Cathode Rays
What is a Cathode?
- An electrode is a conductor through which electricity passes
- A cathode is a negatively charged electrode
- An anode is a positively charged electrode
Discharge Tubes
- In the 1800s, scientists made discharge tubes
- These were glass chambers containing a low pressure gas, with an anode at one end and a cathode at the other, connected to a high voltage supply
- When a potential difference was applied between the anode and cathode, the gas glowed
- It was hypothesised that this glow was caused by emissions from the cathode, called cathode rays
A discharge tube
What are Cathode Rays?
- When a magnetic field was applied to the glass tube, it was found the path of the cathode rays was deflected
- This showed they were made from negatively charged particles
How does the Discharge Tube Conduct?
- The electric field between the electrodes ionises the gas particles in the tube
- This separates atoms into positive ions and electrons
- Negative electrons are attracted to the positive anode
- Positive ions are attracted to the negative cathode
- This can only happen because the pressure of the gas is low enough to allow the charged particles to travel
- Electrons are also emitted from the cathode and travel towards the anode
- Conduction is a result of these electrons and positive ions moving across the tube
Why does the Gas Glow?
- Electrons and positive ions are travelling in opposite directions in the tube
- Due to the low pressure, they have space to gain a large amount of energy in their kinetic store
- When they collide, they recombine in an excited state
- The electrons in atoms de-excite to ground state, emitting visible photons (as well as other frequencies)
Worked example
A discharge tube contains a low pressure gas. An anode is at one end and a cathode at the other with a large potential difference between the two. The gas conducts and also emits light.
(i) Explain how the gas conducts, referring to the charge-carrying particles in your answer.
(ii) Explain why the gas must be at low pressure to emit light.
Answer:
(i)
Step 1: Recall that, for the gas to conduct, we need charged particles
- The electric field ionises gas atoms, removing electrons and forming positive ions
- The cathode also emits electrons (at very high potential difference)
Step 2: Recall why the particles move across the discharge tube
- The electric field accelerates electrons and positive ions, which cause conduction
(ii)
Step 1: Light is emitted when electrons and ions recombine
- Positive ions and electrons collide at high speed and recombine, emitting photons when they de-excite
Step 2: Light is emitted when an atom is excited
- Accelerated electrons collide with gas atoms, exciting them
- The gas atoms emit visible photons when they de-excite
Step 3: Describe a low pressure gas in terms of the distribution of particles
- In a low pressure gas, the particles are widely spaced
Step 4: Recall why this allows atoms to become excited more easily
- There are fewer obstacles for accelerating charged particles, so they can collide with enough energy to produce excited atoms (which then go on to emit light)
