What is Variable: Definition and 1000 Discussions

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.

View More On Wikipedia.org
Recent contents Top users
  1. diracdelta

    Random variable distribution question

    Homework Statement Random variable x is defined on interval (1,3) and it has probability mass function f(x) =A(x2 +1= a) Find PMF, g(y) for y=x2 b)Expectation of y c)Variance of y d)Distribution function of y. e)most probable value of y The Attempt at a Solution As far as a), i integrated from...
  2. thankz

    Example of a variable capacitor input op-amp to voltage....

    example of a varible capacitor input op-amp to voltage level circuit. I'm not sure how to turn reactance to a voltage level using an op-amp? :wink:
  3. A

    Performing integration over field variable

    I have this exercise, where I am to integrate out the fluctuations in the order parameter δρ, but I don't really know how to do this. The integral is over r and imaginary time. Should I try to complete the square, so the correlation term between δρ and Θ drops out? In that case which shift of...
  4. B

    Calculating Gas Pressure: Solving for Pressure in a Variable Volume

    Homework Statement Gas occupies 2m^3 at a pressure of 100kPa. What is the pressure when the volume is 1.5m^3 and the temprature is unchanged? Homework Equations p=nRT/V The Attempt at a Solution As a logic conclusion i took 1.5/2x100 and got 75 but 175 is not the answer.. The answer is...
  5. MexChemE

    Transient state energy balance -- Variable volume system

    Hello PF! I have some questions regarding the accumulation term for the energy balance on a variable volume system. Suppose we have a tank storing a liquid substance. The tank has a moving boundary at the top, which can expand unlimitedly. The system has a mass input \dot{m}_1 and mass output...
  6. C

    Valve lift & variable air flowrate in compressed air

    How we can incorporate valve lift and variable flow rate in compressed air engine Mechanical power = air power torque * angular velocity = pressure * flow rate
  7. B

    Design a coil with a variable pitch but a constant length?

    I believe what I am attempting to do is not possible but here goes. I want to design a fixed overall length coil which I can manually adjust the pitch on. Optimally one would be able to grab the coil and by twisting it in place would be able to change it's pitch by changing its diameter but not...
  8. CAH

    Power dissipated in variable resistor?

    See photo! Why does the power dissipated increase and then decrease (see graph)? Is it something to do with emf? Also can internal resistance change as the variable resistor increases/decreases or is it always constant? - this isn't one of the questions they ask I'm just wondering
  9. CAH

    Variable resistor and thermistors?

    If your doing an experiment to test how the temperature affects a thermistor (increase temp measure v and I...) why do you need a variable resistor in your circuit? Or if you don't NEED one what beneficial use may it pose anyway? Is it something to do with the current? Thanks
  10. shanepitts

    Extracting a constant variable from integral

    Sorry for the simplicity of the question but I encountered a simple math problem whilst autodidacting myself on classical physics. I seen that a surplus of a specific constant variable "b" was extracted from an integral after a bit of manipulation. ∫xdx was turned into b2∫(x/b)d(x/b) Does the...
  11. J

    Which is more optimal for changing a variable from 1 to 0?

    I'm micro-optimizing my program written in classic C. When I use and unsigned short int for a boolean value, I need to switch between values of 0 and 1, so what I need to know is whether it's more efficient to use bitshift operators or reassignment. In other words, is unsigned short int i = 1...
  12. L

    C/C++ C++ Boolean variable - clarity sought

    This is an example from a study guide I'm using at the moment in learning C++. //Test whether a number is prime #include <iostream> using namespace std; int main () { int x, y; bool factorFound = false; cout << "Enter a positive integer: "; cin >> y; x = 2; while (x != y &&...
  13. S

    MHB Function of Sawtooth Wave with Variable Peaks

    Hi everybody I want to know what's the function of sinusoidal curve but with sharp peaks both positive and negative? In the other word, peaks (positive and negative) be as follows: /\/\/\/\/\/\/\ I mean sawtooth wave, please give an answer that what's the function of the sawtooth wave as...
  14. J

    MHB Free variables and binding occurrence of each variable.

