Stephanie Understanding how electrostatic forces cancel Each sketch below shows three objects with an electric charge. In each case, decide whether there is a net force acting on the object outlined in green. If there is a net force, decide whether it pushes the green-ring object to the left or right. Then select the appropriate button under the sketch. For example, if there is a net force pushing the green-ring object in the first sketch to the left, select the left button under the first sketch. There is no net force on the green-ring object in the second sketch. Select the middle button under the second sketch. And so on. 90
The Correct Answer and Explanation is:
Like charges repel, unlike charges attract, and the force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.
General Strategy:
- Identify the Charges: Determine the sign (positive or negative) of each charge.
- Look at Distances: Electrostatic force decreases with distance, so closer charges exert stronger forces.
- Direction of Force: Remember, attraction pulls the green object toward the opposite charge; repulsion pushes it away from the same charge.
- Balance of Forces: If forces from both sides are equal in magnitude and opposite in direction, they cancel out — net force is zero.
Example Explanation (for a typical sketch):
Suppose you have three objects in a straight line:
- Left object: Positive charge
- Middle object (green-ringed): Negative charge
- Right object: Negative charge
Let’s analyze:
- The green-ringed negative charge attracts the positive charge on the left → force pulls left.
- The green-ringed negative charge repels the negative charge on the right → force pushes left.
- Both forces act leftward, so there is a net force to the left.
However, if:
- The green-ringed object is equally distant from two equal charges of the same sign on either side, forces cancel.
- For example, positive on left and positive on right, same distance → green-ringed charge is pushed equally left and right → no net force.
Summary:
You must:
- Compare charge signs
- Compare distances
- Understand direction of attraction/repulsion
If all forces on the green-ringed object are equal and opposite, there’s no net force. Otherwise, it moves toward the stronger force — either left or right.
