# Cathode rays

## Main Question or Discussion Point

During discharge tube experiment, the cathode rays are emitted from cathode at very low pressure...it is said that these rays move at very high velocity(if we apply high Potential Difference between 2 electrodes)...do these rays( stream of electrons) travel in uniform velocity or aren't they accelerated due to presence of external electric field???why do we directly find out the velocity of electron by the formula v=sq.rt(2*p.d.*e/m) ??? i am confused whether the velocity of electrons is uniform or it is being accelerated by the external electric field??

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ZapperZ
Staff Emeritus
During discharge tube experiment, the cathode rays are emitted from cathode at very low pressure...it is said that these rays move at very high velocity(if we apply high Potential Difference between 2 electrodes)...do these rays( stream of electrons) travel in uniform velocity or aren't they accelerated due to presence of external electric field???why do we directly find out the velocity of electron by the formula v=sq.rt(2*p.d.*e/m) ??? i am confused whether the velocity of electrons is uniform or it is being accelerated by the external electric field??
While the electrons are emitted with some velocity, the overwhelming majority of the energy/speed picked up by the electrons are from the applied field. Remember, these are electrons! They are charged particles. Charged particles can interact with an external electric field. So you apply such a field to cause them to speed up.

If you apply a potential difference of V, then the energy gained by the electrons is eV = 1/2 mv^2 (assuming non-relativistic). So calculate your speed from there (assuming it started with 0 or negligible speed, which is a valid assumption in most cases).

P.S. This is not a "Quantum Physics" topic and has been moved out of that forum.

Zz.

dont they accelerate in the electric field???every charged particle in electric field is accelerated....then what and at what time is the velocity we calculate from above formula??

NascentOxygen
Staff Emeritus
dont they accelerate in the electric field???every charged particle in electric field is accelerated....then what and at what time is the velocity we calculate from above formula??
You have a tube with a cathode at one end. Where is the anode?

ZapperZ
Staff Emeritus