Electric FieldsElectric FieldsPhysics AStatic #4

Learning Targets0I can calculate the strength of an electric field,

using the electric force and magnitude of the test charge.

0I can calculate the strength of an electric field, using the magnitude of the charge and the distance between the object and a test charge.

0I can attribute the change in the electric field strength with the changes in the distance from and/or the magnitude of the charge.

Electric Forces as Non-Contact Forces0There are two categories of forces - contact forces

and non-contact forces (or long range forces).0Electrical force and gravitational force are both

listed as non-contact forces because they act on objects even though there is no physical contact between the two objects.

The Electric Field Concept0Contact forces are familiar and not

surprising to us.0One object physically pushes or pulls

another object that it is in contact with.

0Non-contact forces are more perplexing.0How can one balloon reach across

space and pull a second balloon towards it or push it away?

The Electric Field Concept0Non-contact forces are

sometimes referred to as field forces.

0The concept of a field force is utilized by scientists to explain this rather unusual force phenomenon that occurs in the absence of physical contact.

The Electric Field Concept0A charged object creates an electric field - an

alteration of the space in the region that surrounds it. 0Other charges in that field would feel the unusual

alteration of the space. 0Whether a charged object enters that space or not,

the electric field exists. 0Space is altered by the presence of a charged object. 0Other objects in that space experience the strange

and mysterious qualities of the space.

The Force per Charge Ratio0Electric field strength is a vector quantity.0It has both magnitude and direction.

0The magnitude of the electric field is simply defined as the force per charge on the test charge.

The Force per Charge Ratio0The magnitude of the source charge's electric field

could be measured by any other charge placed somewhere in its surroundings. 0The charge that is used to measure the electric field

strength is referred to as a test charge since it is used to test the field strength.

0When placed within the electric field, the test charge will experience an electric force - either attractive or repulsive.

Electric Field Equation0If the electric field strength is denoted by the

symbol E, then the equation can be rewritten in symbolic form as

0F is the force between the charges. Q2 is the magnitude of the test charge.

0The standard metric units for electric field strength are Newton/Coulomb or N/C.

E1 =FQ2

Electric Field Question0If the electric field caused by a charged object does

NOT depend on the charged objects around it, why is the value of the test charge in the equation?

E1 =FQ2

Electric Field Question0Remember that the force between the two charged

objects is dependent on the magnitude of both charges.

0Why does that matter?

E1 =FQ2

F =kQ1Q2d 2

Electric Field Equation…Another One

E1 =FQ2

F =kQ1Q2d 2

E1 =

kQ1Q2

d2

Q2 E1 =kQ1

d2

AND

SO…

SIMPLIFIED

The Direction of the Electric Field0The precise direction of the field is dependent upon

whether the test charge and the source charge have the same type of charge or the opposite type of charge.

0The worldwide convention that is used by scientists is to define the direction of the electric field vector as the direction that a positive test charge is pushed or pulled when in the presence of the electric field.

The Direction of the Electric Field0Given this convention of a positive test charge,

several generalities can be made about the direction of the electric field vector.

0A positive source charge would create an electric field that would exert a repulsive effect upon a positive test charge. 0Thus, the electric field vector would always be

directed away from positively charged objects. 0On the other hand, a positive test charge would be

attracted to a negative source charge.

Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What is the magnitude of the electric field strength that you would expect to be measured at a distance of 60 cm away?

Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What is the magnitude of the electric field strength that you would expect to be measured at a distance of 3 cm away?

Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What would be the electric field strength 30 cm away from a source with charge 2Q?

Charge Q acts as a point charge to create an electric field. Its strength, measured a distance of 30 cm away, is 40 N/C. What would be the electric field strength 15 cm away from a source with charge 2Q?

It is observed that Balloon A is charged negatively. Balloon B exerts a repulsive effect upon balloon A. Would the electric field vector created by balloon B be directed towards B or away from B? ___________ Explain your reasoning.

A negative source charge (Q) is shown in the diagram below. This source charge can create an electric field. Various locations within the field are labeled. For each location, draw an electric field vector in the appropriate direction with the appropriate relative magnitude. That is, draw the length of the E vector long wherever the magnitude is large and short wherever the magnitude is small.

Learning Targets0I can determine the charge of objects based on

an electric field line configuration.0I can rank the charges of objects based on an

electric field line configuration.0I can construct an electric field line

configuration involving multiple charges.

Electric Field Vector Arrows0Since electric field is a vector quantity, it can be

represented by a vector arrow. 0At any location, the arrows point in the direction of the

electric field and their length is proportional to the strength of the electric field at that location.

Electric Field Lines0Rather than draw countless vector arrows in the

space surrounding a source charge, it is more useful to draw a pattern of several lines that extend between infinity and the source charge.

0These pattern of lines, referred to as electric field lines, point in the direction that a positive test charge would accelerate if placed upon the line.

0The lines are directed away from positively charged source charges and toward negatively charged source charges.

Electric Field Lines0Each line must include an arrowhead that points in

the appropriate direction. 0An electric field line pattern could include an

infinite number of lines. Because drawing such large quantities of lines tends to decrease the readability of the patterns, the number of lines is usually limited.

Rules for Drawing Electric Field Patterns1. Surround more charged objects by more lines.

Objects with greater charge create stronger electric fields.

By surrounding a highly charged object with more lines, one can communicate the strength of an electric field in the space surrounding a charged object by the line density.

Rules for Drawing Electric Field Patterns0The density of lines at a specific location in space

reveals information about the strength of the field at that location.

0Consider the diagram.0The field lines are closer together in the regions closest to

the charge; and they are spread further apart in the regions furthest from the charge.

0The electric field is greatest at locations closest to the surface of the charge and least at locations further from the surface of the charge.

Rules for Drawing Electric Field Patterns2. Draw the electric field lines perpendicular to the

surfaces of objects at the locations where the lines connect to object's surfaces. At the surface of both symmetrically shaped and

irregularly shaped objects, there is never a component of electric force that is directed parallel to the surface.

The electric force, and thus the electric field, is always directed perpendicular to the surface of an object.

Once a line of force leaves the surface of an object, it will often alter its direction.

Rules for Drawing Electric Field Patterns3. Electric field lines should never cross.

If the lines cross each other at a given location, then there must be two distinctly different values of electric field with their own individual direction at that given location.

This could never be the case. Every single location in space has its own electric field

strength and direction associated with it.

Several electric field line patterns are shown in the diagrams below. Which of these patterns are incorrect? _________ Explain what is wrong with all incorrect diagrams.

Electric Field Lines for Configurations of Two or More Charges

0What if a region of space contains more than one point charge?

0How can the electric field in the space surrounding a configuration of two or more charges be described by electric field lines?

Electric Field Lines for Configurations of Two or More Charges

Electric Field Lines for Configurations of Two or More Charges

Summary0Electric field lines always extend from a positively

charged object to a negatively charged object, from a positively charged object to infinity, or from infinity to a negatively charged object.

0Electric field lines never cross each other.0Electric field lines are most dense around objects

with the greatest amount of charge.0At locations where electric field lines meet the

surface of an object, the lines are perpendicular to the surface.

Erin Agin drew the following electric field lines for a configuration of two charges. What did Erin do wrong? Explain.

Consider the electric field lines shown in the diagram below. From the diagram, it is apparent that object A is ____ and object B is ____.

a. +, +b. -, -c. +, -d. -, +e. insufficient info

Consider the electric field lines drawn for a configuration of two charges. Several locations are labeled on the diagram. Rank these locations in order of the electric field strength - from smallest to largest.