Field Patterns
Hang up three small paper balls, and hold a big paper ball among them. Rub them with a plastic bag. All the paper balls will gain positive charges.
Look at the big paper ball. This repels each one of the small balls. The direction of force on each small ball is always along the line joining the big ball and small ball.
If we move a small ball around the big ball, the direction of the force from the big ball is always along the line joining them. So we can draw lines radiating from the big ball to show the possible directions of the forces.
This picture is called a field pattern. There is not actually any line around the positive charge. The picture is just a way to help us tell the direction of the force at each point, when we put a small ball there.
We add an arrow to each line to show the direcion of this force.
If we put a small ball with a positive charge at a point in between two field lines, then the direction of the force from the big ball is along a line that is roughly in between those two lines.
What if the small ball has a negative charge? Then it is attracted by the big ball. The force is still along the field line, but the direction now points towards the big ball instead of away from it.
So just remember that if you put a negative charge in a field pattern, the direction of the force is opposite to thhe direction of the arrow.
The field pattern for a negative charge looks the same as that for a positive charge, except that the arrow directions must be reversed. This is because the rule is that the arrow always points in the direction of the force if you put a small positive charge in the field.
What if we have two large balls that are positively charged? The field lines from one ball crosses the field lines from the other ball. If we put a small positive charge in the field, there is a force from one large ball, and a force from the other large ball. These two forces are in different directions, and give a resultant force that points somewhere in between.
It is the resultant force that determines the combined field pattern. At each point, The new field lines must go in the direction of the resultant force on a small charge at that point. This is what the field pattern for two positive balls should look like.
For one positive and one negative ball, the field pattern looks like this.
