PHYSICS F5 : ELECTRONICS (5.1 Electron)

What is thermionic emission?

The emission of free electrons from a heated metal surface.

How does thermionic emission occur?

• A tungsten filament is heated by a 6 V DC power supply.

• The temperature of the filament increases.

• Free electrons gain sufficient kinetic energy.

• Electrons escape from the surface of the metal.

• These emitted electrons can be accelerated towards the anode.

Why is tungsten used as a filament?

• It has a high melting point.

• It can withstand high temperatures without melting easily.

How can thermionic emission be improved?

Coating the cathode with metal oxides such as barium oxide or strontium oxide reduces the temperature needed to release electrons.

What are cathode rays?

A beam of high-speed electrons produced in a vacuum tube.

How are cathode rays produced?

• The heating filament heats the cathode.

• Thermionic emission releases electrons.

• The positively charged anode attracts the electrons.

• The electrons accelerate towards the anode.

• The moving electrons form a high-speed electron beam called cathode rays.

Func. of each component in a cathode ray tube

Heating filament	Heats the cathode
Cathode	Emits electrons
Anode	Attracts and accelerates electrons
Fluorescent screen	Produces a visible glow when struck by electrons
Vacuum tube	Allows electrons to travel without colliding with air molecules

Why must a cathode ray tube be in a vacuum?

• Prevents electrons from colliding with air molecules.

• Prevents energy loss through collisions.

• Allows electrons to travel at high speed.

What are the characteristics of cathode rays?

• Travel in straight lines.

• Carry negative electric charges.

• Can be deflected by an electric field.

• Can be deflected by a magnetic field.

• Possess kinetic energy and momentum.

• Produce a fluorescent effect when they strike certain materials.

• Can be blocked by opaque objects to produce shadows.

How do cathode rays behave in an electric field?

• Cathode rays are negatively charged.

• They are attracted towards the positively charged plate.

• They are repelled by the negatively charged plate.

• Their path bends towards the positive plate in a parabolic path.

What happens when the electric field is reversed?

The direction of deflection is reversed because the positions of the positive and negative plates are exchanged.

How do cathode rays behave in a magnetic field?

• Cathode rays experience a magnetic force.

• Their direction changes when they enter a magnetic field.

• The direction of deflection can be determined using Fleming’s left-hand rule, with conventional current taken opposite to electron motion.

Func. of a Maltese cross tube

To demonstrate that cathode rays travel in straight lines by producing a shadow of the Maltese cross on a fluorescent screen.

What does the shadow produced by the Maltese cross prove?

Cathode rays travel in straight lines and can be blocked by an opaque object.

Why does a fluorescent screen glow?

Because cathode rays transfer energy to the fluorescent material when they strike it.

Velocity of electrons in a cathode ray tube

When electrons move from the cathode towards the anode, electrical potential energy is converted into kinetic energy.

What happens to electrons when the E.H.T. power supply is switched on?

Electrons accelerate towards the positively charged anode and reach maximum velocity at the anode.

What is the energy transformation?

Electrical potential energy → Kinetic energy.

What is the formula for electrical potential energy?

E = eV

E = electrical potential energy (J)

e = charge of an electron (C)

V = potential difference (V)

State the principle of conservation of energy for an electron in a cathode ray tube.

Electrical potential energy = Maximum kinetic energy.

What happens when the E.H.T. voltage increases?

The maximum velocity of electrons increases.