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Electric Fields. Chapter 16. The Electric Field. Objectives. Calculate electric field strength. Draw and interpret electric field lines. Identify the four properties associated with a conductor in electrostatic equilibrium. Chapter 16. The Electric Field. Electric Field Strength.

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Chapter 16

The Electric Field


Calculateelectric field strength.

Draw and interpretelectric field lines.

Identifythe four properties associated with a conductor in electrostatic equilibrium.

Electric field strength

Chapter 16

The Electric Field

Electric Field Strength

Anelectric fieldis a region where an electric force on a test charge can be detected.

The SI units of the electric field, E, are newtons per coulomb (N/C).

Thedirection of the electric field vector, E,is in the direction of the electric force that would be exerted on a small positive test charge.

Electric fields and test charges
Electric Fields and Test Charges

Click below to watch the Visual Concept.

Visual Concept

Electric field strength1

Chapter 16

The Electric Field

Electric Field Strength

Electric field strength depends on charge and distance. An electric field exists in the region around a charged object.

The strength of an electric field is defined as the force placed on a “test charge” divided by the size of the charge. E = Fon q’ / q’

Electric Field Strength Due to a Point Charge

Sample problem
Sample Problem

Electric Field Strength

A positive charge of 3.2 x 10-6 C experiences a force of 0.10 N when located at a certain point. What is the electric field intensity at that point?

Electric field lines
Electric Field Lines

  • The number of electric field lines is proportional to the electric field strength.

  • Electric field lines are tangent to the electric field vector at any point.

Rules for drawing electric field lines
Rules for Drawing Electric Field Lines

1. The lines begin at positive charges and end on negative charges. (Arrows pointing to negative)

2. The number of lines reflects the strength of the charge. More charge = more lines

3. Field lines should not cross in charge-free spaces.

4. The lines should be perpendicular to conducting surfaces.

Click below to watch the Visual Concept.

Visual Concept

Rules for sketching fields created by several charges
Rules for Sketching Fields Created by Several Charges

Click below to watch the Visual Concept.

Visual Concept

Conductors in electrostatic equilibrium
Conductors in Electrostatic Equilibrium

The electric field is zero everywhere inside the conductor.

Anyexcess charge on an isolated conductor resides entirely on the conductor’s outer surface.

The electric field just outside a charged conductor is perpendicular to the conductor’s surface.

On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature of the surface is smallest, that is, at sharp points.

Electric potential energy
Electric Potential Energy

  • Potential Energy is energy due to the location (stored energy).

  • You have electrical potential energy when:

    • positive and negative charges are separated.

    • like charges are pushed together.

  • This is a total amount of energy for a group of charges.

Electrical potential energy
Electrical Potential Energy

+ +

+ +

+ +




Work is done to move the positively charged particle towards the positively charged source. This results in electrical potential energy.

Electric potential
Electric Potential

  • Electric potential energy per charge

  • Often called voltage

  • This is different from electrical potential energy. The EPE is a total for a group, EP is per unit of charge.

V = Ed

Electric potential is measured in volts
Electric potential is measured in volts.

electric potential energy

electric potential = --------------------------------



1 Volt = --------------------------------



  • Stores electrical energy

  • Two plates separated by an insulating space

  • Positive charge on one plate

  • Negative charge on the other plate

  • Potential energy is in the form of the electric field between the plates

  • It takes work to separate the charges

C = q/V