Lesson: Electric Fields of Point Charges Physics • 9th Grade

In this lesson, we will learn how to relate electric forces to electric fields and describe the electric field lines produced by groups of charges.

Lesson Plan

Students will be able to

relate the electric force, , on an object that has a charge of due to the presence of an electric field, , using the equation ,
determine the electric field, , at a distance of from a point charge , applying the equation, , where is the Coulomb constant,
recognize that an electric field can be represented by field lines,
recognize that an electric field line must start at a positively charged object and terminate at a negatively charged object,
recognize that an isolated positive charge has electric field lines starting from it that extend to infinity in all directions,
recognize that an isolated negative charge has electric field lines terminating at it that extend from infinity in all directions,
recognize that the directions of electric field lines in a region represent the directions of electric field vectors in the region,
recognize that the density of electric field lines in a region represents the magnitude of the electric field in the region,
recognize that the electric field lines of an isolated point charge are radial lines,
rank and/or estimate the electric charge values of point charges by inspecting the diagrams of the field lines of the resultant electric fields around the point charges.

Lesson Worksheet

Q1:

A charged object produces an electric field strength of magnitude N/C at a distance of 2.6 m from its center. Find the charge of the object. Use N⋅m²/C² for the value of Coulomb’s constant. Answer to three decimal places.

Q2:

An object with a charge of 1.5 C is at a point in an electric field where the field strength is 12 N/C. What is the magnitude of the force on the object?

Q3:

An object has a charge of 0.335 nC. At a point near the object, the strength of the electric field from the object has a magnitude of 5.65 N/C. Find the distance from the object to the point. Use N⋅m²/C² for the value of Coulomb’s constant. Give your answer to one decimal place.