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I'm studying electronics and circuits.
I'm trying to understand how electrons behave as they move in a circuit, but there's something that still bothers me.
Electrons have some potential energy (the voltage between terminals) when leaving the cathode of the battery, but as they move in the circuit under the effect of the electric field, it gets transformed in kinetic enrgy ( or heat/sound...).
Now, Do the electron arrive at the anode always with zero kinetic energy(they consume it)? or do they still have some?
I will try to make a better argument with the help of an analogy.
Suppose I consider the hydraulic analogy with a pump the makes work and transfer the water molecules at higher (gravitational) potential.
If I imagine a very low friction tube, almost all potential energy will be converted to kinetic energy. The water molecules will arrive at the pump, with a velocity and they will be given a push by the pump.
So it seems they will go faster and faster till they might hit a limit speed due to the shape of the tube.
Will the same argument make sense for electrons? What if they don't consume all their kinetic energy?
I was reading an interesting related question, and some comment stated that :"A vacuum tube is an almost perfect "frictionless place to 'drop' electric charge". When the electrons hit the anode they are going very fast - and turn into heat (emitting bremsstrahlung)."

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  • $\begingroup$ In high power RF tubes elaborate focusing techniques make sure that electrons do not reflect back from the anode but completely absorbed closing the DC bias circuit. Such scattering would not only increase the noise but also would complicate cooling as the electrons would lose their kinetic energy at the walls or on the inside metal parts. $\endgroup$ – hyportnex Apr 20 '18 at 17:24
  • $\begingroup$ have a look here hyperphysics.phy-astr.gsu.edu/hbase/electric/miccur.html . cathode ray tubes generate beams of electrons, but electrons within a lattice move with an average drift velocity. $\endgroup$ – anna v Apr 20 '18 at 18:18
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When electrons leave cathode, they accelerate and gain a very high speed, many orders of magnitude greater than the speed of current (drifting electrons) in a wire.

So, when the electrons hit the anode, all their kinetic energy turns into heat, which makes anode very hot, i.e., there is not speed or kinetic energy left to somehow affect the speed of the electrons inside the anode.

The analogy with the water pump would better reflect the process in the vacuum tube, if the water was pumped from a lake up to some very high level and then would fall down back to the lake, making a good splash and losing all its kinetic energy on the impact.

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