Physics Forums http://www.physicsforums.com Scientific physics forums for professionals and students to discuss serious issues and topics regarding physics in theory, practice, study and reality. Students can also find help for homework and career perspectives en Tue, 09 Feb 2010 16:46:22 GMT vBulletin 60 http://www.physicsforums.com/Nexus/misc/rss.jpg Physics Forums http://www.physicsforums.com Flow through a pipe http://www.physicsforums.com/showthread.php?t=376787&goto=newpost Tue, 09 Feb 2010 16:23:29 GMT Hello guys, I need to model a pipe in modelica. Now modelica has its own fluid library where i can find a model of a pipe. But i need to model a pipe which has unequal inlet and outlet area. The... Hello guys,

I need to model a pipe in modelica. Now modelica has its own fluid library where i can find a model of a pipe. But i need to model a pipe which has unequal inlet and outlet area. The outlet of the pipe is smaller than the inlet. Since mass going in the pipe=mass going out, hence the velocity of the fluid at the outlet should increase. I need a detail mathematical expression of this behavior. Please help me out. ]]> polezero http://www.physicsforums.com/showthread.php?t=376787 9/11 http://www.physicsforums.com/showthread.php?t=376786&goto=newpost Tue, 09 Feb 2010 16:22:17 GMT Was the report commissioned by the US government accurate to a degree or not at all, in accordance with the laws of physics? Was the report commissioned by the US government accurate to a degree or not at all, in accordance with the laws of physics? ]]> bonjour http://www.physicsforums.com/showthread.php?t=376786 Spin states of the addition of 2 spin 1/2 particles http://www.physicsforums.com/showthread.php?t=376785&goto=newpost Tue, 09 Feb 2010 16:17:57 GMT Hi everyone, A QM question that im stuck on: Consider the addition of the spin of 2 electrons, s1=s2=½. Show that there are one ms=1; two ms=0 and one ms=-1 states. Show the 4 spin states... Hi everyone,

A QM question that im stuck on:

Consider the addition of the spin of 2 electrons, s1=s2=½. Show
that there are one ms=1; two ms=0 and one ms=-1 states. Show the
4 spin states are given by:

a1a2
b1b2
(1/(sqrt(2)))(a1b2+b1a2)
(1/(sqrt(2)))(a1b2-b1a2)

I've used a and b in place of alpha and beta, which represent the spin up and spin down states.

I can see why there are one ms=1 and one ms=-1 states, and 2 ms=0 states, from adding the values of ms for individual particles, but am not sure on how to proceed to the second part.

Any nudge in the right direction would be great.

Cheers,

Simon. ]]> Quantum Physics SimonRoberts http://www.physicsforums.com/showthread.php?t=376785 Ethics of withholding science data from the public http://www.physicsforums.com/showthread.php?t=376782&goto=newpost Tue, 09 Feb 2010 16:14:52 GMT I am a graduate student who works with a large science collaboration. The collaboration has built a very useful set of data which will, we are told, be released to the public at some future time... I am a graduate student who works with a large science collaboration. The collaboration has built a very useful set of data which will, we are told, be released to the public at some future time when the collaborators are sure it is sufficiently free of errors. However, it is more or less an open secret that the data is really being withheld from the public so that the collaborators can publish their analysis of the data before anyone else has a chance.

I strongly believe this policy is unethical, and detrimental to science as a whole. The more scientists who have access to data, the sooner it can be used to draw conclusions about nature, and perhaps guide the next generation of research projects in the most fruitful direction. Restricting data to the smallest possible audience may benefit a few in the short term, but ultimately retards the progress of science as a whole.

The practice of withholding science data is also unethical from a financial point of view. Unlike the private sector, where companies can justify keeping their research secret for business reasons (and because they pay for it), most basic science research is funded by the public and is meant to benefit all humankind, not the careers of specific researchers.

Do you readers agree that the current practice of withholding science data is unethical, and what is being done to change this practice? ]]> General Discussion nickyrtr http://www.physicsforums.com/showthread.php?t=376782 Math equation electronic document http://www.physicsforums.com/showthread.php?t=376781&goto=newpost Tue, 09 Feb 2010 16:14:02 GMT Hi, I wish to do math equations electronically. Can someone help me to find a good software? I heard there is such a thing called latex. Where can I find it? Thanks Hi, I wish to do math equations electronically.
Can someone help me to find a good software?
I heard there is such a thing called latex. Where can I find it?

