Learning Targets
0 I can attribute the change in the electric force between
charged objects with the changes in the distance
between them and/or the magnitude of the charges.
0 I can calculate the electric force between two charged
objects when given the charges of the objects and the
distance between them.
0 I can explain why acquiring a large excess static charge
affects your hair.
Force as a Vector Quantity
0 The electrical force is expressed using the unit Newton.
0 Being a force, the strength of the electrical interaction
is a vector quantity that has both magnitude and
direction.
0 The direction of the electrical force is dependent upon
whether the charged objects are charged with like charge
or opposite charge.
Force as a Vector Quantity
0 Electrical force has a magnitude or strength.
0 Electric force can be altered by changing three
variables.
0 First, the quantity of charge on one of the objects
will affect the strength of the repulsive force.
0 The more charged a balloon is, the greater the repulsive
force.
Force as a Vector Quantity
0 Second, the quantity of charge on the second object
will affect the strength of the force.
0 Finally, the distance between the two objects will
have a significant and noticeable effect upon the
repulsive force.
0 The electrical force is strongest when the objects are closest
together.
0 The magnitude of the force and the distance between the two
objects is said to be inversely related.
Coulomb’s Law Equation
0 Coulomb's law states that the electrical force between two
charged objects is directly proportional to the product of
the quantity of charge on the objects and inversely
proportional to the square of the separation distance
between the two objects.
kqa qb
F= 2
d
0 The symbol k is a proportionality constant known as the
Coulomb's law constant.
0 The value is approximately 9.0 x 109 N • m2 / C2.
Coulomb’s Law Equation
0 The Coulomb's law equation provides an accurate
description of the force between two point
charges.
0 The symbols q1 and q2 represent the quantities of
charge on the two interacting objects.
0 Since an object can be charged positively or
negatively, these quantities are often expressed as
"+" or "-" values.
0 DO NOT USE THE “+” OR “-” VALUES IN THE
EQUATION!
Suppose that two point charges, each with a
charge of +1.00 Coulomb are separated by a
distance of 1.00 meter. Determine the magnitude
of the electrical force of repulsion between them.
Two balloons are charged with an identical
quantity and type of charge: -6.25 nC. They are
held apart at a separation distance of 61.7 cm.
Determine the magnitude of the electrical force
of repulsion between them.
Two balloons with charges of +3.37 µC and -8.21
µC attract each other with a force of 0.0626
Newton. Determine the separation distance
between the two balloons.
Comparing Electrical and
Gravitational Forces
0 Electrical force and gravitational force are the two
non-contact forces.
0 How are they similar?
Felect
kqa qb
= 2
d
k = 9.0 *10 N * m / C
9
2
Gm1m2
Fgrav =
2
d
2
G = 6.67*10-11 N * m2 / kg2
The Q in Coulomb's law equation stands for the
_____.
a. mass of a charged object
b. # of excess electrons on the object
c. the current of a charged object
d. the distance between charged objects
e. charge of a charged object
The symbol d in Coulomb's law equation
represents the distance from ___.
a. A to B
b. A to D
c. B to C
d. B to D
e. C to D
f. A to G
g. B to F
h. C to E
Determine the electrical force of attraction between
two balloons that are charged with the opposite type
of charge but the same quantity of charge. The
charge on the balloons is 6.0 x 10-7 C and they are
separated by a distance of 0.50 m.
Joann has rubbed a balloon with wool to give it a
charge of -1.0 x 10-6 C. She then acquires a plastic
golf tube with a charge of +4.0 x 10-6 C localized at a
given position. She holds the location of charge on
the plastic golf tube a distance of 50.0 cm above the
balloon. Determine the electrical force of attraction
between the golf tube and the balloon.
A balloon with a charge of 4.0 µC is held a distance of
0.70 m from a second balloon having the same
charge. Calculate the magnitude of the repulsive
force.
At what distance of separation must two 1.00microCoulomb charges be positioned in order for the
repulsive force between them to be equivalent to the
weight (on Earth) of a 1.00-kg mass?
Cause & Effect Relationships
0 Careful observation and measurement might
indicate that a pattern exists in which an
increase in one variable always causes another
measurable quantity to increase.
0 This type of cause-effect relationship is described as
being a direct relationship.
0 In electrostatics, the electrical force between two
charged objects is directly related to the magnitude
of charge of the two objects.
0 Increasing the charge of the objects increases the
force of attraction or repulsion between the objects.
Cause & Effect Relationships
0 Observation might also indicate that an increase
in one variable always causes another
measurable quantity to decrease.
0 This type of cause-effect relationship is described as
being an inverse relationship.
0 In electrostatics, the electrical force between two
charged objects is inversely related to the distance of
separation between the two objects.
0 Increasing the separation distance between objects
decreases the force of attraction or repulsion
between the objects.
Cause & Effect Relationships
0 Electric force is very sensitive to distance.
0 The pattern between electrostatic force and distance
can be further characterized as an inverse square
relationship.
0 The factor by which the electrostatic force is changed is
the inverse of the square of the factor by which the
separation distance is changed.
0 If the separation distance is doubled, then the
electrostatic force is decreased by a factor of four (22).
0 If the separation distance is tripled, then the electrostatic
force is decreased by a factor of nine.
Two charged objects have a repulsive force of
0.10 N. If the charge of one of the objects is
doubled, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the charge of both of the objects is
doubled, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the distance separating the objects is
doubled, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the distance separating the objects is
tripled, then what is the new force?
Two charged objects have an attractive force of
0.10 N. If the distance separating the objects is
quadrupled, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the distance separating the objects is
halved, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the charge of one of the objects is
doubled, and the distance separating the objects
is doubled, then what is the new force?
Two charged objects have a repulsive force of
0.10 N. If the charge of both of the objects is
doubled and the distance separating the objects
is doubled, then what is the new force?
Two charged objects have an attractive force of
0.10 N. If the charge of one of the objects is
tripled and the distance separating the objects is
tripled, then what is the new force?