Review What You`ve Learned – About Coulomb’s Law – Chapter 7.2
1. The q in Coulomb`s law equation stands for the:
a. Mass of a charged object
b. Number of excess electrons on the object
c. Current of a charged object
d. Distance between charged objects
e. Charge of a charged object
2. The symbol r 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
3. Determine the electrical force of attraction between two balloons with separate
charges of +3.5 × 10−8 𝐶 and −2.9 × 10−8 𝐶 when separated a distance of 0.65 m.
4. 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 × 10−7 𝐶 and they are separated by a distance of
0.50 m.
5. Joann has rubbed a balloon with wool to give it a charge of −1.0 × 10−6 𝐶. She
then acquires a plastic golf tube with a charge of +4.0 × 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.
6. 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.
7. At what distance of separation must two 1.00-microCoulomb 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?
8. Two balloons are charged with an identical quantity and type of charge: -6.25
nC. They are held apart at a separation distance of 61.7cm. Determine the
magnitude of the electrical force of repulsion between them.
Answer Key to problems:
Check Your Understanding
Use your understanding to answer the following questions. When finished, click the button to
view the answers.
1. 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
Answer: E
In the equation Felect = k • Q1 • Q2 / d2 , the symbol Felect represents the electrostatic force of
attraction or repulsion between objects 1 and 2. The symbol k is Coulomb's law constant (9 x
109N • m2 / C2), Q1 and Q2 represent the quantity of charge on object 1 and object 2,
and drepresents the separation distance between the objects' centers.
2. 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
Answer: G
In the equation Felect = k • Q1 • Q2 / d2 , the symbol Felect represents the electrostatic force of
attraction or repulsion between objects 1 and 2. The symbol k is Coulomb's law constant (9 x
109N • m2/ C2), Q1 and Q2 represent the quantity of charge on object 1 and object 2,
and drepresents the separation distance between the objects' centers.
3. Determine the electrical force of attraction between two balloons with separate charges of
+3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.65 m.
Answer: 2.16 x 10-5 N, attractive
Step 1: Identify known values in variable form.
Q1 = +3.5 x 10-8 C and Q2 = -2.9 x 10-8 C
d = 0.65 m
Step 2: Identify the requested value
F = ???
Step 3: Substitute and solve
4. 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.
Answer: 1.3 x 10-2 N, attractive (rounded from 1.296 x 10-2 N)
Step 1: Identify known values in variable form.
Q1 = +6.0 x 10-7 C and Q2 = -6.0 x 10-7 C
d = 0.50 m
Step 2: Identify the requested value
F = ???
Step 3: Substitute and solve
5. 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.
Answer: 0.144 N
Step 1: Identify known values in variable form.
Q1 = -1.0 x 10^-6 C and Q2 = +4.0 x 10-6 C
d = 50.0 cm = 0.50 m.
Step 2: Identify requested information
F = ???
Step 3: Substitute and solve.
6. 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.
Answer: 0.29 N
Step 1: Identify known values in variable form.
Q1 = -4.0 x 10^-6 C and Q2 = +4.0 x 10-6 C
d = 0.70 m.
Step 2: Identify requested information
F = ???
Step 3: Substitute and solve.
7. At what distance of separation must two 1.00-microCoulomb 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?
Answer: 0.030 m or 3.0 cm
Step 1: Identify known values in variable form.
Q1 = 1.0 x 10-6 C and Q2 = 1.0 x 10-6 C
Felect = Fgrav = mg = 1.0 • 9.8 m/s/s = 9.8 N
Step 2: Identify requested information
d = ???
Step 3: Substitute and solve.
8.
Example B – Question 8 on the worksheet
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.
The problem states the value of Q1 and Q2. Since these values are expressed in units of
nanoCoulombs (nC), the conversion to Coulombs must be made. The problem also states the
separation distance (d). Since distance is given in units of centimeters (cm), the conversion
to meters must also be made. These conversions are required since the units of charge and
distance in the Coulomb's constant are Coulombs and meters. The unknown quantity is the
electrical force (F). The results of the first two steps are shown in the table below.
Given:
Find:
Q1 = -6.25 nC = -6.25 x 10-9C
Q2 = -6.25 nC = -6.25 x 10-9C
Felect = ???
d = 61.7 cm = 0.617 m
The final step of the strategy involves substituting known values into the Coulomb's law
equation and using proper algebraic steps to solve for the unknown information. This
substitution and algebra is shown below.
Felect
Felect = k • Q1 • Q2 / d2
= (9.0 x 109 N•m2/C2) • (6.25 x 10-9 C) • (6.25 x 10-9 C) / (0.617 m)2
Felect = 9.23 x 10-7 N
Note that the "-" sign was dropped from the Q1 and Q2 values prior to substitution into the
Coulomb's law equation. As mentioned above, the use of "+" and "-" signs in the equation
would result in a positive force value if Q1and Q2 are like charged and a negative force value
if Q1 and Q2 are oppositely charged. The resulting "+" and "-" signs on F signifies whether the
force is attractive (a "-" F value) or repulsive (a "+" F value).