- #1
Alexandre
- 29
- 0
I got this Lagrangian on the exam and it just seems weird to me:
L = [itex]\frac{m}{2}[/itex](ẋ²+ẏ²) – eBẋy
What I mean by "asymmetric" is that it doesn't seem to behave in a same way on x as on y, because there is ẋ in the last term and y but there is no ẏ and x.
Deriving Euler-Lagrange equations I get:
ẍ = ωẏ
ÿ = –ωẋ
(Where ω=[itex]\frac{eB}{m}[/itex])
Solution to the equations as I calculated must be:
x(t) = [itex]\frac{c_{2}}{ω}[/itex]sin(ωt) – [itex]\frac{c_{1}}{ω}[/itex]cos(ωt) + x0
y(t) = [itex]\frac{c_{2}}{ω}[/itex]cos(ωt) + [itex]\frac{c_{1}}{ω}[/itex]sin(ωt) + y0
(c1 and c2 depend on initial velocities along x and y, x0 and y0 are initial positions)
This must be the Lorentz force acting on a particle with elementary charge moving with velocity v perpendicular to magnetic field B.
Here's the animation
https://www.desmos.com/calculator/r3mzstju9x
So to clarify my question and extend it a little bit, how is the Lagrantian derived (the last term obviously)? Why is there ẋy and not ẏx or yx for instance? Can eBẋy be a potential energy?
This is not a homework, I'm posting this for my curiosity. And also it doesn't seem to match the strict guidelines provided in homework subforum.
L = [itex]\frac{m}{2}[/itex](ẋ²+ẏ²) – eBẋy
What I mean by "asymmetric" is that it doesn't seem to behave in a same way on x as on y, because there is ẋ in the last term and y but there is no ẏ and x.
Deriving Euler-Lagrange equations I get:
ẍ = ωẏ
ÿ = –ωẋ
(Where ω=[itex]\frac{eB}{m}[/itex])
Solution to the equations as I calculated must be:
x(t) = [itex]\frac{c_{2}}{ω}[/itex]sin(ωt) – [itex]\frac{c_{1}}{ω}[/itex]cos(ωt) + x0
y(t) = [itex]\frac{c_{2}}{ω}[/itex]cos(ωt) + [itex]\frac{c_{1}}{ω}[/itex]sin(ωt) + y0
(c1 and c2 depend on initial velocities along x and y, x0 and y0 are initial positions)
This must be the Lorentz force acting on a particle with elementary charge moving with velocity v perpendicular to magnetic field B.
Here's the animation
https://www.desmos.com/calculator/r3mzstju9x
So to clarify my question and extend it a little bit, how is the Lagrantian derived (the last term obviously)? Why is there ẋy and not ẏx or yx for instance? Can eBẋy be a potential energy?
This is not a homework, I'm posting this for my curiosity. And also it doesn't seem to match the strict guidelines provided in homework subforum.
Last edited: