Metals are made up of atoms vibrating in a lattice structure, within a sea of electrons.When the metal is heated up all of the particles (atoms and free electrons) gain kinetic energy. If given enough heat, which equates to enough kinetic energy, some of the electrons at the surface of the metal are given enough energy to leave the metal.
Work function WORK FUNCTION
The work function of a metal is the minimum energy needed to remove the loosest electrons from the surface of that metal. £
Fig 21.2: Lattice structure of atoms in a metal, showing the free electrons
Free electrons
Atoms
For thermionic emission, the metal usually needs to be heated to a temperature in excess of 800 C.
Cathode Ray Tube A cathode ray tube is made up of:
• A glass tube from which most of the air has been evacuated, i.e. a vacuum • A thin filament through which a small current flows; the hot filament heats the cathode
• Two electrodes; the anode has a hole in it and is positive with respect to the cathode, which is near the filament
• A fluorescent screen, which shows where the cathode ray strikes.
Fluorescent screen
Heated filament
Low voltage 0 Fig 21.3: Cathode ray tube A cathode ray tube operates in the following manner
• A low voltage (e.g. 6 V) is placed across the filament, current flows through the filament and the filament becomes hot (in accordance with Joule’s law) and heats the cathode.
• Thermionic emission occurs at the cathode.The cathode is therefore the source of the electrons.
