Electronvolts are a unit particularly useful in atomic and nuclear physics and can be defined as follows:
Electronvolt (eV) – one electronvolt is the energy change of an electron when it passes through a potential difference of 1V.
As an electron accelerates through a potential difference the electric field does work on the electron that is equal to the charge of the electron (e) multiplied by the change in voltage (V).
The charge of an electron (e) is approximately 1.6 x10-19C and so when an electron passes through a potential difference of 1V (this is equivalent to 1JC-1) the total work done (W) is:
The work done is equal to one electronvolt and so we know that:
1eV = 1.6 x10-19J
How to Convert Between Electronvolts and Joules
To convert from electronvolts (eV) to joules (J) you simply need to multiply by 1.6×10-19.
To convert from joules (J) to electronvolts (eV) you do the opposite and divide by 1.6×10-19.
The image below is a useful reminder of how to convert between the two:
Worked Examples
Example 1
Convert each of the below values:
- 2.5eV to joules
- 1.8 x10-19J to electronvolts
- 7eV to joules
Example 2
Calculate the energy of a photon of UV radiation with a frequency of 3 x1016Hz, giving your answer in electronvolts. Use 6.63 x10-34Js for Planck’s constant.
This question requires knowledge of photons and Planck’s equation.
Example 3
A photon releases an electron from an iron plate. The kinetic energy of the electron is 5eV and the work function of iron is 4.5eV. Calculate the energy of the photon in joules.
This question requires knowledge of Einstein’s Photoelectric Equation.
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