Draw gravitational field lines between 2 masses, electric field lines between a positive and a negative charge, electric field lines between 2 positive charges and magnetic field lines around a magnet. Qualitatively describe the differences between the fields and the entities responsible for the field lines.

A gravitational field is caused by any object with mass, and gravitational field lines point radially inward towards the center and extend back to infinity.

Notice that the more densely spaced field lines are associated with the more massive body, i.e., the field strength is proportional to the density of lines, shown in the image below.

An electric field is caused by an electric charge, positive or negative. The electric field lines originate from a positive charge and terminate in a negative charge. Images below illustrate the electric field of two unlike and like point charges, respectively.

A magnetic field describes the magnetic influence on moving electric charges and magnetic materials. The electric current is the source of all magnetism, which essentially means that a magnetic field is caused by electric currents. Furthermore, the north and south magnetic poles cannot be separated, and a magnetic monopole is (as of yet) a theoretical entity. Magnetic field lines form a closed loop, emerging out of the north poles and converging at the south poles.