Thanks ]]> quantum123 http://www.physicsforums.com/showthread.php?t=376781 Hand-cranked battery charger,looking the helps... http://www.physicsforums.com/showthread.php?t=376780&goto=newpost Tue, 09 Feb 2010 16:13:30 GMT Hye guys,

I'm just a beginner in the area..i have a project of hand-cranked battery charger..
I supposed to improve the current charger in every way possible..
How could i the miniature generator, to generate the energy by turning the crank..
Is there any diagram that i can study or any other way so that I can get the clear view about the principle of the charger..

Looking for your expertise! ]]> Electrical Engineering quartz69 http://www.physicsforums.com/showthread.php?t=376780 tension in a string at an angle http://www.physicsforums.com/showthread.php?t=376779&goto=newpost Tue, 09 Feb 2010 15:52:59 GMT *1. The problem statement, all variables and given/known data* A wire of negligible mass is used to hang a 2.15 kg picture. The ends of the wire are attached to the top 2 corners of the picture.... 1. The problem statement, all variables and given/known data
A wire of negligible mass is used to hang a 2.15 kg picture. The ends of the wire are attached to the top 2 corners of the picture. Each end of the wire forms an angle of 15.0 degrees from the horizontal. What is the tension in the wire?



2. Relevant equations
LaTeX Code: \\Sigma F=ma


3. The attempt at a solution
This is my latest and posiibly best attmept so far, but I don't have anyone to tell me if its right or not.
LaTeX Code: \\Sigma F+(-ma)=2TsinLaTeX Code: \\theta -ma=0
2Tsin15-19.737=0
T=38.1N ]]> Introductory Physics BikeSmoth http://www.physicsforums.com/showthread.php?t=376779 Evaluating the non-relativistic Action http://www.physicsforums.com/showthread.php?t=376778&goto=newpost Tue, 09 Feb 2010 15:52:57 GMT 1. The problem statement, all variables and given/known data

Consider the classical action LaTeX Code: (m/2) \\int dt \\dot{x}^{2} where m is the mass of the non-relativistic point particle, and LaTeX Code: \\dot{x} = dx/dt .

This classical path starts at LaTeX Code: (x_{1}, t_{1}) and ends at LaTeX Code: (x_{2}, t_{2}) . Could someone help me evaluate this classical action explicitly please?


2. Relevant equations



3. The attempt at a solution

Thanks a lot! ]]> Advanced Physics wam_mi http://www.physicsforums.com/showthread.php?t=376778 Sums and Limits http://www.physicsforums.com/showthread.php?t=376777&goto=newpost Tue, 09 Feb 2010 15:51:26 GMT lim_{n \\rightarrow \\infty} \\sum_{k=0}^n f(k,x)
')"> I've been trying to tackle a problem of the following form
LaTeX Code: <BR>lim_{n \\rightarrow \\infty} \\sum_{k=0}^n f(k,x)<BR>

I know that the limit of each function is zero as n goes to infinity.
ie. LaTeX Code: <BR>lim_{n \\rightarrow \\infty} f(n,x) =0<BR>

But I'm not sure how to approach the problem above. I would greatly appreciate any thoughts/suggestions.

Thanks ]]> Apteronotus http://www.physicsforums.com/showthread.php?t=376777 Electric fields between point charges http://www.physicsforums.com/showthread.php?t=376774&goto=newpost Tue, 09 Feb 2010 15:20:49 GMT I am struggling to grasp the concept of the electric field between two opposite charges. I know that the field is strongest closest to the charge and gets weaker further away from the charge. I also... I am struggling to grasp the concept of the electric field between two opposite charges. I know that the field is strongest closest to the charge and gets weaker further away from the charge. I also know that the electric field lines go from positive to negative and that they are perpendicular to the equipotential lines.

My question is: Is the electric field on the center line (shortest distance) between two opposite charges constant? If not, how can it be constant everywhere in a parallel plate capacitor? ]]> Classical Physics schiasso http://www.physicsforums.com/showthread.php?t=376774 electron speed due to magnetic field. http://www.physicsforums.com/showthread.php?t=376773&goto=newpost Tue, 09 Feb 2010 15:12:52 GMT *1. The problem statement, all variables and given/known data* electrons emitted by a certain device are observed to travel in a circular path with a radius of 2.0cm when plaves in a uniform... 1. The problem statement, all variables and given/known data

electrons emitted by a certain device are observed to travel in a circular path with a radius of 2.0cm when plaves in a uniform magnetic field whose strength is 10MN/C. What is the speed of the elctrons emitted by this device?

