16.1 Thermionic Emission
SLO 16.1.1 — Explain the process of thermionic emission
emitted from a filament
🔎 1. What happens inside a metal (Before heating)?
A metal contains:
Free electrons (conduction electrons)
Positive metal ions fixed in lattice
These electrons:
Move randomly inside the metal
But cannot escape from the surface
Why?
Because they are held inside by attractive electrostatic forces of positive ions.
To escape, an electron must overcome a minimum energy barrier called:
🔹 Work Function (Φ)
Work function = minimum energy required to remove an electron from metal surface.
Unit: electron volt (eV)
Different metals have different work functions.
🔥 2. What happens when filament is heated?
When we heat a metallic filament (usually tungsten):
Heat energy is supplied
Kinetic energy of electrons increases
Electrons move faster
, Some electrons gain enough energy to overcome work function
Those electrons escape from the surface.
This emission of electrons due to heating is called:
✅ Thermionic Emission
⚡ Step-by-Step Process
1. Filament is connected to power supply.
2. Current flows → filament becomes hot.
3. Thermal energy increases electron kinetic energy.
4. Some electrons overcome work function.
5. Electrons leave surface into surrounding space (vacuum).
🎯 Important Conditions
1⃣ High Temperature Required
,Higher temperature → more electrons emitted.
2️⃣ Low Work Function Metal Preferred
Lower work function → easier emission.
Example metals:
Tungsten (high melting point)
Oxide-coated cathodes (lower work function)
3️⃣ Vacuum Required
Why vacuum?
If air is present:
Emitted electrons collide with air molecules
Energy is lost
Beam cannot form properly
So thermionic emission occurs inside vacuum tubes.
🧠 Deep Conceptual Understanding (AKUEB Level)
🔹 Does heating create electrons?
No.
Electrons already exist in metal.
Heating only gives them energy to escape.
🔹 Does increasing voltage cause thermionic emission?
No.
Temperature controls emission.
Voltage only attracts emitted electrons.
🔹 What if temperature increases?
, Number of emitted electrons increases.
Emission rate increases exponentially.
🔹 What if work function increases?
Harder for electrons to escape.
Emission decreases.
📊 Relationship Concept
Emission depends on:
Temperature (T)
Work function (Φ)
Higher T → More emission
Higher Φ → Less emission
🔬 Why Tungsten Filament?
Very high melting point (~3400°C)
Can withstand high temperature
Strong metal
⚠️ Very Important Differences
Thermionic emission ≠ Photoelectric effect
Thermionic Emission Photoelectric Effect
Caused by heat Caused by light
Depends on temperature Depends on frequency
Used in vacuum tubes Used in solar cells
AKUEB loves this comparison.
SLO 16.1.1 — Explain the process of thermionic emission
emitted from a filament
🔎 1. What happens inside a metal (Before heating)?
A metal contains:
Free electrons (conduction electrons)
Positive metal ions fixed in lattice
These electrons:
Move randomly inside the metal
But cannot escape from the surface
Why?
Because they are held inside by attractive electrostatic forces of positive ions.
To escape, an electron must overcome a minimum energy barrier called:
🔹 Work Function (Φ)
Work function = minimum energy required to remove an electron from metal surface.
Unit: electron volt (eV)
Different metals have different work functions.
🔥 2. What happens when filament is heated?
When we heat a metallic filament (usually tungsten):
Heat energy is supplied
Kinetic energy of electrons increases
Electrons move faster
, Some electrons gain enough energy to overcome work function
Those electrons escape from the surface.
This emission of electrons due to heating is called:
✅ Thermionic Emission
⚡ Step-by-Step Process
1. Filament is connected to power supply.
2. Current flows → filament becomes hot.
3. Thermal energy increases electron kinetic energy.
4. Some electrons overcome work function.
5. Electrons leave surface into surrounding space (vacuum).
🎯 Important Conditions
1⃣ High Temperature Required
,Higher temperature → more electrons emitted.
2️⃣ Low Work Function Metal Preferred
Lower work function → easier emission.
Example metals:
Tungsten (high melting point)
Oxide-coated cathodes (lower work function)
3️⃣ Vacuum Required
Why vacuum?
If air is present:
Emitted electrons collide with air molecules
Energy is lost
Beam cannot form properly
So thermionic emission occurs inside vacuum tubes.
🧠 Deep Conceptual Understanding (AKUEB Level)
🔹 Does heating create electrons?
No.
Electrons already exist in metal.
Heating only gives them energy to escape.
🔹 Does increasing voltage cause thermionic emission?
No.
Temperature controls emission.
Voltage only attracts emitted electrons.
🔹 What if temperature increases?
, Number of emitted electrons increases.
Emission rate increases exponentially.
🔹 What if work function increases?
Harder for electrons to escape.
Emission decreases.
📊 Relationship Concept
Emission depends on:
Temperature (T)
Work function (Φ)
Higher T → More emission
Higher Φ → Less emission
🔬 Why Tungsten Filament?
Very high melting point (~3400°C)
Can withstand high temperature
Strong metal
⚠️ Very Important Differences
Thermionic emission ≠ Photoelectric effect
Thermionic Emission Photoelectric Effect
Caused by heat Caused by light
Depends on temperature Depends on frequency
Used in vacuum tubes Used in solar cells
AKUEB loves this comparison.
