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.

View More On Wikipedia.org
Recent contents Top users
  1. johann1301

    Creating Wave Functions With One Variable: Can It Be Done?

    With the number i, its possible to create a wave function with just one variable. example: f(x)=i^x or.. f(x)=2^ix But is there any wave function without the number i AND with only one(x and y) variable? can this be created?
  2. J

    Variable Resistror, Calculating Vo

    Homework Statement A variable resistor, R, is connected to the terminals of a battery (dashed box in the diagram). The graph below shows data collected in the circuit as R is varied. What is V0, in volts? What is the internal resistance, r, in ohms? Homework Equations Ohm's Law The...
  3. T

    Finding displacement from variable acceleration

    Hello, I've been staring at this question for an hour with no luck.. if anyone could point me in the right direction that would greatly appreciated! Homework Statement a = acceleration v = velocity x = displacement m and n are constants vt = terminal velocity Show that after...
  4. P

    How Do You Calculate Average Capacitance of a Variable Separation Capacitor?

    Hi there, I'm trying to calculate the average capacitance of a "variable separation capacitor." Google (and all my textbooks) are not helping me at all. The capacitor has two parallel plates, one of which is hinged at one side. Sort of like this... V Basically I need an expression for the...
  5. I

    Simple work and variable position question

    Homework Statement A one-dimensional force acts on a particle of mass m = 6.26 kg in such a way that its position is given by: x = 0.484t^3 - 33.6t Find W, the work done by this force during the first 1.49 s. Homework Equations W=F*s W=mgs Integration The Attempt at a...
  6. A

    Derivative wrt one variable of an integral wrt another

    I can't seem to find this anywhere. What's d/dn(∫ n(w) dw)? That's the derivative wrt n of the integral of n over w (note: n is a function of w). Seems straightforward enough. Is there a simplification? Thanks
  7. 1

    Work done by General Variable force

    Homework Statement The force on a particle is directed along an x-axis and is given by F = F0(x/x0 - 1). Find the work done by the force in moving the particle from x = 0 to x = 2x0 Homework Equations The Attempt at a Solution It looks like the force is a recurrence relation or...
  8. T

    Probability - Poisson Random Variable?

    1. Homework Statement During a typical Pennsylvania winter, I80 averages 1.6 potholes per 10 miles. A certain county is responsible for repairing potholes in a 30 mile stretch of the interstate. Let X denote the number of potholes the county will have to repair at the end of next winter. 1...
  9. F

    Expectation of a Random Variable

    I know the E[X] = Integral between [-inf,inf] of X*f(x) dx Where X is normally distributed and f(x) is the PDF How do I find the expectation of X4? Bare with me because I'm useless in Latex So far what I've done is written the integral as Integral between [-inf,inf] of X4*f(x) dx...
  10. R

    Solving ODE with variable coefficients

    Homework Statement I wanted to solve a ode which has Brownian motion as a variable coefficient Homework Equations 2x2y'' + y' -ρy = 0 where x is the Brownian motion with respect to time ρ is a constant The Attempt at a Solution I have tried power series with no solution. Is there a...
  11. E

    Creating Flange or Variable Delay DSP model

    Homework Statement Im trying to model this in simulink. I am having a little trouble getting things to work. I need to create a variable delay model or basically implement a "flanger" effect. Flange A flange is an audio effect, which is achieved by combining two identical signals with...
  12. Z

    C/C++ C++ how to check for variable type?

    So, I want to use cin >> to get a user input that must only be int. How can I check the type of the input? Thanks.
  13. J

    Separable equations: How do you know which variable to solve for?

