The discharge tube
A discharge tube is a glass tube containing gas at low pressure, with a negative cathode and positive anode connected to a large potential difference.
- The electric field ionises gas atoms, producing positive ions and free electrons.
- Electrons accelerate towards the positive anode.
- Positive ions accelerate towards the negative cathode.
- When positive ions strike the cathode, they can release further electrons from it.
- These electrons are accelerated towards the anode and form the cathode ray.
Why the gas conducts
The charged particles produced by ionisation move through the tube under the action of the electric field. Electrons travel towards the anode while positive ions travel towards the cathode, so charge is transported through the gas.
The pressure must be low enough for the charged particles to travel significant distances between collisions. This allows them to gain substantial kinetic energy as they accelerate.
Why the gas glows
- Accelerated electrons collide with gas atoms and transfer energy to them.
- The gas atoms are left in excited states.
- Electrons within the atoms later fall to lower energy states and photons are emitted.
- Some of the emitted photons are in the visible part of the spectrum, producing the glow.
Electrons and positive ions may also recombine, with photons emitted as the resulting excited atoms de-excite.
Exam Tip: For an explanation of the glow, link low pressure to greater distance between collisions, then to increased kinetic energy, excitation of gas atoms and photon emission during de-excitation.