 Now that we know what the electric field surrounding charged particles looks like, let's try to quantify these fields. Coulomb's law describes the force a charged particle will experience due to another charged particle. First, let's look at how we expect the equation to behave. In previous videos, we discussed how the force gets bigger as you get closer to the charge and smaller as you get further away from the charge. Let's say you had three negative particles. Particle A and B have charged negative one coulombs, while particle C has charged of negative five coulombs. Note that I've remembered to include the units on these charges. In case one, we have particle A and particle B separated by a distance of D. In case two, we have particle A and C also separated by the same distance. The force that B exerts on A will be smaller than the force C exerts on A. Remember back to the definition of electric fields. The electric force exerted on a charged particle per unit charge. The per unit charge is because the force depends on the charge the particle has, as demonstrated in this case. Newton's third law states that every action has an equal and opposite reaction. So A will exert the same amount of force back on B as B exerted on A, except in the opposite direction. Same with particle A and C. So coulombs' law has to model force getting smaller as distance gets larger and force getting larger as charge gets larger. Coulomb's law is given as F equals K times Q1 times Q2 divided by D squared, where F is force measured in Newton's. K is Coulomb's constant measured in Newton's times meter squared divided by Coulomb's squared. Q1 is the charge of the first particle measured in Coulomb's. Q2 is the charge of the second particle also measured in Coulomb's. And D is the distance between the particles measured in meters. As distance increases force decreases. And as the charge on particle 1 or particle 2 increases force also increases. This fits with our previous analysis of the situation. Coulomb's constant K is given by 1 over 4 pi epsilon naught, where epsilon naught is a constant. 8.854 times 10 to the negative 12 farads per meter to be precise. It's known as the permittivity of free space. But writing out a fraction every time is rather tedious, so we can just use K instead. However, both versions of Coulomb's law are equally valid.