1/r2. Gravitation obeys this law. The force between charges obeys this law. Why? Physicists in the 19th century used the sprinkler analogy. Imagine water coming out of a sprinkler going in all directions. The amount of water crossing an area obeys this, as the area of a sphere grows as r2.

 

The lines of force of masses (gravitation) or charges (electric forces) converge to a source.

 

Magnetic fields produced by currents are rings around the current. If you look at pictures of the surface of the sun, you will see giant circles of flame. These are due to the magnetic fields.

 

Here how the lines of force from an isolated positive electric charge look:

 

We see that it looks like force squirting out of the charge.

 

Here are lines of magnetic forces around a current. The current flows in the direction of the blue arrow. The magnetic lines of force are given by the red lines B:

 

Magnetic forces are loops, not force lines squirting out.

 

SEE THE BOOK FOR MORE DETAILS.