In article ,
clifto wrote:

isw wrote:
With electrostatic (but not magnetic) deflection, there is an increase
in the momentum of the electrons -- they are accelerated sideways while
traversing the deflection plates.

I'll show my ignorance: why doesn't magnetic deflection cause an increase
in momentum? Seems to me the electrons would be accelerated sideways while
traversing the magnetic field.

Fundamentally, magnetic fields are conservative; you can't get work out
of them. If an electron came out of the field moving faster than when it
went in, work must have been done on it. That's why those
geometry-correcting magnets stuck all over CRTs don't "run down".

Think of it this way:

The effect of magnetic deflection is a function of the electron's
velocity -- a stationary electron will not be deflected (accelerated) at
all, and high velocity ones are deflected faster than low velocity ones
(because a moving electron acts like a current-carrying conductor).
Assuming a uniform field (and in a well-designed yoke, it's pretty
close), the math works out such that while a slower electron spends more
time in the field than a fast one, they both wind up ultimately being
deflected by the same angle.

With electrostatic deflection, the lateral force an electron feels is
unrelated to its forward velocity -- a stationary one will still be
accelerated towards the positive plate. The result is that slower
electrons spend more time under the influence of the plates, and so are
deflected more than faster ones.

Even if all the electrons in the beam left the cathode with precisely
the same velocity (and despite best effort, they don't), they'd still be
moving in different directions -- remember that cone? It's the "forward
vector", not the absolute velocity, that determines how long an electron
is influenced by the deflection field, so all electrons not on the axis
of the cone spend more time being deflected.

Isaac