 You probably hear the word field in your physics class a lot and maybe you've been hearing about electric fields a lot lately. So what is an electric field and what are they even used for? First of all, an electric field is a model for explaining how electric forces work. Electric forces are pretty different than a push or pull kind of force because they act over a distance, they get stronger the closer you are and they can attract a repel. So how can you explain all that? Well, first you have to come up with a set of rules called electric field theory. Here are the basics. 1. All charged objects create an electric field. 2. The electric field can be represented by field lines, which are these arrows that start at the object and go out to infinity or start at infinity and eventually run into the object. And 3. The electric field lines move towards negative objects and away from positive objects. We can also say that the field lines move in the direction a positive test charge would move. That is, if you were to put a little tiny positive charge near the source of charge you're studying, whatever way that positive would move, either towards or away from the source charge, because of attraction or repulsion, is the direction of the electric field in that area. The field lines are sort of like vectors, but their lengths don't indicate their strengths, like a regular vector. Recall these lines actually go on forever. So we use the density of the lines or how closely they're spaced to indicate strength. Close together electric field lines means a stronger electric field. There are lots of different shapes of electric fields, but a few common ones are non-uniform radial fields made by point charges like this. You might recognize that these look very similar to the gravitational field lines that we talked about back in the gravitational field line video of physics 20. We have concentrated fields at points like this. And the exact opposite, if we have nice flat charged objects like these charged parallel plates, we get uniform electric fields, where the field strength is the same absolutely everywhere and it always points in the same direction. Note again that these electric field lines move from positive to negative. The same direction a positive test charge would move if we put it inside the plates and just let it go. We have three different formulas to mathematically describe electric fields. The first one is electric field strength equals the electric force divided by charge. And it tells us that the electric field acting on a test charge, Q, is based on the electric force acting on that test charge. It's important to remember that this charge Q is the charge of the test charge. The object sort of feeling the electric field. This isn't the charge of the source which is creating the electric field. And you might note that if we rearrange this formula, it looks a lot like the force of gravity formula. Force of gravity equals mass times acceleration due to gravity. This idea of electric field theory is really closely related to gravitational field theory from back in physics 20. A second formula to describe electric fields is the absolute value of the electric field equals KQ over R squared. This formula comes out of Coulomb's law and only describes non-uniform electric fields produced by point charges. In this formula, K is Coulomb's constant, 8.99 times 10 to the 9 newton meter squared per Coulomb squared. And Q is the charge of the source. That's the object that's producing the electric field. R is the separation from that source charge to the point in space where you're measuring the electric field. Again, it's really important to note here that this Q, this charge, is the charge of the object making the electric field. This is not the charge of the test charge. The third formula only applies to uniform electric fields. The absolute value of the electric field strength equals the electric potential difference divided by D where V is the electric potential difference between any two points inside of the parallel plates and D is the distance between those two points and meters. This formula only applies in these uniform electric fields. So why do we even care about electric fields? Well, one common application is using them to describe the movement of charged particles. Let's take our uniform electric field and put a positive charged particle in there. Note the electric field between these plates is pointing down away from the positive top plate and toward the negative bottom plate. The positive charge we put in the electric field experiences an electric force downwards in the downwards electric field. This makes sense. The positive object is attracted to the negative bottom plate and repelled from the positive top plate. So moving downwards seems logical. So what we've learned here is that for a positive particle, the electric field and electric force are always in the same direction. A negative particle will do the opposite to the positive particle. For a negative, the electric force and the electric field are always in opposite directions. These are really important statements to remember. Electric fields help us work out how electric forces work. And from a study of forces, we can work out many other aspects of the movement of these charges. Check out these videos to see more of the calculations you can get into to describe charged particles in electric fields.