AQA A Level Physics

Revision Notes

2.4.1 The Electronvolt

The Electronvolt

  • 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:

4. The Electronvolt equation 1

  • 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:

E = QV = 1.6 × 10-19 C × 1 V = 1.6 × 10-19 J

  • Therefore, an electronvolt is defined as:

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

1 eV = 1.6 × 10-19 J

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:

eV = ½ mv2

  • Rearranging the equation gives the speed of the electron:

4. The Electronvolt equation 2

Worked Example

Show that the photon energy of light with wavelength 700nm is about 1.8 eV.

Step 1: Write the equations for wave speed and photon energy

The Electronvolt Worked Example equation 1

The Electronvolt Worked Example equation 2

Step 2: Calculate the photon energy in Joules

The Electronvolt Worked Example equation 3

Step 3: Convert the photon energy into electronvolts

1 eV = 1.6 × 10-19 J                 J → eV: divide by 1.6 × 10-19

The Electronvolt Worked Example equation 4

Exam Tip

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

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