   Next: Example 3.2: Electrostatic force between Up: Electricity Previous: Worked Examples

## Example 3.1: Electrostatic force between three colinear point charges

Question:  A particle of charge is located on the -axis at coordinate . A second particle of charge is placed on the -axis at . What is the magnitude and direction of the total electrostatic force acting on a third particle of charge placed at the origin ( )?

Solution: The force acting between charges 1 and 3 is given by Since , the force is repulsive. This means that the force exerted by charge 1 on charge 3 is directed along the -axis (i.e., from charge 1 towards charge 3), and is of magnitude . Thus, . Here, we adopt the convention that forces directed along the -axis are positive, and vice versa. The force acting between charges 2 and 3 is given by Since , the force is attractive. This means that the force exerted by charge 2 on charge 3 is directed along the -axis (i.e., from charge 3 towards charge 2), and is of magnitude . Thus, .

The resultant force acting on charge 3 is the algebraic sum of the forces exerted by charges 1 and 2 separately (the sum is algebraic because all the forces act along the -axis). It follows that Thus, the magnitude of the total force acting on charge 3 is , and the force is directed along the -axis (since ).   Next: Example 3.2: Electrostatic force between Up: Electricity Previous: Worked Examples
Richard Fitzpatrick 2007-07-14