2. Relevant equations
I know that 10MN/C is "b-bar" as it is called in the book. But i'm not sure what equation to use.

3. The attempt at a solution
I attempted to use the equation Fm=q(velocity/c x B).... but im not sure what to use for the magnetic force. ]]> Introductory Physics sarahaha288 http://www.physicsforums.com/showthread.php?t=376773 Solving 1d Helmholtz with boundary conditions http://www.physicsforums.com/showthread.php?t=376772&goto=newpost Tue, 09 Feb 2010 15:03:29 GMT Hello all,
This is to do with forced longitudinal vibration of a rod (bar).
It's basically a problem to do with the linearised plane wave equation (1d).

The rod is fixed firmly at one end, and excited at the other by a harmonic force.

The wave equation (with constant rho/E instead of 1/c^2) is reduced to the helmholtz equation, which is fine. But the boundary conditions which exist (in this example) are at x=0, u=0 (u=displacement) and at x=L, (AE)*du(x,t)/dx=Fcos(wt)

This leads to the solution of the plane wave equation which is:
u(x,t)=Fsin(ax)cos(wt)/(AEcos(aL))
anyway, there is a time dependence there which I'm not really wanting.
How do I remove this? Basically, i don't know if you noticed but I am lost!

At the end of it all i'm looking for the point mobility. ]]> Differential Equations ptptaylor http://www.physicsforums.com/showthread.php?t=376772 Matlab help, function handle http://www.physicsforums.com/showthread.php?t=376771&goto=newpost Tue, 09 Feb 2010 14:56:56 GMT 1. The problem statement, all variables and given/known data
Hi,

I'm writing a little Matlab function that solves an equation with the help of a function handle.The equation is

x(t) = exp(-a*t)*x0 + exp(-a*t) * int(exp(a*s)*y(s), s=0..t)

I have tried it with two following functions

2. The attempt at a solution

function #1
function ode = ode1sol(a, x0, y, tol)

% if tol is not given
if nargin == 3
tol = 1e-6;
end

t = 0:1:10;

% sub3 being a subfunction
ode = sub3(a,x0,y,t,tol);

function #2
function x = sub3(a,x0,y,t,tol)

f = @(s) exp(a*s) .* y(s);

for i = 1:length(t)
x(i) = exp(-a*t(i))*x0 + exp(-a*t(i)) .* quad(f,0,t(i),tol);
end

--
when I write
t=0:1:10;
ode1sol(1,1,sin(6*t))

Matlab gives me this error
??? Subscript indices must either be real positive integers or logicals.

Error in ==> sub3>@(s)exp(a*s).*y(s) at 5
f = @(s) exp(a*s) .* y(s);

Error in ==> quad at 77
y = f(x, varargin{:});

Error in ==> sub3 at 8
x(i) = exp(-a*t(i))*x0 + exp(-a*t(i)) .* quad(f,0,t(i),tol);

Error in ==> ode1sol at 13
ode = sub3(a,x0,y,t,tol);

I'm confusing the indices somehow but I don't know why... ]]> Jackk http://www.physicsforums.com/showthread.php?t=376771 Writing out to a directory in python http://www.physicsforums.com/showthread.php?t=376769&goto=newpost Tue, 09 Feb 2010 14:45:51 GMT Hi, I have a program which is reading in line by line information, saving that, and writing it out to a file. How do I change where the files are created? Also, can I create a folder to write the... Hi, I have a program which is reading in line by line information, saving that, and writing it out to a file. How do I change where the files are created? Also, can I create a folder to write the files out to in my program?

The second issue I have is this. right now my program opens the file, reads a line, writes a line, reads a line, writes a line, until its finished. I want it to read all of the lines, then open the file, then write all of them. Would I save all my lines to an array then fout.write(array[0:x],'w')?? ]]> dacruick http://www.physicsforums.com/showthread.php?t=376769 I failed for a job in astronomy http://www.physicsforums.com/showthread.php?t=376768&goto=newpost Tue, 09 Feb 2010 14:35:51 GMT Yesterday I got an interview After that, they told me to try next time if this time I was not accepted and wait for their call if they want to contact. When someone told you so, does that mean you... Yesterday I got an interview
After that, they told me to try next time if this time I was not accepted and wait for their call if they want to contact.
When someone told you so, does that mean you failed ? ]]> General Discussion Dolphiney http://www.physicsforums.com/showthread.php?t=376768