    Hi, I'm trying to identify the free variable and binding occurrence but I'm not sure if they're right in lambda calculus. $x$ $\lambda x.x$ $(\lambda a.z)a$ $\lambda a.za$ $(\lambda n.n)z$ $\lambda z.(\lambda y.(\lambda x.x)y)z$ $(\lambda t.((\lambda t.(\lambda t.t)t)t))t$ Then for free...
  15. jdawg

    How to Calculate Fixed and Variable Cost for Shampoo and Conditioner Bottles?

    Hi! I'm having some trouble with this problem! A small convenience store pays $100 for a constant supply of shampoo bottles. 20 bottles are shipped to the small business. The convenience store then purchases 30 bottles of conditioner for $6 per bottle. What is the fixed and variable cost for...
  16. T

    Entropy State Variable: Thermal Equilibrium Questions

    I have a few questions about entropy and path Scenario I have a closed system of Ideal Gas- Volume is Fixed , Mol Vol is fixed. At room temperature the system is in thermal equilibrium with its surroundings I then cool this system removing Q . To this system I then re-add Q until it is again in...
  17. W

    Independence from a variable - Graphical confusion

    http://postimg.org/image/pb0eu3ap3/ So I've got this graph, and magnitude, M, depends on the number of points, N,... but it shouldn't. http://postimg.org/image/pb0eu3ap3/ The line of best fit is... M = a*N ^b It's been a while since I studied, but knowing their relationship would it be...
  18. D

    Minimum of two random variable

    Hello, want to know if it's correct 1. Homework Statement X and Y two random variables iid of common density f and f(x)=x*exp(-x²/2) if x≥0 and f(x)=0 if x≤0 and Z=min(X,Y) Find -The density of Z -The density of Z² - E[Z²] Homework EquationsThe Attempt at a Solution 1.[/B] FZ(u) = P(min(X,Y...
  19. A

    Age of radioactive sample, single variable equation.

    I'm taking an introduction course for simple physics. All problems are solved with simple single variable analysis. My problem is that this is an introduction where you don't need any advanced math to solve the problems, but I can't remember how to solve the last equation. Would you mind helping...
  20. C

    Variable resistor & inductor connected to alternator

    A simple 2 pole alternator (a synchronous generator), is powered by a steam turbine. The amount of steam to the turbine is kept constant with a constant pressure ##p##, flow rate ##\dot{q}##, and power ##P##. If a variable resistor, inductor, and capacitor are connected in series to the...
  21. T

    Angular velocity by time variable force

    Homework Statement Force represented by F(t)=20+t+5t^2 [N] acts upon the rim of a disk. How much time has to pass before the disk has angular velocity of 200 revs\sec? R and I are known. Homework Equations \begin{equation*} \tau=I\alpha \end{equation*} \begin{equation*} \tau=R \times F...
  22. E

    How to Solve Exponents that also has a variable with it

    When one was reading some review books for engineering mathematics one has come across that kind of notation, below, and where to find books or materials that deals with these kinds of exponents, they look like this>> x^3x+2.
  23. END

    Reducing the function of a function to the ind. variable

    Hey, PF: If I have a function ##f(x)## where ##x## is itself a function of another variable (say time), is the following then true? $$f(x)=f(x(t))=f(t)$$ I ask this because if I have the following system of differential equations $$\frac{dA}{dt}=-Bb$$ $$\frac{dB}{dt}=-Aa$$ where litte ##a##...
  24. perplexabot

    Double sum of same variable simplification help

    Hello all, I have been meditating on this for a while, but can't seem to understand how this simplification came to be. Any help will be greatly appreciated. So, here is what we start with: ##\mathop{\sum_{k=0}^m\sum_{l=0}^n}_{m{\geq}n} x(k,l)## We also know that: l (lower case L) = n-m+k and...
  25. rayne1

