 What is an electron volt? Well, it's a little tiny unit of energy. It's equivalent to 1.60 times 10 to the negative 19 joules. But it's also the amount of energy that an electron, or any other particle with an elementary charge, that's a charge of 1.60 times 10 to the negative 19 coulombs, would receive if it moved through an electric field that was made by a power source of exactly 1 volt. Let's look at an example. Here's a uniform electric field made by connecting two parallel plates to a 20 volt power source. The electron is placed at the negative plate, and it will accelerate towards the positive plate because of the electric force. And this means that the electron will gain some kinetic energy because it starts moving. We can calculate the amount of energy gained by the electron by using the formula electric potential difference equals change in energy over charge. The potential difference is 20 volts. The charge is 1.6 times 10 to the negative 19 coulombs. So the energy gain of the electron in joules is 3.2 times 10 to the negative 18 joules. So what is that kinetic energy in electron volts? We need to take this energy in joules and divide it by the conversion factor of 1.60 times 10 to the negative 19 joules per electron volt. This gives us 20 electron volts. But wait, that's the exact same number we had when we started. So that means that an electron or any particle with an elementary charge that moves through an electric field made by, say, a 20 volt power supply gains 20 electron volts of energy. And this works for more than just electrons. If a proton is moved through this same electric field, it would also gain 20 electron volts of energy, since a proton has an elementary charge just like an electron. If we use a particle with double the elementary charge, like this alpha particle, it will gain double the amount of energy. So a total of 40 electron volts when moving through the electric field created by a power source of 20 volts.