Indiana Science HS-PSII-4.3
The Standard
Using Coulomb's law, pictorially and mathematically describe the force on a stationary charge due to other stationary charges. Understand that these forces are equal and opposite as described by Newton’s third law and compare and contrast the strength of this force to the force due to gravity.
Indiana Academic Standards for Science (2023) · Electricity
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students draw force arrows on a chosen charge based on the signs and positions of nearby charges. They use Coulomb's law to calculate each force and combine the forces to find the net force. They compare electric force with gravity.
What Mastery Looks Like
- A student draws arrows in the right directions and calculates the net force in newtons. They identify the equal, opposite force on the other charge, not on the chosen charge. They explain that electric force can attract or repel and is much stronger than gravity between two protons.
Common Misconceptions
- Students may add force strengths without checking directions. They may think Newton's third-law forces cancel because they act on the same charge. Some think gravity can repel or that it is about as strong as electric force.
How to Assess It
- Exit ticket: Put +2 nC at 0 cm, +1 nC at +10 cm, and −1 nC at −10 cm. Draw and calculate the net force on +2 nC, then draw its equal, opposite force on +1 nC.
Lesson moves
Ways to Teach It
Hang two rubbed balloons from strings, mark their positions, and draw equal, opposite force arrows on a sketch of both balloons.
Ask partners to write why the equal, opposite forces between two charges do not cancel the force on either charge.
Place +1 nC, +2 nC, and −1 nC cards 10 cm apart in a row; race to calculate the middle card's net force.
Show a video of a charged balloon lifting paper scraps; sketch electric and gravitational force arrows on one scrap and explain why it rises.
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Printable HS-PSII-4.3 Worksheet
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