    MHB Expected value of a continuous random variable

    Given the PDF: f(x) = 1/12 , 0 < x <= 3 x/18, 3 < x <= 6 0, otherwise find the expected value, E(x). I know how to find the expected value if there was only one interval, but don't how to do it for two.
  26. J

    Stuck getting derivative when can't isolate my variable

    Hi. This is not a homework assignment. I am working to get an extrema on a graph that involves a bunch of functions and got stuck on one step: How to get the derivative of: \frac{dy}{dn} = \frac{nc(a+b)}{nc+a} I can't get "n" in a place where I recognize how to get the derivative of it. I...
  27. A

    MHB Variable Pi Formula: Diameter (mm) Calculation

    variable pi formula Diameter (mm) = 0.0000003 : (pi of D - 3.1416 )^2 Pi of D is variable from 3.1416 to 3.1589...
  28. R

    Maximum and minimum values of a variable capacitor

    Homework Statement A parallel circuit like that in the figure forms the tuning circuit for an AM radio. If the inductor has a value of 8.6×10-6 H, what must be the maximum and minimum values of the variable capacitor if the radio receives frequencies from 590k Hz to 1670k Hz? a) Maximum C? b)...
  29. S

    Significance of Y = X^2 + 1 as random variable instead of X

    Homework Statement Let X be a random variable with the following probability distribution X 0 1 2 3 4 f(x) 1/16 1/4 3/8 1/4 1/16 If another random variable Y = X^2 + 1 is formed, find the mean E[Y]. 2. Relevant equation...
  30. A

    Fortran Fortran 77 variable declaration issue

    Hi I've been struggling with a fortran 77 code that I need for my astrophysics thesis... Since it's quite old, I'm trying to rewrite some features in order to make it more readable, and I'm trying to make it more rigorous by declaring all variables (I'm declaring "implicit none" everywhere)...
  31. Math Amateur

    MHB Inverse Function Theorem for One Real Variable

    I am reading Manfred Stoll's book: Introduction to Real Analysis. I need help with Stoll's proof of the Inverse Function Theorem (IFT) for real-valued functions of one real variable. Stoll's statement of the IFT for Derivatives and its proof read as follows: In the above proof we read: "...
  32. E

    Variable mass calculation of thrust for a water rocket

    Homework Statement I am trying to derive the formula for time varying thrust given that I know the initial mass/volume of water in a water rocket. Knowns will include initial pressure, initial volume of water, and nozzle cross sectional area. Homework Equations Bernoullis equation: (p/ρg) +...
  33. J

    Solving for a variable. Hard to do in this simple equation.

    Hi. This isn't for any sort of homework. I've run into a deadend where I have a situation like this where I need to solve for "a": a(a+b+c) =d You'd think it would be easy but I can't seem to get "a" by itself. It always end up on both sides of the equation at best which is starting to...
  34. Somefantastik

    Fortran Clear a variable in fortran subroutine

    Hello, I am trying to figure out how to clear a variable inside a subroutine from a calling function. The variable in the subroutine acts as a persistent(MATLAB) or static(C), but once in so many calls I want it to be reset to empty as if the subroutine is being called for the first time. I...
  35. Jameson

    MHB Estimating variance of Poisson random variable

    I am trying to use a generated random sample in R to estimate the mean and variance for a Poisson random variable. The first one is a Poisson random variable with mean 5. To estimate the above I generate a random sample in R with the following code: P5 <- rpois(100,5) Given the above I want to...
  36. thegreengineer

    Integral calculus: integral variable substitution confusion

    Recently I started seeing integral calculus and right now we are covering the topic of the antiderivative. At first sign it was not very difficult, until we started seeing integral variable substitution. The problem starts right here: Let's suppose that we have a function like this: \int...
  37. L

    Is the gravitational constant actually a variable?

