"Today I'm going to work with you on a new concept and that is the concept of what we call electric field. We spend the whole lecture on electric fields. If I have a -- a charge, I just choose Q, capital Q and plus at a particular location and at another location I have another charge little Q, I think of that as my test charge. And there is a separation between the two which is R. The unit vector from capital Q to li- little Q is this vector. And so now I know that the two charges if they were positive -- let's suppose that little Q is positive, they would repel each other. Little Q is negative they would attract each other.

And let this force be F and last time we introduced Coulomb's law that force equals little Q times capital Q times Coulomb's constant divided by R squared in the direction of R roof. The two have the same sign. It's in this direction. If they have opposite sign it's in the other direction. And now I introduce the idea of electric field for which we write the symbol capital E..."