Concept of Field Lines, how Electricity vs Gravity

[flyingpig]

Homework Statement

Take the Earth for instance, if I were to draw field lines, would it be field lines coming out of the mass or into the mass? I initially thought it would be going out because we could look at it as a "positive charge", but then I thought that was silly because gravity is going "down", so which is right...?

I mean if I were to use a "positive test charge" (a mass), the mass would attract to the core of the Earth regardless of the "charge" of the mass.

 

[AlexChandler]

The direction of the field lines by convention have the same direction as the field vectors. So that would be towards the earth. And you can't really make the comparison to electric fields. There is a fundamental difference. You cannot think of gravity as something like electricity with only one charge. Remember like charges repel.

The big difference between gravitational field lines and electric field lines is that gravitational field lines come from infinity and end at the "source". Electric field lines always start at a positive charge and end at a negative charge, they never come from infinity.