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  1. C

    Finding the potential inside a semiconductor

    Homework Statement My question is more about understanding the task itself, not about calculation. I am supposed to use the poisson equation, to derive the potential inside a semiconductor for a barrier with potential height ##\phi_B## and a donator doping with ##N1 > N2##. Then I should use...
  2. C

    Collision of two particles

    But it has acceleration in the x- and y- direction namely a_x = sin(\theta)*0,4 and a_x = cos(\theta)*0,4 . Particle B starts with zero velocity and starts the origin so \vec{x_0} = 0 , \vec{v_0} = 0
  3. C

    Collision of two particles

    Homework Statement I am currently solving a problem and I am not sure if it is correct. There are two particles A and B. A has a constant velocity with |\vec{v}| = 3 and starts from y = 30 B has constant acceleration with |\vec{a}| = 0,4 The goal is to find the angle between the...
  4. C

    I What happens mathematically when a wave function collapses

    In Shankars "Principle of Quantum Mechanics" in Chapter 4, page 122, he explains what the "Collapse of the State Vector" means. I get that upon measurement, the wave function can be written as a linear combination of the eigenvectors belonging to a operator which corresponds to the...
  5. C

    Question regarding an integral

    Ah I see, thanks very much both of you!
  6. C

    Question regarding an integral

    I did perform that step as I wrote in my post and I get the same equation, though my derivative might be wrong. Let's differentiate the solution from the book with respect to x: \frac{d}{dx}\left(\frac{l^2a^2x^2}{12x^2+l^2}\right) = l^2a^2 \frac{d}{dx}\left(\frac{x^2}{12x^2+l^2}\right) Using...
  7. C

    Question regarding an integral

    It means :\frac{d\dot{x}^2}{dx} = \frac{d}{dx}\left(\frac{dx}{dt}\right)^2 For the result from the book there where no steps shown. For my result: \frac{d\dot{x}^2}{dx} = \frac{2l^4a^2x}{(12x^2+l^2)^2} \Longrightarrow \int d\dot{x}^2 =2 l^4a^2\int \frac{x}{(12x^2+l^2)^2} dx Apply...
  8. C

    Question regarding an integral

    Homework Statement I've got an equation which I need to integrate. However, integrating it and checking with the solutions, I get two different results. I get the same result as using wolfram alpha, but a different result from the book. If I differentiate both results, I get back to the orginal...
  9. C

    Finding the geodesic equation from a given line element

    \begin{equation}p_u = \frac{\partial L}{\partial \dot{u}} = m\dot{u}f\end{equation} \begin{equation}\frac{\partial L}{\partial u} = 0\end{equation} \begin{equation}\frac{dp_u}{dt} = \frac{d}{dt}(m\dot{u}f) = m\ddot{u}f + m\dot{u}\frac{\partial f}{\partial x}\dot{x} = 0\end{equation} Dividing by...
  10. C

    Finding the geodesic equation from a given line element

    Homework Statement We've got a line element ds^2 = f(x) du^2 + dx^2 From that we should find the geodesic equation Homework Equations Line Element: ds^2 = dq^j g_{jk} dq^k Geodesic Equation: \ddot{q}^j = -\Gamma_{km}^j \dot{q}^k \dot{q}^m Christoffel Symbol: \Gamma_{km}^j = \frac{g^{jl}}{2}...
  11. C

    Lagrangian mechanics: Bar connected to a spring

    Thank you very much!! I'll try to solve it now
  12. C

    Lagrangian mechanics: Bar connected to a spring

    Homework Statement Mass 1 can slide on a vertical rod under the influence of a constant gravitational force and and is connected to the rod via a spring with the spring konstant k and rest length 0. A mass 2 is connected to mass 1 via a rod of length L (forms a 90 degree angel with the first...
  13. C

    Discrete Lagrangian

    Hey thank you very much! Now the discrete equation looks like newtons 2nd law just as was asked :')
  14. C

    Discrete Lagrangian

    Wouldn't it reduce to \frac{\partial U(r_{k})}{\partial r_k} j = r_{k+1} because only for j=k+1 the kroenecker delta would give me a 1? And when you would have \frac{\partial U(r_{k+1-1})}{\partial r_k} = \frac{\partial U(r_{k})}{\partial r_k} and then I would have two \frac{\partial...
  15. C

    Discrete Lagrangian

    Homework Statement In this exercise, we are given a discrete Lagrangian which looks like this: http://imgur.com/TL0P61r. We have to minimize the discrete S with fixed point r_i and r_f and find the the discrete equations of motions. In the second part we should derive a discrete trajectory for...
  16. C

    2-D elastic collision circle

    There is a factor 2 at m2. I don't see where it is wrong. (I guess you corrected it) And we'll you can write p1 to m1*v1. Look I tried all steps for hours now and I don't get it. I know that it is just simple steps but by now I would learn much more, if somebody could post the solution.
  17. C

    2-D elastic collision circle

    I suppose that the m1m2 in U comes from adding the two fractions and the m1+m2 comes from dividing a m1+m2. I still don't get it though.
  18. C

    2-D elastic collision circle

    Yes , sorry for that, it's ((p1-x)^2+y^2)/(2m1) like you said
  19. C

    2-D elastic collision circle

    Homework Statement Hi there! In this exercise, we are supposed to derive this formula for a 2-D elastic with two different masses: (x-U*v1)^2 + y^2 = (Uv1)^2 (example, two billiard balls), the second mass is at rest. It's a equation which leads to a circle where all of the possible p2' lie...
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