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)=g→rr3 has due to the Lorentz force →FL=−e→v×→B the following equation of motion:
m¨→r=−egr3˙→r×→r.
Since →J=→r×→p+eg→rr is constant, the electron moves on the surface of a cone, which tip is in the origin at →r=0. Mathematically, →r⋅→J=egr=rJcosϑ, thus the angle ϑ between →r and →J is constant.
Now I use spherical coordinates:
→r=rer
and obtain the following equations of motion (using ˙ϑ=0):
¨r=r˙φ2sin2ϑ(1)
In order to solve (1) for r(t), one has to use the fact that the kinetic energy 12m→v2 is conserved. This holds due to the vector potential →A=12→B×→r=0. Therefore, →v2=˙r2+r2˙φ2sin2ϑ≡u2=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)=r0√1+(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(e2g2tan2ϑm2−1r2)
Thus, a simpler differential equation is obtained (C=const):
˙r2=e2g2tan2ϑm2−1r2+C
and can be solved by separation of variables:
dt=r dr√Cr2−e2g2tan2ϑm2
t=1C√Cr2−e2g2tan2ϑm2+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)=r0√tan2(sinϑφ)+1=r0cos(sinϑφ)
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