Unit 18: Coulomb’s Law & Electric Force
Practice Problems
Note: problem difficulty is ranked using a star system.
(*) One-star problems are fundamental to the unit, and can be done relatively quickly. Use these problems to introduce yourself to the material.
(**) Two-star problems are more difficult, and require an understanding of one or two key concepts. Use these problems to test your understanding of the material.
(***) Three-star problems are the most difficult, and require some creative thinking in addition to a deep familiarity with multiple key concepts. Use these problems to challenge yourself; if you can complete one of these, you’re on your way to mastering the material.
*Q18.1) What force would an electron feel if it were placed in a northward 100 N/C electric field?
*Q18.2) What force would an electron feel if it were placed 20nm from a calcium ion?
**Q18.3) A charged metal sphere of mass 50g is placed between charged parallel plates so that it hovers perfectly in place (the weight of the sphere is perfectly balanced by the electric force acting on it). If the electric field between the plates is 300 N/C, what’s the charge of the sphere?
***Q18.5) Two identical 100g metal spheres of charge q are suspended from cables as shown below. The spheres repel each other electrostatically and move apart until an equilibrium is reached with both cables angled 25 degrees from the vertical. If the cables are 1m in length, what’s the magnitude of the charge q?
**Q18.4) Three ions are placed at the vertices of an equilateral triangle of sidelength 10nm as shown below. Find the magnitude and direction of the force on the chlorine ion.
