5.1 - Electron

Thermionic Emission and Cathode Rays

• Thermionic emission releases free electrons from a heated metal surface. Heating a tungsten filament gives its free electrons enough kinetic energy to escape.
• Cathode rays are beams of electrons travelling at high speed through a vacuum.

Component	Function
6 V d.c. supply and tungsten filament	Heat the cathode to release electrons.
Extra high tension (E.H.T.) supply	Establish a potential difference between the cathode and anode. The positive anode attracts and accelerates emitted electrons.
Evacuated glass tube	Allows electrons to accelerate without collisions with air molecules and the resulting energy loss.
Milliammeter	Detects current when the electron beam completes the E.H.T. circuit.

• Reversing the E.H.T. connections gives no milliammeter reading.
• Coating the cathode with barium oxide or strontium oxide lowers the temperature needed to release electrons.
• A thermionic diode is a non-ohmic component; its current–voltage graph is not a straight line through the origin.

Electric Field and the Deflection Tube

• The deflection tube contains a cathode, anode, deflection plates and fluorescent grid. Switch $S_1$ controls the 6 V heater supply, $S_2$ controls the accelerating E.H.T. supply, and $S_3$ controls the E.H.T. supply across the plates.

Condition	Observation and interpretation
$S_1$ and $S_2$ on	Cathode rays travel in a straight line.
$S_1$, $S_2$ and $S_3$ on	The beam follows a parabolic path towards the positive plate, showing that cathode rays carry negative charge.
Deflection-plate polarity reversed	The beam deflects towards the newly positive plate.

• Precautions: Do not touch metal parts while the E.H.T. supply is on. Switch off the E.H.T. supply when observations are not being made.

Magnetic Field and the Maltese Cross Tube

• The Maltese cross tube contains an opaque cross and fluorescent screen. A 6 V supply heats the cathode; an E.H.T. supply accelerates the electrons. A bar magnet near the tube produces the magnetic field.

Condition	Observation and interpretation
Heater only	The cross blocks light from the hot filament and casts a shadow: light travels in a straight line.
Heater and E.H.T. supply on	The cross blocks cathode rays and casts a shadow; fluorescence appears around it. Cathode rays travel straight and possess momentum and kinetic energy.
Magnet brought near the operating tube	Two shadows appear: one from filament light and another from the magnetically deflected cathode rays.

• Use Fleming’s left-hand rule to determine the cathode-ray deflection direction.
• Precautions: Earth the Maltese cross tube properly. Do not touch metal parts while using E.H.T.; switch off the E.H.T. supply between observations.

Electron Energy and Maximum Velocity

• An electron accelerated towards a positive anode through potential difference $V$ gains electrical energy $E=eV$.
• In the vacuum tube, electrons accelerate without collisions and reach their maximum velocity at the anode.
• By conservation of energy, electrical potential energy becomes maximum kinetic energy:

• $e=1.6\times 10^{-19}\mathrm{C}$ is the magnitude of an electron’s charge; $m=9.11\times 10^{-31}\mathrm{kg}$ is its mass.
• $V$ is the cathode–anode potential difference in volts; $E$ is in joules and $v_{\max }$ in $\mathrm{m}{\mathrm{s}}^{-1}$.
• For a fixed electron mass and charge, $v_{\max }\propto \sqrt{V}$.

Worked Example: Acceleration Through 550 V

• An electron beam accelerates through 550 V in a vacuum.
• Electrical energy gained:

• Maximum kinetic energy is also $8.8\times 10^{-17}\mathrm{J}$.
• Maximum velocity: