c) energy transfer

• explain how an electron gun produces a beam of electrons

• a metal filament is heated by an electric current

  • the electrons in the metal filament gain kinetic energy

• some electrons gain enough kinetic energy to escape the surface of the metal filament

  • thermionic emission = the emission of electrons through heat

• a high potential difference is applied between the metal filament and an anode

  • the metal filament acts as a cathode

  • the free electrons accelerate through the vacuum towards the anode

  • the free electrons gain kinetic energy

• the free electrons in line with a hole in the anode can travel through it

  • a beam of electrons with a specific kinetic energy is produced

• define accelerating potential difference

• the potential difference used to increase the speed of a charge carrier

• describe how a change in the accelerating potential difference changes the speed of a charge carrier

• the greater the potential difference

• the more energy is transferred to the charge carriers

• the faster their speed

• explain how the formula for energy transfer is derived from the formula for work done

• W = VQ

  • work done on one coulomb of charge (J) = potential difference (V) x charge (C)

• W = eV

  • work done on one electron = accelerating potential difference (V) x elementary charge

• due to the law of the conservation of energy:

• W = KE

  • work done on one electron = gain in kinetic energy

• eV = (1/2)mv^2