    Separable equations: How do you know which variable to solve for? + extra question Homework Statement I attached a sample problem with variables u and t. How do I know what the answer should be at the end? In terms of u or in terms of t or it doesn't matter? Homework Statement I also...
  14. R

    Find the Mean and Variance of Random Variable Z = (5x+3)

    Homework Statement Find the Mean and Variance of Random Variable Z = (5x+3) Using data set:Using: & The Attempt at a Solution
  15. W

    Partial derivative of a single variable function

    So I don't understand why if you have something like U(x,y) = f(y+2x) and you take \frac{\partial U}{\partial x} = \frac{\partial f}{\partial x} you get \frac{df}{d(y+2x)} * \frac{d(y+2x)}{dx} Why does the partial derivative just change to the total derivative for one variable? It...
  16. V

    What does it mean variable Direct Current ?

    what does it mean "variable Direct Current"? I am confused on the point where i see the word variable DC in a question(see attachment). If a DC vary like a curve then in some cases it can be like AC. How is it varying (Don't tell me answers.)
  17. S

    Heat conduction in a beam with variable x-section

    Say we have a laterally insulated beam and some boundary conditions at either end, be it convective or fixed-temperature, but the cross-sectional area is variable. If the cross-section were constant I'd just say it were a 1-D problem, but I'd imagine that having the cross-sectional area be a...
  18. L

    Problem with variable capacitor and changing angles

    Homework Statement Here is the problem: A variable capacitor has seven armatures in the shape of half-circles with a radius of 2 cm. The armatures have a distance of 1 mm separating them. Find the capacity when the angle θ is: (a) 0 degrees; (b) 45 degrees; (c) 135 degrees. Homework...
  19. G

    Proof of variance for functions of variable

    http://bildr.no/view/1115383 i wonder if anyone could explain the proof for theorem 4.3 i have understood definition 4.3
  20. L

    Torsion on materials of variable cross section

    New here, sorry if I'm in the wrong place or unclear, etc. I've been looking at torsion on solid plates and the torsion constant. I was wondering how to handle the torsion constant on a plate of changing cross section (e.g. there are holes drilled in it, changing the area of the cross section...
  21. F

    Shooter hitting targets of variable heights

    I have a shooter that shoots a ball at a fixed angle of 65 degrees from horizontal. The shooter is aimed at 3 targets of different but fixed heights. The top target is at 1.536 meters above the shooter, the middle target is at 0.571 meters, and the bottom target is -0.318 below the shooters...
  22. C

    How to Derive Upper and Lower Bounds for a Random Variable?

    Dears, If a random variable is generated with the pdf of p(f) = 1/(f^x), how can I derive the upper bound or lower bound of the random variable? Thanks,
  23. C

    Upper bound of random variable

    Dears, If a random variable is generated with the pdf of p(f) = 1/(f^x), how can I derive the upper bound or lower bound of the random variable? Thanks,
  24. G

    MATLAB Matlab input string as variable name (or other ways)

    Hi, I am writing a Matlab function(script), which take some input. I have several datasets in the workspace, say dataset1,dataset2,dataset3, ect In my function, I have tried to use data = input('Please input your dataset', 's') and then use 'data' as a variable for some computation, say...
  25. F

    Sign of u and v in a complex variable

    Homework Statement if z=x+iy and sqrt(z) = u+iv u=±(1/√2)*√(x+√(x^2+y^2 )) v=±(1/√2)*√(-x+√(x^2+y^2 )) How must the signs on these result be chosen? 2. The attempt at a solution I wondered if it was something to do with ensuring √(z) was always positive, but then ruled this out as...
  26. A

    Two Variable Limit: Solve xy²/(sin(x²+y²))

    Hi, Homework Statement lim (xy²)/(sin(x² + y²)) (x,y) -> (0,0) The Attempt at a Solution I think the answer is 0, but I don't know how to get rid of the sin (x² + y²). I thought about using taylor series but I'm not sure if that works with two variables. Can someone help me out?
  27. A

    Name for binning a variable with respect to another variable(s)

    Hi, Sorry if it's not the right place as this question is a bit generic. Let say we have N datapoint triplets xi, yi, zi. If we ask "how many points" there are in the interval X < x < X+ΔX and Y < y < Y+ΔY (where X and Y are some gridpoints) -- then this is a 2D histogram. But in each...
  28. A

