really know where else to turn. I was reading through a derivation and one of the steps goes from \frac{\sqrt{a}}{n} = \frac{\sqrt{A}}{n+m} to n = \frac{\sqrt{a}*m}{\sqrt{A}-\sqrt{a}} I'm getting really...
  25. Calpalned

    Functions of more than one variable nomenclature

    Homework Statement Homework Equations n/a The Attempt at a Solution ##y'=f(x.y)## is a function of two variables. ##y=y(x)## is a function of only one variable. How can they be related? Clearly ##y(x) = f(x) \neq f(x,y)## Thanks
  26. T

    Analyzing a Continuous Random Variable in a Coin-Operated Target Game

    Homework Statement Suppose the distance X between a point target and a shot aimed at the point in a coin-operated target game is a continuous random variable with pdf f(x) = { k(1−x^2), −1≤x≤1 0, otherwise. (a) Find the value of k. (b) Find the cdf of X. (c) Compute P (−.5...
  27. Vaibhav Sahu

    Snell's law for an interface with variable refractive index

    Consider an interface along x-axis which separates two media. The medium below y = 0 is air or vacuum and light is incident from this medium onto the surface. The refractive index of the medium above y = 0 varies with x as some function of x : μ = f(x). Does the Snell's law still hold good ?? If...
  28. J

    Concerning the Airfoil of Variable Pitch propellers

    Hello, I am not an expert in aerospace engineering, merely a hobbyist so bear with my ignorance. lately I have been thinking of creating my own propeller and variable pitch system using a CNC machine as a longer term goal. This desire arose due to the fact that there are no commercially...
  29. L

    Constant and variable specific heat assumptions

    1. Homework Statement A gas (treated as air) powered turbine provides power to a compressor which then sends the compressed air through an intercooler (heat exchanger). Turbine: Gas enters @ 0.03kg/s, 370 degrees Celsius Gas leaves @ 300 degrees Celsius and a "lower pressure" Compressor...
  30. Bassa

    Change of Variable With Legendre Equation

    Homework Statement [/B] Change the independent variable from x to θ by x=cosθ and show that the Legendre equation (1-x^2)(d^2y/dx^2)-2x(dy/dx)+2y=0 becomes (d^2/dθ^2)+cotθ(dy/dθ)+2y=0 2. Homework Equations The Attempt at a Solution [/B] I did get the exact form of what the equation...
  31. W

    Expectation of a function of a continuous random variable

    Homework Statement X ~ Uniform (0,1) Y = e-X Find FY (y) - or the CDF Find fY(y) - or the PDF Find E[Y] 2. Homework Equations E[Y] = E[e-X] = ∫0 , 1 e-xfx(x)dx FY(y) = P(Y < y) fY(y) = F'Y(y) The Attempt at a Solution FX(x) = { 0 for x<0 x for 0<x<1 1 for 1<x } fX(x) = { 1 for...
  32. C

    Why don't you put a variable before an else statment?

    I'm too lazy to see the big picture. Here is an example to show you what I'm talking about: x = str(input()) if x == "that's a nice meme sir": print("thanks friend") elif x == "crap meme stop posting anytime": print("oh") else: print("okay") Why don't you put the variable in...
  33. MichaelWReed

    Expandable Gear CVT: Michael Warren Reed's Project

    Hi there, I started working on an idea for an expandable gear CVT about 3 years ago, while out of work for a few months. I had some exciting progress on an idea that I think might be worth pursuing more, but I now have 2 kids, 2 jobs, and no time. I have brought up the project idea with a few...
  34. Alan I

    Work on a charge inside a Variable Electric Field

    Homework Statement A variable electric field permeates all space: E= (5.04x+ 84.2)x104i where x is in meters and E is in N/C How much work is done to move point charge q = 8.03 μC at constant velocity along the x-axis from point A at (3.63 m,0) to (7.67 m,0)? The sign will indicate who...
  35. T

    Can variable refrigerant flow outdoor units be combined

    Hey, I noticed that some manufacturers have provided data for pre-combined (2 or 3 modules) to achieve higher loads (limited to about 60 refrigeration tons).I was wondering if a designer could combine modules to achieve a greater even larger(different) loads or am i limited to the combinations...
  36. Jarvis323

    Change independent variable in magnetic field line equations

    How can I convert this system of equations : $$\frac{dz}{d\phi} = \frac{rB_z}{B_{\phi}}, \frac{dr}{d\phi} = \frac{rB_r}{B_{\phi}}, \frac{d\phi}{d\phi} = 1$$ to $$\frac{dr}{dz}, \frac{d\phi}{dz}, \frac{dz}{dz}$$
  37. L

    MHB Trying to reverse structure a calculation to arrive at original variable

    Trying to reverse-calculate an equation to achieve one of the original variables when not present. 1) You have a number... A (which is always a positive number) 2) A is multiplied by 1.75% 3) This result is then added to A, and the resulting sum being B, i.e. (A x 0.0175) + A = B In my...
  38. S

    Key difference between two real and single complex variable?

