Download Two equally charges particles are 3 cm apart and repel each other

Physics QOD 12.3
Two equally charges particles are 3 cm apart and repel each other
with a force of 4 x10-5 N. Compute the charge on each particle.
q1 = q2 = 2 x 10-9 C
12.3
ELECTRIC FIELDS
Electric Fields
The space around a concentration of
electric charge is different from how it
would be if the charge were not there.
If you walk by the charged dome of an
electrostatic machine—a Van de Graaff
generator, for example—you can sense
the charge. Hair on your body stands
out—just a tiny bit if you’re more than
a meter away, and more if you’re
closer.
The space is said to contain a Electric
Force field.
Electric Fields
§  An electric field is a force field that surrounds an
electric charge or group of charges.
Electric Fields
§  A gravitational force holds a satellite in orbit about a
planet, and an electrical force holds an electron in orbit
about a proton.
§  The force that one electric charge exerts on another is the
interaction between one charge and the electric field of the
other.
Electric Fields
§  You can use electric field lines (also called lines of force) to
represent an electric field. Where the lines are farther apart,
the field is weaker.
§  An electric field has both magnitude and direction. Since
an electric field has both magnitude and direction, it is a
vector quantity and can be represented by vectors.
Electric Field Lines
The direction of an electric field at any point, by convention, is
the direction of the electrical force on a small positive test
charge.
§  If the charge that sets up the field is positive, the field
points away from that charge.
§  If the charge that sets up the field is negative, the field
points toward that charge.
§  Magnitude of the field is indicated by the vector length.
The electric field is greater where the vectors are longer.
Electric Field Lines
You can use electric field lines to represent an electric field.
§ 
Where the lines are farther apart, the field is weaker.
§ 
For an isolated charge, the lines extend to infinity.
§ 
For two or more opposite charges, the lines begin at a
positive charge and terminate on a negative charge.
Electric Field Lines - Vectors
a.  In a vector representation of
an electric field, the length of
the vectors indicates the
magnitude of the field.
b.  In a lines-of-force
representation, the distance
between field lines indicates
magnitudes.
Electric Field Lines - Direction
a.  The field lines around a single positive charge extend to
infinity.
b.  For a pair of equal but opposite charges, the field lines
emanate from the positive charge and terminate on the
negative charge.
c.  Field lines are evenly spaced between two oppositely
charged plates.
Electric Field Lines
Electrical Field Lines – Single Point Charge
Electric Field Lines
Electrical Field Lines – positive and negative charge
Electric Field Lines
The electric field inside a conductor is zero – if it were not, the
charges would move.
Electric Field Lines
You can demonstrate electric field
patterns by suspending fine thread in
an oil bath with charged conductors.
The photos show patterns for:
a.  equal and opposite charges;
b. equal like charges;
c.  oppositely charged plates;
d. oppositely charged cylinder and
plate.
Electric Field Lines
opposite charges
Electric Field Lines
like charges
Electric Field Lines
oppositely charged plates
Electric Field Lines
oppositely charged cylinder and plate.
Electric Field Lines
How are the magnitude and direction of an electric
field determined?
•  The length of the field lines and how far apart they are
determine magnitude.
•  The type of charge determines the direction of the field.
Electric Field Strength
Electric Field Strength (E) is the amount of electrostatic force
observed per charge.
Formula:
E = F/q
§ 
§ 
§ 
E = Electric Field Strength (N/C)
F = Electrostatic Force (Newton's)
q = charge (Coulombs)
Electric Fields – Assessment Questions
Example #1
Two oppositely charged parallel metal plates, 1 cm apart,
exert a force with a magnitude of 3.60 x 10-15 N on an electron
placed between the plates. Calculate the magnitude of the
electric field strength between the plates.
Electric Fields – Assessment Questions
Example #2
What is the magnitude of the electric field intensity at a
point where a proton experiences an electrostatic force of
magnitude 2.3 x 10-25 N?
Electric Fields – Assessment Questions
Example #3
The diagram represents an electron within an electric field
between two parallel plates that are charged with a potential
difference of 40 volts.
If the magnitude of the electric force on the electron is 2 x 10-15
N, find magnitude of the electric field strength between the
charged plates.
Electric Fields – Assessment Questions
Example #4
Based on the diagram below, find:
a.  The magnitude of the electrostatic force between the spheres
b.  Direction of the electric field at point P (Right, Left, Up or Down?)
Electric Fields – Assessment Questions
Example #5
In the diagram below, P is a point near a negatively charged
sphere. What direction best represents the direction of the
electric field at point P? (Left, Right, Up or Down)
Electric Fields – Assessment Questions
Example #6
Sketch at least four electric field lines with arrowheads that
represent the electric field around a negatively charged
particle.
Electric Fields – Assessment Questions
Example #7
An electric field has:
a. 
b. 
c. 
d. 
no direction.
only magnitude.
both magnitude and direction.
a uniformed strength throughout.
Electric Fields – Assessment Questions
Example #8
In the electric field surrounding a group of charged
particles, field strength is greater where field lines are:
a. 
b. 
c. 
d. 
thickest.
longest.
farthest apart.
closest.
Electric Fields – Assessment Questions
Example #9
A beam of electrons is produced at one end of a glass tube (as
shown) and lights up a phosphor screen at the other end. If the
beam passes through the electric field of a pair of oppositely
charged plates, how is the beam deflected? (Up, down, into the
page, out of the page)