What is an electron volt?

Electron volts, like joules, are a unit of energy. This may sound confusing- why two? But think of it this way; length has two possible units- metric (metres) and imperial (inches). We can use either units to measure length and can switch between the two. The same is true of joules and electron volts. We define electron volts as 'the energy required to accelerate an electron across a potential difference of 1 volt'. From this definition, we can use an equation from electronics W=VQ, where W is work done (energy), V is potential difference and Q is charge. In our definition, our energy is electron volts (which we use the symbol eV for), the potential difference is 1V and our charge is the charge of an electron, e (which has a value of 1.6 x 10^-19 C). Substituting these values into W=VQ we get 1 eV = 1.6 x 10^-19 J. We now have a way of converting between the two units of energy! If we want to find what 10 eV is as joules, we just multiply both sides of the equation by 10. As you can see, an electron volt is a very small unit of energy so is only really used in particle and nuclear physics.