Read Unit Introduction
You can learn more about electric fields and how they differ from gravitational fields by reading "Magnitude and Direction of an Electric Field" on pages 546 and 547 of your textbook.


Self-Check

Answer the following self-check (SC) question then click the "Check your work" bar to assess your responses.

 DID YOU KNOW?

The symbol for energy is  E  which is a scalar quantity measured in joules.  The symbol for electric field strength is   which is vector quantity measured in   .  The absolute value signs used around the   indicate that the equation does not solve for the direction as in   .

SC 6.

  1. Explain why gravitational field lines are always drawn toward the centre of the source.
  1. Explain why this simple rule cannot always apply in the case of electric fields.
  1. Explain how the direction of an electric field vector is determined for a given location outside a source charge.
  1. State the equation for the electric field. Label each of the variables in this equation.

   Self-Check Answer

SC 6.

  1. Gravitational field lines are always drawn toward the centre of the source because this is the direction of the gravitational force on test masses. 

  2. In the case of electric phenomena, the electrostatic force is not always directed toward the source. Electrostatic forces can attract or repel test charges depending upon the sign of the test charge and the sign of the source charge. 

  3. The direction of an electric field vector is determined by the direction of force on a small positive test charge for a given location outside a source charge. 

      


Read Unit Introduction
Read "Example 11.1" on page 548 of your textbook.  As you go through this example, first identify the key steps and then focus on the details in each step.

 
Self-Check

Answer the following self-check (SC) questions then click the "Check your work" bar to assess your responses.

 

SC 7 . 

Try "Practice Problems" 1 and 2 on page 548 of your textbook.


   Self-Check Answer

SC 7. 


Page 548 Practice Problem 1

 

Given

 

Required

The magnitude of the electrostatic force.

 

Analysis

 

Paraphrase

The force is 1.60 × 10 −16  N.

 

Page 548 Practice Problem 2.

 

Given

 

Required

The magnitude of the charge on the small charged sphere.

 

Analysis

 

Paraphrase

The magnitude of the charge on the small sphere is 6.40 × 10- 19  C.

 

The magnitude of the electric field at some distance from a point charge can be determined from another equation that is derived from Coulomb's law.  The process of deriving this equation is similar to your earlier work deriving a second equation for gravitational field from Newton's law of universal gravitation.

 


Read Unit Introduction
To learn how a second equation for electric field is derived and applied, read from the bottom of page 548 to the end of "Example 11.2" on page 549.  As you read, first identify the key steps and then focus on the details in each step.


Try This
Use the Electric Field Lines simulation to complete the following two questions.  Note, the answers are given (in brackets), so you can confirm your responses.


1. Calculate and illustrate the electric field strength at a distance of 4.25 × 10 -1 m from a +3.00 μ C charged particle. Use the simulation to verify your field diagram. (1.49 x 10 5 N/C away from charge)

 

2. Calculate the electric field strength 6.50 × 10 -2 m from a -5.00 μ C charged particle. Sketch the electric field lines around the source. Use the simulation to verify the directions of your field lines. (1.06 x 10 7 N/C toward charge)