    Hi, as an engineer i don't have much formal training in theoretical physics so bare with me. As most of you know, we can only understand 4% of our universe with the current models we have. The rest of the 96% is dark matter and dark energy (23% and 73%, respectively). Could these unexplained...
  38. TrickyDicky

    Local hidden variable theories

    Are many-particles systems theories like the Standard model of particle physics examples of local hidden variable theories?
  39. S

    Calculating velocity with variable acceleration

    This is not so much coursework as it is a thought experiment that has been bugging me for a while now. 1. Homework Statement Find the equation for velocity either as a function of distance (x) or time (t). I have an object whose acceleration is given by the rational function a(x) =...
  40. P

    Citizen SR-270x unknown variable

    Good day everyone, i have one question. Is it possible to calculate unknown variable on Citizen SR-270x?
  41. A

    What is the independent variable in Boyle's Law?

    I asked my teacher, and I was told that volume is the independent variable, and that pressure is dependent. The textbook I'm using said the same thing. After looking up some graphs of the law, I found that both pressure and volume were used as independent variables. It seems counter-intuitive...
  42. Mr Davis 97

    What is the significance of using a variable as an input to a function?

    For a generic function ##f##, why is that we use the variable ##x## as the argument of the function? What is the significance of this variable in regards to the object that is the function? Does it denote what the function is able to transform? i.e., ##f(z)## usually denotes a function that...
  43. S

    Transformer core variable reluctance

    Hi , When DC is applied to a transformer there is no secondary induction because DC current creates no changing flux in the Core. If I could change the reluctance, or permeability is another word I guess, of the transformer core while still having DC applied to the...
  44. M

    Mass, Momentum, and Velocity: Exploring the Dynamics of Change

    Hey guys, I'm reading my modern physics book over break and I remember hearing that mass changes as you approach the speed of light. But is it really the mass that is changing or just the amount of momentum a certain mass can have. So is mass really varying or is it the energy capacity of mass...
  45. rajeshputta

    Variable AC (0 to 240) source how to take fixed DC (110 or 220)

    sir, i am looking for fixed DC (110VDC and 220VDC) from my variable AC source (0 to 220V). please support me. rajesh.p 9010770003.
  46. ELB27

    Finding resistance of cylinders with variable conductivity

    Homework Statement Suppose the conductivity of the material separating two coaxial cylinders, of radius a and b (a < b) and held at a potential difference ##V##, would not be uniform. Specifically, ##\sigma(s) = k/s##, for some constant ##k##. Find the resistance between the cylinders. [Hint...
  47. C

    How TO make a variable 3D metal box

    Hi i want to know how to make a 3D metal box whose length, width, and height i can change. For example, i have a box of 3 m x 3 m x 3 m. i should be able to extend any dimension by a maximum of 1 meter i.e 4 meter (say). I want to use this to store items whose size is unknown, but the limits of...
  48. topsquark

    MHB Change of Variable: Physics Math in GR Problem

    What I am about to do is write an example of what I call "Physics Math." I am working on a GR problem backward: I have a motion I want to exhibit and am trying to work out the connection coefficients. The problem is that they are in terms of the wrong variable...The equations are expressed in...
  49. T

    Showing Chi squared is independent with another variable

    So I have X1 and X2 are iid standard normal. Then I have Y=X1^2+ X2^2 and Z=X1/(X1^2+x2^2) I'm supposed to find the distribution of Y and Z and then determine if they are independent. Clearly Y is chi squared with degrees of freedom 2. But I have no idea how to find the distribution of Z...
  50. C

    Solving an ODE with variable coefficients

    Homework Statement Solve the following: [/B] y'' = c2 / (x2 + c1*x) * y c1, c2 are constants, x is variableHomework Equations As above The Attempt at a Solution I have used the method of Frobenius and regular power series and obtained an infinite series on top of an infinite series, which is...
Back
Top