Learn how electrons are released from a heated metal surface through thermionic emission, how cathode rays are produced and accelerated in a vacuum tube, and how their behaviour in electric and magnetic fields reveals their properties.
See how electrons travel in straight lines, deflect towards a positively charged plate in an electric field, and deflect in a magnetic field according to Fleming's left-hand rule.
When the deflection plates are switched on, cathode rays curve towards the positive plate in a parabolic path. This proves that cathode rays are negatively charged.
A sharp shadow of the Maltese cross is formed on the fluorescent screen. When a magnet is brought near, the cathode ray shadow shifts while the light shadow stays centred.
Thermionic emission is the emission of free electrons from a heated metal surface. When a tungsten filament is heated by a 6 V power supply, free electrons gain enough kinetic energy to escape from the metal surface.
Cathode rays are high-velocity electron beams formed when electrons emitted from the cathode are attracted to the anode in a vacuum tube.
The vacuum state allows electrons to accelerate towards the anode without colliding with air molecules. This prevents energy loss and allows electrons to achieve maximum velocity.
The E.H.T. power supply provides a high potential difference across the cathode and anode to accelerate electrons toward the anode.
Electrical potential energy of an electron
where \(e\) is the charge of an electron and \(V\) is the potential difference between the cathode and anode.
Conservation of energy in a cathode ray tube
Electrical potential energy is converted into maximum kinetic energy of the electron.
Maximum velocity of an electron
where \(m\) is the mass of an electron. The maximum velocity is directly proportional to the square root of the potential difference: \(v_{\text{max}} \propto \sqrt{V}\).
Answer in your mind, then press “Check Answer”.
What is thermionic emission, and how does coating the filament with barium oxide affect the process?
Why must the cathode ray tube be evacuated to a state of vacuum?
An accelerating potential difference of \(800\text{ V}\) is applied across the cathode and anode. What is the maximum velocity of an electron? (\(e = 1.6 \times 10^{-19}\text{ C}\), \(m = 9.11 \times 10^{-31}\text{ kg}\))
Cathode rays are electromagnetic waves like X-rays or gamma rays.
Cathode rays are fast-moving streams of negatively charged particles (electrons) with mass, charge, and momentum.
Both shadows in the Maltese cross experiment are caused by cathode rays.
One shadow is caused by light emitted from the hot glowing filament, which is unaffected by magnetic fields. The second shadow is formed by cathode rays, which are deflected by a magnetic field.
Objective: Identify thermionic emission, cathode ray properties, and electron velocity calculations.
1 Which statement correctly explains why cathode rays deflect towards the positive plate in a deflection tube?
2 What happens to the maximum velocity of electrons in a cathode ray tube if the potential difference across the anode and cathode is quadrupled?
3 The process of emitting free electrons from a heated metal surface is called emission.
4 In a cathode ray tube, the electrical potential energy converted into kinetic energy is given by the formula \(E =\) .
Drag each component to the correct function. If using a phone, tap the answer first, then tap the matching box.
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