Transcript Electric Potential Difference

J. Pulickeel
November 2008
SPH 3U1
Contents of this Presentation
In this presentation we will learn
 What are Electric Force Fields?
 What are the properties of Electric Force Fields?
 How do we calculate the magnitude of the Force
between two charged particles?
 How do we measure the charge of a particle?
Electric Force Fields
 A field force is sometimes referred to as an action at a
distance
 The space surrounding a charged object is affected by
the presence of the force or charge.
Gravitational Fields
Magnetic Fields
Electrical Fields
Electric Field Lines
 Electric Field Lines are sometimes called lines of
force. The lines are drawn to show the direction of the
force, due to the electric field, as it would
 act on a test charge.
 Generally we state that the field starts at the positive
charge and goes to the negative charge
Electric Field Lines
 All charged objects create electric field lines.
Similar charges attract each other
Like charges will repel each other.
Drawing Electric Field Lines
 Field Lines always go from positive to negative
 Field lines NEVER cross. They represent the net force on a
test charge. If they were to cross it would mean that the
charge had two different net forces with different directions.
This is not possible. The charge will experience a single net
force in the direction of the field.
 The number of field lines leaving the (+) is proportional to
the magnitude of the charge.
 The density of field lines in a given area can represents the
relative strength of the field
Field Strength
 A measure of the
strength of a field is how
dense the field lines in a
certain area are.
 As you get closer to the
source of the field the
force gets stronger, and
the field line density
increases.
Drawing Electric Field Lines
The number of field lines leaving the (+) is proportional to the magnitude of
the charge. Thus there are more field lines around the 4q charge that the q
charge.
The density of field lines does not always indicate relative strength. In this
case, color indicates the relative strength of the field.
Calculating the Charge…
 How do we calculate the charge (Q) for each particle?
 The charge of 1 electron is 1.60
x 10-19 C
 Therefore the charge of a particle is
 Where N is the number of electrons
Coulombs (C)
 The charge of a particle is measured in Coulombs.
 A Coulomb is approximately the amount of electric
charge that passes through a 100 light bulb in 1 second
 The earth stores approximately 400 000 C of charge
 A person walking across a woollen rug can store 10-8 C
Drawing Electric (and Magnetic) Field Lines
Recall: Gravitational Force Fields
 The Universal Law of Gravitation states that all objects
with mass exert a force of attraction on others.
 The force is dependant on the

Mass of the two objects (F  m)

The distance between them (F  /d)
1
Electric and Gravitational Forces
Gravitational Forces
Electric Forces
 A Force of attraction between
 A force between two charged
two objects.
particles. It can be attractive
or repulsive.
Electric Force Fields
 Charles de Coulomb (19th Century) discovered a similar
relationship between charged particles
 Coulomb’s Law states that all objects with charge exert a
force of attraction on others.
 The force is dependant on the

Charge of the two objects (F  q)

The distance between them (F  /d)
1
True/ False Questions
1.
If electrons are added to an object, it will be charged negatively.
True
2.
False
Answer T
A negatively charged ebonite rod is brought near a neutral, metalliccoated pith ball. Some of the electrons are repelled by the ebonite rod
and move to the far side of the pith ball. This process is called charging
by induction
True
False
Answer F
True/ False Questions
3.
Coulomb’s law states that the electric force is directly proportion to the
product of the charges on each sphere and inversely proportional to the
distance between the charges.
True
4.
Answer
F
When the distance between two charged spheres is decreased by a
factor of 2, then the electric force decreases by a factor of 2.
True
5.
False
False
Answer
F
One of the ways in which Newton’s law of universal gravitation differs
from Coulomb’s law is that gravitational force can only attract,
whereas the electric force can only repel.
True
False
Answer
F
Multiple Choice
A negatively charged rod is held near, but does not touch the knob
of an electroscope. The leaves of the electroscope move apart from
one another. A wire is connected to the knob and to a water tap
with the negatively charged rod staying in the same position. Which
of the following would occur?
A. Electrons flow from the earth through the wire to the
B.
C.
D.
E.
electroscope.
No electron flow takes place.
The leaves of the electroscope remain still.
The leaves of the electroscope move closer together.
Electrons flow from the electroscope through the
wire to the earth.
My Answer
Official Answer
E
Multiple Choice
When a charge separation has been induced on an object, the
charge on the object
A. is opposite to the charge on the charging object
B. is proportional to the size of the object being
charged
C. is permanent
D. is the same as the charge on the charging object
E. is inversely proportional to the size of the object
being charged
My Answer
Official Answer
A
Multiple Choice
To charge an electroscope positively by induction you need
A. a negatively charged rod
B. two objects of the same charge
C. a negatively charged rod and a ground
D. two objects with opposite charges
E. a positively charged rod and a ground
My Answer
Official Answer C
Multiple Choice
The law of electric charges states that opposite charges
A. attract each other, similar charges attract neutral objects, and
B.
C.
D.
E.
charged objects repel one another
repel each other, similar charges attract neutral objects, and
charged objects attract one another
attract neutral objects, similar charges repel each other, and
charged objects attract one another
attract each other, similar charges repel one another, and
charged objects attract some neutral objects
attract neutral objects, similar charges attract each other, and
charged objects repel one another
My Answer
Official Answer
D
Sample Question 1
How many electrons have been removed from a positively
charged pith ball electroscope if it ahs a charge of 7.5 x10-11C?
 Q = 7.5 x10-11 C
 e = 1.6 x10-19C
 N= ?
Q = Ne
N= 4.7 x 108 electrons
Sample Question 2
Two charged spheres are 2.00 m apart. One sphere has a charge of 5.7 x10-5C
and the other sphere has a charge of 1.6 x10-1C . What is the electric force
between the two spheres?
The electric force between the two spheres is 2.05 x 104 N [APART].