Cathode Rays (AQA A Level Physics)
Revision Note
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:
Part (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
Part (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)
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