    ODE with a function of variable crazy equ

    How to solve a equation of this kind T' = a*(T^4 - r^4) + b*(T^4 - s^4) + P*(1 - η(T)) The above equation is driving me nuts... the 'η' is a function of T(Temperature) the efficiency and initial value of T is known. Say at t = 0 T is 298 Need help! Please! I need to find the...
  29. K

    Finding X and Y with respect to third variable if having two sets of X and Y

    My problem At temperature 5 degrees the Y=1.00228594 for X=435 and Y=1.000986038 for X=449 and Y=0.999760292 for X=463 At temperature 7 degrees the Y=1.002094781 for X=435 and Y=1.00079709 for X=449 and Y=0.999573015 for X =463 For a new temperature of 9.6 degrees how to find the Y...
  30. M

    Determining the distribution of a variable

    Hi, I am trying to understand how to predict the distribution of a variable that is a combination of other random variables. For example, how should I determine the distribution of Variable A, which is the sum of Variable X and Variable Y? Variable X has a normal distribution and Variable...
  31. A

    When can I decompose a random variable $Y=X'-X''$?

    I am wondering if I can find a decomposition of Y that is absolutely continuous nto two i.i.d. random variables X' and X'' such that Y=X'-X'', where X' is also Lebesgue measure with an almost everywhere positive density w.r.t to the Lebesge mesure. My main intent is to come up with two i.i.d...
  32. N

    What kinda of Variable is this?

    The following values are tonnes of a specific animal feed for a specific consumer. Jan - 13.97 Feb - 12 Mar - 11 Apr - 5.87 May - 10 Jun - 13.95 Jul - 15.96 Are these variables best described as "quantitative ratio variables" and why? If not what would be the most accurate class...
  33. T

    1500w 120vac variable power control

    Hello, I've got an extremely limited background in electronics, so I'd like to ask for some advice regarding this circuit: I found the diagram online, It's supposed to allow variable control of a load up to 1500w at 120v, using an ordinary light dimmer and a 600V / 40A triac. After...
  34. R

    MATLAB MATLAB - Dynamic variable name for Data Store/Read/Write

    Hello, I am trying to create a subsystem with parameters shown here: I want the parameter name to auto-populate the 'Timer Name' in the blocks of Data Store Memory/Read/Write, but it just leaves it as 'nam'. The subsystem blocks are shown below. So basically, I was to set the Timer...
  35. D

    Trouble understanding how to choose a free variable (linear algebra)

    Homework Statement I just don't understand how one chooses a free variable when using Gaussian elimination and back substitution to solve linear equations Homework Equations The Attempt at a Solution One particular example here is provided (already in row echelon form) -2...
  36. A

    Confusing variable change calculation

    Suppose you start with a system of N particles identified by position vectors r_1, r_2, \ldots, r_N and masses \mu_1, \mu_2, \ldots, \mu_N. Then (quantum mechanically) the kinetic energy operator for this system is given by (assuming \hbar = 1) T = \sum_{i = 1}^N -\frac{\Delta_i}{2\mu_i}...
  37. Z

    Any function is not a Random Variable

    There are plenty example of functions are random variables from my class note. I only interested of thinking up functions are not random variables. If you know functions are not random variables please please reply this post. This class is about set theory, probability measure, Borel...
  38. S

    Variable Acceleration - Clueless

    Homework Statement The problem is labeled Variable Acceleration. The acceleration of a marble in a certain fluid is proportional to the speed of the marble squared and is given (in SI units) by the expression Ax = -3.00 Vx^2. If the marble enters this fluid with a speed of 1.50 m/s, how long...
  39. B

    Calculate work =variable force & variable distance

    Dear All I would like to calculate work done on the system by variable force & distance at each point. Kindly let me know to the formula how to calculate work in excel sheet (because I'm having huge data) I have attached my copy of data for your reference. Kindly advice. Many...
  40. M

    MHB Can Analytic Functions in the Unit Disk Meet Specific Modulus Conditions?

