Thermionic emission

Question 1

What is thermionic emission?

Answer 1

Thermionic emission is the process whereby a material releases electrons when heated.

Question 2

How are thermionic emitters compared?

Answer 2

Thermionic emitters are compared in terms of their work function, which is the work that must be performed by an electron in escaping from the body, expressed in units of electron volts (eV).

Question 3

What determines the effectiveness of thermionic emitters?

Answer 3

Substances that are good thermionic emitters (metals) have a lower work function than those that are poor thermionic emitters. TE depends on the temperature, material, and surface area of the body.

Question 4

What are examples of good thermionic emitters and their work functions?

Answer 4

Tungsten: 4.5 eV

Thoriated Tungsten: 2.5 eV

Oxides of Alkaline Earths (e.g., Calcium, Barium): 1.0 – 1.3 eV

Question 5

How is thermionic emission used in radiography?

Answer 5

• The filament of the X-ray tube is heated by passing an electric current through it.
• It releases thermionic electrons and forms a space charge around the filament.
• A positive potential is applied to the anode of the X-ray tube, attracting the space charge and producing X-rays.

Question 6

What is the role of thermionic emission in a vacuum diode?

Answer 6

TE ensures that electric current will only pass in one direction (rectifier).

Question 7

How is TE used in exposure switching circuits and constant potential circuits?

Answer 7

Specialized vacuum or gas-filled valves such as the triode or thyratron use the principle of TE.

Question 8

What is the space charge effect?

Answer 8

The space charge effect refers to the collection of electrons emitted from the metal surface, forming a cloud of negative charge that repels and limits further emission of electrons.

Question 9

What is a vacuum diode?

Answer 9

A vacuum diode is a device that allows electric current to pass in one direction only, from the cathode to the anode, while blocking current from anode to cathode.

Question 10

Define work function

Answer 10

The work function is the minimum energy needed to remove an electron from the surface of a material.

Question 10

How does a vacuum diode work?

Answer 10

It works on the principle of thermionic emission. When the cathode is heated, electrons are emitted and attracted to the anode, allowing current to pass through.

Question 11

what are the factors that facilitates the number of generated electrons at the cathode

Answer 11

• Amount of heat applied
• Work function