Transcript Electrical Charges - Southgate Schools

Learning Targets
0 I can calculate the strength of an electric field, using
the electric force and magnitude of the test charge.
0 I can calculate the strength of an electric field, using
the magnitude of the charge and the distance
between the object and a test charge.
0 I 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 Forces
0 There are two categories of forces - contact forces
and non-contact forces (or long range forces).
0 Electrical 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 Concept
0 Contact forces are familiar and not
surprising to us.
0 One object physically pushes or pulls
another object that it is in contact
with.
0 Non-contact forces are more
perplexing.
0 How can one balloon reach across
space and pull a second balloon
towards it or push it away?
The Electric Field Concept
0 Non-contact forces are
sometimes referred to as
field forces.
0 The 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 Concept
0 A charged object creates an electric field - an
alteration of the space in the region that surrounds
it.
0 Other charges in that field would feel the unusual
alteration of the space.
0 Whether a charged object enters that space or not,
the electric field exists.
0 Space is altered by the presence of a charged object.
0 Other objects in that space experience the strange
and mysterious qualities of the space.
The Force per Charge Ratio
0 Electric field strength is a vector quantity.
0 It has both magnitude and direction.
0 The magnitude of the electric field is simply defined
as the force per charge on the test charge.
The Force per Charge Ratio
0 The magnitude of the source charge's electric field
could be measured by any other charge placed
somewhere in its surroundings.
0 The 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.
0 When placed within the electric field, the test charge
will experience an electric force - either attractive or
repulsive.
Electric Field Equation
0 If the electric field strength is denoted by the
symbol E, then the equation can be rewritten in
symbolic form as
F
E1 =
Q2
0 F is the force between the charges. Q2 is the
magnitude of the test charge.
0 The standard metric units for electric field strength
are Newton/Coulomb or N/C.
Electric Field Question
0 If 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?
F
E1 =
Q2
Electric Field Question
0 Remember that the force between the two charged
objects is dependent on the magnitude of both
charges.
kQ1Q2
F=
2
d
0 Why does that matter?
F
E1 =
Q2
Electric Field Equation…Another One
kQ1Q2
F=
2
d
AND
F
E1 =
Q2
SO…
kQ1Q2
2
d
E1 =
Q2
SIMPLIFIED
kQ1
E1 = 2
d
The Direction of the Electric Field
0 The 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.
0 The 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 Field
0 Given this convention of a positive test charge,
several generalities can be made about the
direction of the electric field vector.
0 A positive source charge would create an electric
field that would exert a repulsive effect upon a
positive test charge.
0 Thus, the electric field vector would always be
directed away from positively charged objects.
0 On 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 Targets
0 I can determine the charge of objects based on
an electric field line configuration.
0 I can rank the charges of objects based on an
electric field line configuration.
0 I can construct an electric field line
configuration involving multiple charges.
Electric Field Vector Arrows
0 Since electric field is a vector quantity, it can be
represented by a vector arrow.
0 At 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 Lines
0 Rather 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.
0 These 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.
0 The lines are directed away from positively charged
source charges and toward negatively charged
source charges.
Electric Field Lines
0 Each line must include an arrowhead that points in
the appropriate direction.
0 An 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 Patterns
1. 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 Patterns
0 The density of lines at a specific location in space
reveals information about the strength of the field at
that location.
0 Consider the diagram.
0 The 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.
0 The 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 Patterns
2. 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 Patterns
3. 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
0 What if a region of space contains more than one
point charge?
0 How 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
Summary
0 Electric 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.
0 Electric field lines never cross each other.
0 Electric field lines are most dense around objects
with the greatest amount of charge.
0 At 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.