Coulomb Force (Static)
Vocabulary: Coulomb’s law, electrostatic force, vector
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
Have you ever taken clothes out of the dryer and found a sock stuck to your underwear? Static cling is an example of electrostatic forces, or the forces that exist between charged objects.
How do you think the sock and underwear became charged?
Suppose two socks acquire the same charge. Do you think they would stick together?
As clothes are tumbled in a dryer, electrons are rubbed off some items, giving them a positive charge, and deposited on other items, giving them a negative charge. These charged items exert electrostatic forces on one another. You can explore these forces with the Coulomb Force (Static) Gizmo™.
In its initial settings, the Gizmo shows two objects that each have a charge (q) of 10.0 × 10-4 C (coulombs). Turn on the Show force vector checkboxes for objects A and B. The arrows coming from each object are vectors that represent the electrostatic force. The direction and length of each vector show the direction and magnitude (strength) of each force.
Are the vectors for objects A and B pointing together or away from each other?
Are objects A and B attracted together or repelled apart?
Compare the lengths of the vectors. What do you notice?
The effect of charge
Get the Gizmo ready:
Turn on Show grid. Place object A on the x-axis at -5 and object B on the x-axis at +5.
Check that Show force vector is turned on for each object.
Question: How does charge affect the strength of the electrostatic force?
Observe: You can change the charge of each object by entering the desired value in the qA and qB boxes. Observe the force vectors for each of the situations listed in the table below. Based on the force vectors, state whether the objects are repelled from one another, attracted to one another, or if there is no force at all.
Make a rule: Complete the following sentences with the words “attract,” “repel,” or “zero.”
When the charges are the same, the two objects one another.
When the charges are opposite, the two objects one another.
When one of the objects has no charge, the resulting force is.
Predict: How do you think the magnitude of the electrostatic force between two objects will change if the charge of each object was doubled?
Measure: Turn on Show vector notation for both objects. Set the charge of objects Aand B to 1.0 × 10-4 C. The force on object A is now -0.9i + 0j N. That means that the force is
-0.9 N in the x direction and 0.0 N in the y direction.
What is the magnitude of the force on object A? |FA| =
What is the magnitude of the force on object B? |FB| =
The force on object A is negative. What does this indicate about the direction of the force? (Activity A continued on next page)
Activity A (continued from previous page)
Gather data: For each charge combination listed in the table below, write magnitude of the force on object A. (Note: The magnitude of the force, or its strength, is always positive.)
Analyze: What patterns do you notice in the data?