The Electronvolt (CIE A Level Physics)

• The electronvolt is a unit which is commonly used to express very small energies
• This is because quantum energies tend to be much smaller than 1 joule
• The electronvolt is derived from the definition of potential difference:

• Where:
  • V = potential difference (V)
  • E = energy (J)
  • Q = charge (C)

• When an electron travels through a potential difference, energy is transferred between two points in a circuit or electric field
• If an electron, with a charge of 1.6 × 10^-19 C, travels through a potential difference of 1 V, the energy transferred is equal to:

• Therefore, an electronvolt is defined as:

The energy gained by an electron travelling through a potential difference of one volt

• To convert between eV and J:
  • eV → J: multiply by 1.6 × 10^-19
  • J → eV: divide by 1.6 × 10^-19

Relation to kinetic energy

• When a charged particle is accelerated through a potential difference, it gains kinetic energy
• If an electron accelerates from rest, an electronvolt is equal to the kinetic energy gained:

• Where:
  • e = charge of an electron (1.60 × 10^–19 C) 
  • V = potential difference (V)
  • m = mass of the particle (kg)
  • v = velocity of the particle (m s^–1)
• Rearranging the equation gives the speed of the electron: