# Electric Forces and Electric Fields

Student Learning Objectives
Lessons / Lecture Notes
Important Equations
Example Problems
Applets and Animations
Videos

Student Learning Objectives

• To become familiar with basic electric phenomena.
• To learn the charge model and apply it to conductors and insulators.
• To understand polarization and the attraction between neutral and charged objects.
• To understand and use Coulomb’s law for point charges.
• To recognize and use the principle of superposition for electric forces.
• To understand the field model and the concept of a field.
• To learn the electric field of the point charge.
• To use the principle of superposition to calculate the electric field of multiple point charges.

Lessons / Lecture Notes

The Physics Classroom (conceptual)

PY106 Notes from Boston University (algebra-based):

Introductory physics notes from University of Winnipeg (algebra-based):

HyperPhysics (calculus-based)

PHY2049 notes from Florida Atlantic University (calculus-based):

PHY2044 notes from Florida Atlantic University (calculus-based)

General Physics II notes from ETSU (calculus-based)

Important Equations

Example Problems

Problem 1
Three point charges are fixed in place in a right triangle. What is the electric force on the –0.60-µC charge due to the other two charges? (Solutions)

Problem 2
(a) A tiny ball (mass = 0.012 kg) carries a charge of –18 µC. What electric field (magnitude and direction) is needed to cause the ball to float above the ground? (Solutions)

(b) Two charges are placed on the x axis. One charge (q1 = +8.5 µC) is at x1 = +3.0 cm and the other (q2 = -21 µC) is at x2 = +9.0 cm. Find the net electric field (magnitude and direction) at x = +6.0 cm. (Solutions)

Applets and Animations

 Balloons and Static Electricity Why does a balloon stick to your sweater? Rub a balloon on a sweater, then let go of the balloon and it flies over and sticks to the sweater. View the charges in the sweater, balloons, and the wall. John Travoltage Make sparks fly with John Travoltage. Wiggle Johnnie's foot and he picks up charges from the carpet. Bring his hand close to the door knob and get rid of the excess charge. Three Charged Particles The Three Charged Particles model displays three charged objects, one at each corner of an equilateral triangle. The charges have different magnitudes - your job is to rank the charges based on their magnitudes, from largest to smallest.  You can drag the charges around to see the effect on the net force on each charge. Coulomb's Law and E-Field Package The Coulomb's Law and Electric Field Package is a collection of models for electrostatics.  You can move charges around and see the force, you can observe the electric field generated by charge configurations and observe the motion of test particles in electric fields. Electric Field Lines Illustrating representing an electric field with field lines. Electric Field of Dreams Play ball! Add charges to the Field of Dreams and see how they react to the electric field. Turn on a background electric field and adjust the direction and magnitude. (Kevin Costner not included). Charges and Fields Move point charges around on the playing field and then view the electric field, voltages, equipotential lines, and more. It's colorful, it's dynamic, it's free. Electric Field Hockey Play hockey with electric charges. Place charges on the ice, then hit start to try to get the puck in the goal. View the electric field. Trace the puck's motion. Make the game harder by placing walls in front of the goal. This is a clone of the popular simulation of the same name marketed by Physics Academic Software and written by Prof. Ruth Chabay of the Dept of Physics at North Carolina State University. E-Field of Oscillating Charge Illustrating representing an electric field with field lines. Electric Field and Potential Model The Electric Field and Potential Model shows the electric field and potential for cylindrical and spherical symmetry. In both symmetries, there are co-centric charged surfaces where the outer surface is grounded. The model reports the force on a test charge (yellow) between the co-centric surfaces as well as the voltage.

Videos

 Download video (1.8 Mb) Watch Streaming This video shows why it can be dangerous to get in and out of your car when you are at the gas station. Charging by friction caused the woman to become charged. She them discharged near the gas tank which ignited the gasoline vapor surrounding the tank.