• When metals are?heated, the conduction electrons within them gain?energy
• If these electrons gain sufficient energy, they are able to leave the surface of the metal
  • This is known as?thermionic emission
  • This is?similar?to the?photoelectric?effect, but the energy absorbed by electrons in this case is due to?thermal?energy, rather than the energy absorbed by incident?photons

• Once electrons are released from a metal surface they may be accelerated by?electric?or?magnetic fields

Electrons are emitted from the (negative) cathode and accelerated to the (positive) anode

Step 1: List the known quantities

• • Potential difference = 5 kV = 5000 V
  • Mass of an electron,?me?= 9.11 × 10–31?kg
  • Electron charge?e?= 1.6 × 10–19?C

Step 2: Equate the kinetic energy gained by the electron to the energy transferred across the potential difference

• • Potential difference?V?is the energy transferred?W?per unit of charge?Q, or
  • Since the charge in this case is an electron,?Q?=?e?and so?W?=?eV
  • Therefore, the kinetic energy gained is equal to?eV?so we can write:

Step 3: Make speed?v?the subject of the equation

• • Rearranging this equation for?v?gives:

Step 4: Substitute quantities and calculate the speed?v

• • Substituting known quantities gives:

= 41 908 313... = 4.2 × 107?m s–1