Cathode rays

Cathode rays are a stream/beam that travels away from the cathode (negative electrode) towards the anode (positive electrode) in an evacuated (or low-pressure) tube when a high voltage is applied.

Early experiments showed these “rays” behave like particles and are the same regardless of the gas used. They were later identified as electrons (negatively charged particles).

Evidence that cathode rays travel in straight lines (Maltese cross)

Set-up: A Maltese cross is placed in the path of the cathode rays inside the tube, with a fluorescent/glowing screen at the far end.

Observation: A sharp shadow of the cross appears on the screen.

Conclusion: Cathode rays must be travelling in straight lines from the cathode to the screen.

Evidence that cathode rays are particles with momentum (paddle wheel)

Set-up: A very light paddle wheel is placed so that the cathode ray beam strikes the paddles.

Observation: The wheel rotates when the tube operates.

Conclusion: The beam must transfer momentum (and kinetic energy), so cathode rays behave like particles.

Evidence cathode rays are charged (deflection by electric and magnetic fields)

Set-up: A cathode ray beam is produced in a tube. The beam passes through a region where an electric field can be applied (e.g. between charged plates). A magnetic field can also be applied across the beam using magnets/coils.

Observation:

• With an electric field, the beam bends toward the positive side.

• With a magnetic field, the beam curves, and the direction changes if the magnetic field direction is reversed.

Conclusion: Electric deflection shows the beam is charged, and bending toward + shows it is negative. Magnetic deflection shows the particles are moving charges. Overall: cathode rays are moving negative particles (electrons).

Electrons can be produced in two key ways:

• Gas discharge / ionization tubes: At low pressure, a high voltage produces a strong electric field which pulls the electrons out of the gas atoms. (i.e. ionises the gas). The freed electrons are then accelerated towards the anode by the electric field.

• Thermionic emission: Heating a metal cathode gives electrons enough energy to escape the surface.

Kinetic energy gained from an accelerating potential difference:

Once electrons are available, a potential difference accelerates them. Assuming the electrons starts at rest, we can find the speed using:

The glow in a discharge tube happens because fast electrons collide with gas atoms.

• High voltage creates some free electrons (ionisation as mentioned above).

• These electrons accelerate and collide with gas atoms, causing excitation (atomic electrons move to higher energy levels).

• When atoms de-excite, they emit photons (light) — many emissions produce the visible glow.

Pressure effect: more gas (higher pressure) causes more frequent collisions (brighter glow), but the beam becomes less sharp because collisions scatter the electrons. Very low pressure gives a clearer beam and sharper shadows/deflection.