# Thermionic Emission(Edexcel International A Level Physics)

Author

Katie M

Expertise

Physics

## Thermionic Emission

• 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

#### Worked example

Electron guns use electric fields to accelerate electrons to very high speeds.

Show that an electron accelerated from rest across a potential difference of 5.0 kV attains a speed of 4.2 × 107 m s–1

Use the following data:

• Mass of an electron me = 9.11 × 10–31 kg
• Electron charge e = 1.6 × 10–19 C

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

#### Exam Tip

Examiners commonly test if candidates can equate the energy gained across a potential difference with the kinetic energy of a particle, as we did here. Make sure you can combine the equations for kinetic energy and potential energy in order to calculate speed like we did here!

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