- #1
Nick Jackson
- 13
- 0
Can anybody out there show me how the sine wave formula [tex] y=Acos(kx - ωt + φ_{0}) [/tex] or [tex] y=Acos(kx + ωt + φ_{0}) [/tex] is the direct solution of the wave equation [tex] \frac{\partial^2 y}{\partial t^2} = v^2 \frac{\partial^2 y}{\partial x^2} [/tex] ? I mean I looked it over on the Internet but everybody keeps showing either the Fourier series solution of the wave equation or another integral equation that is derived by the homogeneous and the inhomogeneous transport equation. Only a few people are approaching this by separation of variables, assuming that [tex] y(x,t)=f(x)*g(t) [/tex] and rearranging the wave equation and setting the result to be a constant k that they assume to be of the form -ω^2, like this: [tex] \frac{1}{v^2} \frac{g^{''}(t)}{g(t)}=\frac{f^{''}(x)}{f(x)}=k [/tex]
The result is immediate and correct.
However what I don't understand is why k has to be negative. Shouldn't [tex] \frac{g^{''}(t)}{g(t)} [/tex] and [tex] \frac{f^{''}(x)}{f(x)} [/tex] be any non-zero number since the change of displacement in oscillation can be either positive or negative? That said, shouldn't the wave equation give also formulas of hyperbolic sines and cosines for specific parts of the oscillation?
P.S. I am most interested in deriving the E/M sine wave formulas:
[tex] E=E_{max}cos(kx - ωt + φ_{0}) [/tex] or [tex] E=E_{max}cos(kx + ωt + φ_{0}) [/tex]
[tex] B=B_{max}cos(kx - ωt + φ_{0}) [/tex] or [tex] B=B_{max}cos(kx + ωt + φ_{0}) [/tex]
Thank you very much!
The result is immediate and correct.
However what I don't understand is why k has to be negative. Shouldn't [tex] \frac{g^{''}(t)}{g(t)} [/tex] and [tex] \frac{f^{''}(x)}{f(x)} [/tex] be any non-zero number since the change of displacement in oscillation can be either positive or negative? That said, shouldn't the wave equation give also formulas of hyperbolic sines and cosines for specific parts of the oscillation?
P.S. I am most interested in deriving the E/M sine wave formulas:
[tex] E=E_{max}cos(kx - ωt + φ_{0}) [/tex] or [tex] E=E_{max}cos(kx + ωt + φ_{0}) [/tex]
[tex] B=B_{max}cos(kx - ωt + φ_{0}) [/tex] or [tex] B=B_{max}cos(kx + ωt + φ_{0}) [/tex]
Thank you very much!