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yusohard
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Homework Statement
A charge q is released from rest at the origin, in the presence of a uniform electric
field and a uniform magnetic field [itex] \underline{E} = E_0 \hat{z} [/itex] and [itex] \underline{B} = B_0 \hat{x} [/itex] in frame S.
In another frame S', moving with velocity along the y-axis with respect to S, the electric field is zero.
What must be the velocity v and the magnetic field in the frame S' ?
Show that the particle moves in S' in a circle of radius [itex] R=m\gamma^2 v / (q B_0) [/itex] What are the equations in [itex] x', y', z, t' [/itex] which describe the trajectory of the particle in the moving frame S' ?
By transforming from frame S’ show that the path of the particle in the original frame S is:
[itex] \gamma^2 (y-vt)^2 + (z-R)^2 = R^2 [/itex]
Homework Equations
Transformations of electric and magnetic fields for boosts in y-direction:
[itex] E'_x = \gamma (E_x + \beta c B_z) [/itex]
[itex] E'_y = E_y [/itex]
[itex] E'_z = \gamma (E_z - \beta c B_x) [/itex]
[itex] B'_x = \gamma (B_x - (\beta / c) E_z ) [/itex]
[itex] B'_y = B_y [/itex]
[itex] B'_z = \gamma (B_z + (\beta / c) E_x ) [/itex]
Lorentz Force:
[itex]\underline{F} = m \gamma \frac{d v}{d x} = m \gamma \frac{v^2}{R} = q \underline{B} \times \underline{v} [/itex]
The Attempt at a Solution
Only the E field in the z-axis exists and, as stated in the problem, is zero:
[itex] E'_z = \gamma (E_0 - \beta c B_0) = 0 \rightarrow v=E_0/B_0 [/itex]
And similarly only the B field in the x-axis has a solution, and using the equation for v above:
[itex] B'_x = \gamma (B_0 - (\beta / c) E_0 ) = \gamma B_0 (1 - E^2_0/c^2 B^2_0 ) = B_0 / \gamma[/itex]
Using the Lorentz force equation above:
[itex] m \gamma \frac{v^2}{R} = q B_0/\gamma \rightarrow R=m\gamma^2 v / (q B_0) [/itex]
Now i doubt how to write equations in [itex] x', y', z, t' [/itex] which describe the trajectory of the particle in the moving frame S'.
I think it should be:
[itex] (y'^2 + z'^2) = R^2 [/itex]
As it is in the yz-plane(right?)
If i transform back with [itex] y' = \gamma (y - vt) [/itex] and [itex] z'=z [/itex] I just get:
[itex] \gamma^2 (y - vt)^2 + z^2 = R^2 [/itex]
Missing the [itex] (z-R)^2 [/itex] term.
Can anyone see where I've gone wrong or what I've missing?
Any help appreciated. Been driving me crazy.
Thanks in advance!