Fields are used in almost every part of physics. In Physics 7C we will concentrate on the gravitational, electric and magnetic fields. We have some experience dealing with gravity from Physics 7A and 7B, so we will gain our intuition on fields from concentrating on gravity. After discussing the gravitational field, we will not be able to do anything we could not before, but it should make discussing less familiar fields easier.
After dealing with gravitational fields we develop electric fields and magnetic fields. Here using fields will be much more useful and easier than thinking about objects interacting directly with one another. The last section in this unit shows that electric and magnetic fields are closely related and can propagate – a phenomenon we commonly refer to as "light”! For this problem, fields are indispensable: neglecting them would lead to a violation of both the conservation of energy and conservation of momentum! So when we are learning how to calculate the same quantity with or without using fields (as we will do here under the names of "direct method" and "field method") it is worth keeping in mind that there is a good reason for going through this!
By the end of this chapter you should have familiarized yourself with the notion of field and potential, and be comfortable with how these things are different from force and potential energy. You should also be familiar with three different representations of the gravitational and electric field: the field map representation, the field line representation and the equipotential representation. The magnetic field is quite different in character. For the magnetic field, only the field map and field line representations will be useful to us.