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Exercises with Magnetic Monopoles - Kurt Nalty

Exercises with Magnetic Monopoles - Kurt Nalty

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Electron Motion in a Radial <strong>Magnetic</strong> Field<br />

The force law for a pure electron in an electromagnetic field is<br />

⃗F = q e<br />

(<br />

⃗E + ⃗v × ⃗ B<br />

)<br />

A pure magnetic field can do no work. Force is at right angles to motion,<br />

and results only in a change of direction. Consequently, the speed is constant,<br />

and the velocity and acceleration are always orthogonal.<br />

To describe the motion of the electron, I will use the Frenet-Serret approach,<br />

and identify the curvature and torsion formulas for the electron in<br />

the radial magnetic field. I assume that Newtonian mass and acceleration<br />

still apply in this scenario.<br />

⃗B = µq m ⃗r<br />

4π r 3<br />

⃗F = q e<br />

(⃗v × B ⃗ )<br />

= µq eq m ⃗v × ⃗r<br />

4π r 3<br />

F<br />

⃗a = ⃗ m = µq eq m 1 ⃗v × ⃗r<br />

4π m r 3<br />

We see above that ⃗a is normal to velocity, and see below that speed will<br />

be constant.<br />

⃗a · ⃗v = 0 = 1 d<br />

(⃗v · ⃗v)<br />

2 dt<br />

= 1 dv 2<br />

2 dt<br />

v 2 = const<br />

The magnitude of curvature in three dimensions is<br />

⃗κ =<br />

⃗a × ⃗v<br />

v 3<br />

av sin θ<br />

κ =<br />

v 3<br />

= a for ṽ ⊥ ã<br />

v 2<br />

21

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