Thermionic Emission

Explain why the anode must be at a positive potential

electrons carry negative charge so anode needs to be positive (to attract them) (1)

why is it essential that the container is evacuated

electrons from wire would be absorbed/scattered/stopped by gas atoms or collide with gas atoms and lose kinetic energy or speed (1)

what is meant by thermionic emission

filament heated by an electric current [or metal heated by nearby hot wire filament] (1) (conduction) electrons in the metal gain sufficient kinetic energy to leave the metal/cathode/filament

Explain why the electron beam curves downwards at an increasing angle to its initial direction

force due to electric field acts (vertically) downwards on electrons vertical (component) of velocity of each electron increases horizontal (component of) velocity unchanged (so angle to initial direction increases) Plates provide resultant force on beam in a downward direction for the entire width of the plate

Explain why the electrons pass undeflected through the fields when their speed v is given by v= V / Bd

magnetic (field) force = Bev and electric (field) force = eV / d (Accept Q or q as symbol for e (charge of electron) Magnetic force=electric force Bev = eV/d (for no deflection) gives v = V / Bd

Explain why thermionic emission is negligible when the filament current is too low.

temperature of filament depends on the current or low current so small heating effect] (1) kinetic energy of electrons depends on temperature of filament (1) electrons must do work (or overcome work function) to leave metal (1) electrons have insufficient (kinetic) energy to leave metal/cathode/filament (or overcome work function) if the current is too low (1)

How would you make the beam have zero deflection

Adjust the strength of the magnetic field so that it's equal to the electric field