Saturday, 13 May 2017

homework and exercises - Electron in the proximity of a magnetic monopole



I am puzzling about an exercise in the book "Electromagnetic Theory" by Ferraro (p.543).


An electron (mass m, charge e) in a monopole magnetic field B(r)=grr3 has due to the Lorentz force FL=ev×B the following equation of motion:


m¨r=egr3˙r×r.


Since J=r×p+egrr is constant, the electron moves on the surface of a cone, which tip is in the origin at r=0. Mathematically, rJ=egr=rJcosϑ, thus the angle ϑ between r and J is constant.


Now I use spherical coordinates:


r=rer

v=˙r=˙rer+r˙ϑeϑ+r˙φsinϑeφ
a=¨r=(¨rr˙ϑ2r˙φ2sin2ϑ)er+(2˙r˙ϑ+r¨ϑr˙φ2sinϑcosϑ)eϑ+(2˙r˙φsinϑ+2r˙ϑ˙φcosϑ+r¨φsinϑ)eφ


and obtain the following equations of motion (using ˙ϑ=0):


¨r=r˙φ2sin2ϑ(1)

r˙φ2cosϑ=egmr˙φ(2)
2˙r˙φ+r¨φ=0(3)
with the boundary conditions: r(0)=r0φ(0)=0.


In order to solve (1) for r(t), one has to use the fact that the kinetic energy 12mv2 is conserved. This holds due to the vector potential A=12B×r=0. Therefore, v2=˙r2+r2˙φ2sin2ϑu2=const


Using this in (1) one gets the simplified differential equation: r¨r=u2˙r2(4)



Does anyone know how to derive the following solution of (4)? r(t)=r01+(utr0)2(5)


Solving (2) for ˙φ: ˙φ=egmr2cosϑ


and inserting (5) I obtain: φ(t)=egmr0ucosϑarctanutr0.


Now the solutions says: r(φ)=r0cos(φsinϑ).


But I get something similar, but not equal: r(φ)=r0cos(mr0ucosϑegφ).


Has anyone an idea where I could have done something wrong?



Answer



Actually, I don't have to use the conservation of the kinetic energy. With a slightly different procedure, one gets another simplified differential equation.


Inserting (2) in (1):


¨r=e2g2tan2ϑm21r3



Now look at:


ddt˙r2=2˙r¨r=e2g2tan2ϑm22˙rr3=ddt(e2g2tan2ϑm21r2)


Thus, a simpler differential equation is obtained (C=const):


˙r2=e2g2tan2ϑm21r2+C


and can be solved by separation of variables:


dt=r drCr2e2g2tan2ϑm2


t=1CCr2e2g2tan2ϑm2+t0


With t0=0 and r(0)=r0:


r(t)=e2g2tan2ϑm2r20t2+r20


Now solving for φ(t) from (2):



˙φ=egmcosϑ1r2


dφ=mcosϑr20egsin2ϑdtt2+(mr20egtanϑ)2


Using the arctangent integral:


dtt2+b2=1barctantb


and φ(0)=0, one obtains:


φ(t)=1sinϑarctanegtanϑ tmr20


Finally,


r(φ)=r0tan2(sinϑφ)+1=r0cos(sinϑφ)


No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...