By looking at the concentration of field lines, you can get an idea of the strength of the field. Look at the last few problems. Note that near the charges the lines are concentrated far from them, they are dispersed. Two large, flat plates are separated by a small distance and are oppositely charged. Find the direction of the field at points A, B, and C. Sketch the total field.
Parallel plates are important in physics, because they can generate uniform electric fields.
Note that the direction and concentration of lines is unchanged from top to bottom and side to side. One large, flat plate is positively charged. Find the direction of the field at points near its surface. Sketch the total field. As long as you stay close to the plate, the direction of the field is always directly away from the plate that's easy to see.