    Let $f:B(0,2)\to B(0,2)$ be an analytic function so that $f(1)=0.$ Prove that for all $z\in B(0,2)$ is $\left| \dfrac{f(z)}{z} \right|\le \left| \dfrac{2(z-1)}{4-z} \right|.$ What's the trick here? I don't see it.
  41. 1

    Variable mass/force oscillations

    Hi, I'm working on a problem that i would like your input on. I have a solution but i would like to see how others would approach it since my solution is more of a hypothesis until we continue with the testing phase. In the most basic form, this project can be described as a hanging container...
  42. M

    Formula that describes a variable magnetic field

    1. I am running a project that encompasses a fixed magnet interacting with a movable metal grid. The gap between the magnet and the metal grid obviously varies therefore the interactive field between these two objects will also vary in strength depending on how close or distant the metal grid...
  43. S

    Vaidity of rotational formulae for variable acceleration

    Homework Statement Which of the following formulas is valid if the angular acceleration of an object is not constant? Explain your reasoning in each case. (a) v = rω; (b) atan = r\alpha; (c) ω = ω0 + \alphat; (d) arad = rω2; (e) K = 0.5Iω2. Homework Equations The Attempt at a...
  44. D

    How Can I Electrically Vary the Inductance in an RF Transformer?

    I have an inductor in a design that is one half of an air coupled RF transformer. The operating frequency is 50 MHz. The purpose is to provide power to the load side. It is for all intents and purposes an RF to DC power supply design over some gap. The primary is the fed radiator inductor, a...
  45. U

    Partial derivative of function w.r.t. the percent change of the variable

    Homework Statement Rewrite this in terms of f, f, ∂f/∂x, and x: ∂f(x,y)/∂(%Δx) = ∂f(x,y)/∂(d log(x) ) Homework Equations ∂(%Δf(x,y))/∂(%Δx) = ∂logf(x,y)/∂log(x)= ∂f(x,y)/∂x*x/f(x,y). ∂f(x,y)/∂log(x)=x∂f(x,y)/∂x The Attempt at a Solution I found that (%Δx) can be written as...
  46. N

    Show that if X is a bounded random variable, then E(X) exists.

    Homework Statement Show that if X is a bounded random variable, then E(X) exists.Homework Equations The Attempt at a Solution I am having trouble of finding out where to begin this proof.This is what I got so far: Suppose X is bounded. Then there exists two numbers a and b such that P(X > b)...
  47. K

    Analytical solution to the diffusion equation with variable diffusivity

    Hi, I'm trying to find an analytical solution (if one exists) to the 1d diffusion equation with variable diffusivity κ(x); \partial_t u(x,t) = \partial_x[\kappa(x) \partial_x u(x,t)] Could someone point me in the right direction to solve this if its possible to do so analytically. I've...
  48. J

    How to get one variable from one equation?

    Homework Statement In manufacture of nitric acid, ammonia and prehated air are mixed to form a gas containing 10 mole %NH3 at 650 C. If ammonia enters the gas blending unit at 25C at a rate o 520kg/? and heat is lost from the mixer to its surroundings at a rate of 7 kW, determine the...
  49. E

    Gauss Law and Flux for variable charge density in a sphere

    Gauss Law and Flux for variable charge density in a sphere Homework Statement The charge density within a sphere varies as a constant, a, times its radius, r. Find an expression for the direction and magnitude of the electric flux, D, within the sphereHomework Equations Gauss' Law sphere...
  50. G

    Normal Distribution: PDF of a Normally Distributed Random Variable

    Homework Statement A Normally distributed random variable with mean μ has a probability density function given by _ρ_...*...((-ρ2(x-μ)2)/2δ) √2∏δ|...e^ Homework Equations Its standard deviation is given by: A)ρ2/δ B)δ/ρ C)√δ|/ρ D)ρ/√δ| E)√δ|/2ρ The Attempt at a Solution...
Back
Top