Hello David,

 >
 > The electrons are travelling in a closed path - part of which
[..]
 > Also, of course, electrons that are travelling around curved
 > parts of the circuit must experience a centripetal force. Overall, the
 > net force on the electrons is zero, so there is no tendency for the
 > vacuum tube to accelerate through space.

If it did that would be a violation of the law of conservation of momentum.

Cheers, Michiel

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