Field transformations in the z-direction

7 replies · 3K views · last reply
milkism
Messages
118
Reaction score
15
Homework Statement
Transformation rules fot the fields in a lorentz boost in the z-direction.
Relevant Equations
Look at attempt at a solution.
Question:
Printed text: Write down the transformation rules for the electric and magnetic fields (i.e. analogues of Eq. 12.109 of the textbook) for a Lorentz boost in the z direction.

Eq. 12.109:
Six field transformation equations from Griffiths, Eq. 12.109: Ex-bar = Ex; Ey-bar = gamma(Ey - v Bz); Ez-bar = gamma(Ez + v By); Bx-bar = Bx; By-bar = gamma(By + (v/c^2) Ez); Bz-bar = gamma(Bz - (v/c^2) Ey).

My solution:
We’ll first use the configuration from figure 12.35 in the book Griffiths. Where the only difference is
that v_0 is in the z-direction. The electric field in the y-direction will be the same.
Two 3D diagrams of parallel charged plates: left shows plates labeled plus and minus sigma-0 with length l-0 and width w, axes y-0, x-0, z-0; right shows plates labeled plus and minus sigma with length l and width w, axes y, x, z, with arrows labeled v-0 pointing left.

$$E_y = \frac{\sigma}{\epsilon _0}$$
Now we're going to derive the general rule, following just like how it's done in the book Griffiths. The surface current will look like:
$$\mathbf{K_{\pm}}=\mp \sigma v_0 \hat{\mathbf{z}}$$
By the right-hand rule the field would point in the positive x direction, with the magnitude as:
$$B_x = \mu_0 \sigma v_0.$$
The third system $\overline{S}$, would ofcourse travel in the z direction instead of the x direction.
Diagram of three coordinate frames S0, S and S-bar, with S moving at v0 relative to S0 and S-bar moving at v-bar relative to S0, drawn in the x-y plane.

Equation 12.101: gamma-bar over gamma-0 equals the square root of (1 minus v0^2/c^2) over the square root of (1 minus vbar^2/c^2), which equals (1 + v v0/c^2) over the square root of (1 minus v^2/c^2), which equals gamma times (1 + v v0/c^2). Equation 12.102: gamma equals 1 over the square root of (1 minus v^2/c^2).

The fields will look like:
$$ \overline{E_y} = \frac{\overline{\sigma}}{\epsilon _0}$$
$$\overline{B_x} = \mu_0 \overline{\sigma} \overline{v_0}$$
Now we just need to express $\overline{\mathbf{E}}$ and $\overline{\mathbf{B}}$ in terms of $\mathbf{E}$ and $\mathbf{B}$.
We will use the same equations 12.98, 12.99, 12.101 and 12.102 to do the algebra because they didn't change.
$$\overline{E_y} = \gamma \left( E_y + vB_x \right)$$
$$\overline{B_x} = \gamma \left( B_x + \frac{v}{c^2}E_y \right)$$
To find $E_x$ and $B_y$, the configuration will be identical to figure 12.36.
Diagram of two large parallel charged plates in 3D axes, with x, y and z axes; left plate labeled +σ, right plate labeled −σ, separated by distance d, with plate width w and length l marked.


The fields in $S$ will be as follows:
$$E_x = \frac{\sigma}{\epsilon_0}$$
$$B_y = \sigma \mu_0 v_0$$
The fields in $\overline{S}$ will look like:
$$\overline{E_x} = \gamma \left( E_x + vB_y \right)$$
$$\overline{B_y} = \gamma \left( B_y + \frac{v}{c^2}E_x \right)$$
For the z component of the electric fields will be same and for the magnetic will also be the same, if we take a solenoid parallel to the z-axis.
Diagram of a long solenoid lying along the x-axis, with coil windings drawn as loops around a cylinder, arrows inside indicating current direction, and the label I below; axes x, y and z are shown.
But the solenoid being on the z-axis.
$$\overline{B_z} = B_z$$
$$\overline{E_z} = E_z$$
I'm not exactly sure if I'm right, because it all depends if your configuration is correct.
 
Trying reading Purcell's as a complement. It uses SR to introduce E&M.
 
Last edited by a moderator:
ChiralSuperfields said:
Trying reading Purcell's as a complement. It uses SR to introduce E&M.
Can't find anything about field transformations in his book, can you point out what I did wrong?
 
Okay I have tried to do it with different configurations.
New solution:
Handwritten notes on squared paper with four sketched configurations of charged plates or sheets and axes, with field relations such as E_y = sigma/epsilon_0, B_x = -mu_0 sigma v_0, and boosted expressions for E and B.

At configuration 1 (top configuration), I concluded the following:
$$E_y = \frac{\sigma}{\epsilon _0}$$
$$\mathbf{K_{\pm}} = \pm \sigma v_0 \mathbf{\hat{z}}$$
$$B_x = -\mu _0 \sigma v_0$$
$$\overline{E_y} = \gamma \left( E_y - vB_x \right)$$
$$\overline{B_x} = \gamma \left( B_x - \frac{v}{c^2} E_y \right)$$
At configuration 2 (middle left), I concluded the following:
$$\overline{E_z}=E_z$$
At configuration 3 (bottom left), I concluded the following:
$$\overline{B_z} = B_z$$
At configuration 4 (bottom right), I concluded the following:
$$E_x = \frac{\sigma}{\epsilon_0}$$
$$B_y = \mu _0 \sigma v_0$$
$$\overline{E_x} = \gamma \left( E_x + vB_y \right)$$
$$\overline{B_y} = \gamma \left( B_y + \frac{v}{c^2} E_x \right)$$

Personal questions:
Since in griffiths they have boosted at a negative x directio, should I also have boosted in a negative z direction?
I think my second solution is more correct than my first one.
 
milkism said:
Can't find anything about field transformations in his book, can you point out what I did wrong?
Sorry I have only read a bit of purcell's so I am not up to that level yet.
 
The transformations in the second solution are correct, but indeed you should have moved the capacitor in the negative z-direction because this corresponds to you moving in the positive z-direction relative to the rest of space and thus a positive boost in the z-direction.