# Recent content by jimmy neutron

1. ### Entropy problem -- heating 2 moles of an ideal gas...

The change in entropy for a reversible process is DS = int (dQrev/T) You should be able to calculate this if you know the equation of state for a perfect gas
2. ### E cross drift calculus

Something else you need to consider: To derive the expression for v you used the result vXB = - E then I guess you took the cross product with B giving...
3. ### E cross drift calculus

No problem about the vector notation. The reason I wondered if there was more to the question because, in general, dv/dt wouldn't be zero. Look up "velocity selector", for example https://en.wikipedia.org/wiki/Wien_filter (must admit I learned something as well, I never realised that velocity...
4. ### E cross drift calculus

I think you first have to realise that you are dealing with vector quantities. With just a magnetic field of strength B you have the force acting on a charge q moving with velocity v is mdv/dt = q vXB (where vectors are bold face). In this situation if you assume that the charge moves...
5. ### Ordinary Differential Equation Problem

I think you have made a mistake in some of your +ve/-ve signs and think that your approach would eventually give y = y. The best way to go about this is to write cosx =(exp(ix) + exp(-ix))/2 (Euler's formula) and then you have to solve a differential equation of the form dy/dx = exp(mx) where...
6. ### Numerically solutions with periodic boundary conditions

It is for the time independent Schrodinger Equation and for ground state solution
7. ### Numerically solutions with periodic boundary conditions

Is anyone aware of how to numerically solve the (1D) SE with periodic boundary conditions?
8. ### Poincare's theorem and 2nd order ode

This is a request about the second order differential equation y'' + (k^2 + f(r))y(r) = 0 (1) where k is a (real) constant and f(r) is a real valued function of r that has some constraints regarding integratability. According to...
9. ### Capacitors in series and electric potential

I belierve there is a pd. One reply suggested no pd because no current: This is wrong because (as pointed out)the resistance is infinite. So, you can have a pd between points but no current.
10. ### Value of i^15

I think part of the problem is that the square root of a number, say x^2, is +/- x depending on whether x is +/-ve. That of use?
11. ### Voltage/Potential Difference

The terms met with in electricity have some historical bias. Voltage appears to come from the name of the scientist Alessandro Giuseppe Antonio Anastasio Volta who was one of the first people to investigate electric circuits (http://en.wikipedia.org/wiki/Alessandro_Volta) One way to introduce...
12. ### Confusion over binding energy

Mass isn't invariant: It is the total mass+energy that is conserved. As well, I'm not sure if you're aware of the well known formula from special relativity that relates the mass of a particle to its speed v: m = m0/sqrt( 1 - v^2/c^2)?
13. ### Vector Potential

You may consider using the following: gradXA = B then use Stoke's theorem to write surface int(gradXA.dS) = line int (A.dl), with the closed curve chosen suitably. This gives line int(A.dl) = int(B.ds). The surface integral is straight forward...
14. ### Waves equation

You dont have a wave equation there, you have solutions to a wave equation. As well, since cos(-z) = cos(z), i.e. cos is an even function you can write the cos solution as A cos (ωt - kx) if you wish. The general solution to the wave equation is Acos(wt - kx) + Bsin(wt -kx) where A and B...
15. ### How much is Special Relativity a needed foundation of General Relativity

By that I assume that you mean that, otherwise, light signals would have to travel at speeds less than c?