Edexcel A Level (IAL) Physics-4.33 Thermionic Emission- Study Notes- New Syllabus
Edexcel A Level (IAL) Physics -4.33 Thermionic Emission- Study Notes- New syllabus
Edexcel A Level (IAL) Physics -4.33 Thermionic Emission- Study Notes -Edexcel A level Physics – per latest Syllabus.
Key Concepts:
- understand that electrons are released in the process of thermionic emission and how these can be accelerated by electric and magnetic fields
Thermionic Emission and Acceleration of Electrons
Thermionic emission is the process by which electrons are released from a metal surface when it is heated. Once emitted, electrons can be accelerated and deflected using electric and magnetic fields.
Thermionic Emission
Definition: Thermionic emission is the emission of electrons from the surface of a metal when thermal energy is supplied.
- Occurs in metals containing free (conduction) electrons.
- Heating the metal increases the kinetic energy of electrons.
- Some electrons gain enough energy to escape the metal surface.
Work Function
- Electrons are held in the metal by attractive forces.
- The work function is the minimum energy needed to remove an electron.
- Thermionic emission occurs when electron energy exceeds the work function.
Thermionic Emission in a Vacuum Tube
- A metal filament (cathode) is heated by an electric current.
- Electrons are emitted into the surrounding vacuum.
- A vacuum prevents collisions with air molecules.
Acceleration of Electrons by an Electric Field
When electrons move through a potential difference, they experience an electric force.
\( F = qE \)
- \( q \) = charge of electron
- \( E \) = electric field strength
Effects of an electric field:
- Electrons accelerate towards the positive terminal (anode).
- Their speed increases.
- Kinetic energy gained equals electrical work done.
\( \text{Gain in kinetic energy} = qV \)
Motion of Electrons in a Magnetic Field
Moving electrons in a magnetic field experience a force perpendicular to their motion.
\( F = Bqv\sin\theta \)
- \( B \) = magnetic flux density
- \( v \) = speed of electron
- \( \theta \) = angle between velocity and field
Important features:
- Magnetic force does not change speed, only direction.
- When motion is perpendicular to the field, electrons move in a circular path.
- The magnetic field provides the centripetal force.
Comparison: Electric vs Magnetic Fields
| Electric Field | Magnetic Field |
|---|---|
| Changes speed of electrons | Changes direction of electrons |
| Force parallel to field direction | Force perpendicular to motion and field |
| Acts on stationary charges | Acts only on moving charges |
Applications
- Cathode ray tubes
- Oscilloscopes
- Electron guns
- Television and display technology
Example (Easy)
Explain why heating a metal causes electrons to be emitted.
▶️ Answer / Explanation
- Heating increases electron kinetic energy.
- Some electrons gain energy greater than the work function.
- These electrons escape the metal surface.
Example (Medium)
Describe the effect of an electric field on electrons emitted from a cathode.
▶️ Answer / Explanation
- Electrons accelerate towards the anode.
- Their speed increases.
- Their kinetic energy increases.
Example (Hard)
An electron enters a magnetic field perpendicular to the field direction. Describe its subsequent motion.
▶️ Answer / Explanation
- A magnetic force acts perpendicular to its velocity.
- The force provides centripetal acceleration.
- The electron moves in a circular path at constant speed.