Point Charge
To help visualize how a charge, or a collection of charges, influences the region around it, the concept of an electric field is used. The electric field E is analogous to g, which we called the acceleration due to gravity but which is really the gravitational field. Everything we learned about gravity, and how masses respond to gravitational forces, can help us understand how electric charges respond to electric forces.
The electric field a distance r away from a point charge Q is given by:
Electric field from a point charge : E = k Q / r2
The electric field from a positive charge points away from the charge; the electric field from a negative charge points toward the charge. Like the electric force, the electric field E is a vector. If the electric field at a particular point is known, the force a charge q experiences when it is placed at that point is given by :
F = qE
If q is positive, the force is in the same direction as the field; if q is negative, the force is in the opposite direction as the field.
Simulation
a. Check the force at point A. Write your observation?
b. Check the force at point B. Write your observation?
c. What is the difference you feel between force A and B.
Check Your Understanding
1. Several electric field line patterns are shown in the diagrams below. Which of these patterns are incorrect? _________ Explain what is wrong with all incorrect diagrams.
Answer: C, D and E
In C, the lines are directed towards a positively charged object.
In D, the lines are not symmetrically positioned despite the fact that the object is a symmetrical sphere.
In E, the lines are directed away from a negative charge.
2. It is observed that Balloon A is charged negatively. Balloon B exerts a repulsive effect upon balloon A. Would the electric field vector created by balloon B be directed towards B or away from B? ___________ Explain your reasoning.
Answer: Towards B
If balloon B repels balloon A then balloon B must be negatively charged. The electric field vectors are always directed towards negatively charged objects. As such, the E vectors must be towards balloon B.