    Notion of differentiability (analyticity) for function of complex variable is normally introduced and illustrated by comparison with function of single real variable. It is stated that there are infinite number of ways to approach any given point of complex plane where function is defined, not...
  39. N

    Can Variable Speed Heat Pumps Provide Different Temperatures at Different Loads?

    Hi! The problem is in connection with variable speed heat pumps used for heating purposes, where the source/reservoir is ground/rock. The ground has temperatures around 5-7 celsius. Is it correct that these systems only can deliver one temperature at one load? E.g.: at 30 % of full power, it...
  40. D

    Feasibility and Advantages of Variable Wing Area Concept in UAV/RC Aircraft

    Hey guys!, this is my final year aeronautical engineering project that me and my group are going to work on! we have this concept of producing a roll in an uav/RC aircraft by reducing the area of the wing on one side while keeping the other side area constant or increasing it(gradually) and...
  41. R

    C/C++ C++ Variable Transfer Help - Ask for Advice

    I am just getting into C++ and I would like to ask you guys for some help regarding transferring variables form one function to another. Is there any way I can declare a variable such that it can be referenced and changed throughout all the programs functions? Thanks.
  42. morrobay

    Can Non Realism = Non Deterministic Hidden Variable Theory

    Given the locality assumption : p(ab|xy,λ) = p(a|x,λ) p(b|y,λ) with λ defining a single valued realism: a,b,a'b' each equal ± 1 the inequality S = (ab) + (ab') + (a'b) - (a'b') ≤ 2 is derived. Previously Bell pointed out that classical indeterminism wouldn't be enough for any hidden...
  43. P

    Variable Speed Of Light In General Relativity

    In his book (Relativity) published in 1920 Einstein said that in General Relativity the speed of light is affected by gravity and therefore isn't a constant: In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of...
  44. C

    Energy consumed by variable drag force when stopping?

    I was trying to roughly calculate the energy that could be recovered by regenerative breaking when bringing a vehicle to a stop, so I calculated to total kinetic energy and then tried to calculate the work done by air resistance that would take away from the energy that could be captured. So I...
  45. Remixex

    Simple question about Variaton of Parameters in 1 variable

    Given a generic second order ODE, normalized that equals to g(x) I saw in a video that, when writing the Wronskian, W1 and W2, for writing W1 you had to "delete" the first column if the Determinant and replace it with zeroes until you reach the end, where you replace it with g(x) Is this always...
  46. ChrisVer

    What does "momentum-weighted 'variable' " mean?

    It's pretty clear by the title... what does it mean to say something like (check http://arxiv.org/pdf/hep-ex/0606027v1.pdf pg5 ends of left column) "a momentum-weighted charge of the decay products of the second b..." ? I don't understand why someone would use momentum-weighted variables and...
  47. T

    Hydraulic Variable displacement piston Pump selection

    Hello everyone, I am designing one hydraulic system. In that system their is total 8 no of component function. When all the component work that time its take 150 Bar pressure and 140 lpm flow. but this all are all time not working together. they are hardly 4 to 5 time working together in day...
  48. matt_crouch

    Integral inconsistency with variable change

    so ## \int wdw = \frac{1}{2}w^{2} ## now if w=x+y, ## \int (x+y)(dx+dw)= \int xdx + \int ydx + \int xdy + \int ydy ## which can be evaluated and gives ##\int(x+y)(dx+dy)=\frac{1}{2}x^{2}+2xy+\frac{1}{2}y^{2}## but ##\frac{1}{2}x^{2}+2xy+\frac{1}{2}y^{2} \neq \frac{1}{2}w^{2}## can...
  49. J

    Commercial transformers for variable frequency & voltage

    I am looking to step up the voltage of a 3 phase PM generator which has variable AC output of 200 - 400 Vl-l, and 15 - 30Hz. I am looking for a step up ratio between 1:1.5 and 1:2, and need this transformer to be a commercial device. What i have been finding is that most commercial...
  50. Kior

    Would anyone explain the solution a little bit to me?

    I am preparing for my math probability class next semester. There is question: Calculate E(x3µσ) Would anyone explain the solution in the picture a little bit to me? 1.Why is the step hold? Is there a formula or something that i can calculate E[X-µ]n? 2.Why is the fourth step hold? Where is...
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