- #1
fluidistic
Gold Member
- 3,923
- 261
Homework Statement
I got no credit in an exam for the following exercise and I've been told what's wrong but even then I am unable to solve it correctly.
A conducting sphere with radius R moves with constant velocity ##\vec v =v \hat x## (v <<c) inside a constant magnetic field ##\vec B =B \hat y##. Find the induced charge distribution on the sphere to 1st order in v/c in the laboratory inertial reference frame.
Homework Equations
Fields transformations: ##\vec E '=\gamma (\vec E +\vec \beta \times \vec B ) - \frac{\gamma^2}{\gamma +1} \vec \beta (\vec \beta \cdot \vec E )##
##\vec B' =\gamma (\vec B -\vec \beta \times \vec E ) - \frac{\gamma ^2}{\gamma +1} \vec \beta (\vec \beta \cdot \vec B )##.
Lorentz boost.
##\rho = \gamma \rho'##. Where rho is the charge distribution in the laboratory K and rho' the charge distribution in another inertial reference frame moving with constant velocity with respect to K.
The Attempt at a Solution
My idea was to consider K', an inertial reference frame that would move alongside the conducting sphere. From there, calculate the E' and B' fields "seen" by the sphere. From there, calculate the induced surface charge density ##\sigma '##. And from there simply convert using the fact that ##\sigma = \gamma \sigma '##.
So here I go:
Using the formulae for fields transformations, ##\vec E'=\frac{\gamma v}{c}\hat z## and ##\vec B'=\gamma B \hat y##. Thus for the reference frame K', there's a static EM field. Now the sphere is a perfect conductor so the E' field inside of it must vanish. So that there's a discontinuity in the E' field that is proportional to the surface charge density sigma'. Mathematically, ##\hat n \cdot (\vec E' - \vec E'_{\text{inside}})=\sigma ' /\varepsilon _0##. Where ##\hat n = \hat r'## and as I said, ##\vec E'_{\text{inside}}=\vec 0##.
Furthermore, ##\hat z' \cdot \hat r' = \cos \theta '##.
Therefore ##\sigma '(\theta ') = \varepsilon_0 \frac{\gamma v}{c}\cos (\theta ')##. Now ##\sigma = \varepsilon_0 \frac{\gamma ^2 v}{c}\cos (\theta ')## but I must transform back ##\theta '## into non primed coordinates. I won't do it though, because this answer is completely wrong according to my professors and I don't deserve any credit.
Instead, according to them, what I should have done is to calculate the electric field generated by the surface charge density. The total electric field seen in K' will be the sum of the field I had calculated plus the field due to this charge density.
That's basically what I have been told. But then what? I wasn't asked to calculate the fields... I was asked the charge density. So if I understand them well, I must calculate this new E' field, and from it recalculate the new charge density? This doesn't make any sense to me, I could do this process forever, i.e. calculate the E field generated by a charge density, then calculate the new charge density, then calculate the new E field, then calculate the new charge density, etc. So why should I stop at step 1 or 2? Is it because they asked "to 1st order in v/c" in the problem statement?