Activity 18 – Coulomb’s Law
Studio Physics I
The General Form of Coulomb’s Law
1. The mathematical expression that describes Coulomb’s Law (Equation 47b) is

1 q 1q 2
F
(r̂ ) . State what each variable in this expression stands for. Hint: You can
4  0 r 2
1
simply call k 
the constant of the electrostatic force and leave it at that until Physics 2.
4  0
(Don’t confuse k in the ideal spring equation with k for the electrostatic force, they are totally
different. It should be clear from the context which is which.)
2. If you did not already address this above, state precisely why the “–” sign is in the equation.
What does that mean for the force between two charges of like sign? Opposite signs? How does
the “–” sign relate to the direction of the unit vector in the statement of Coulomb’s Law?
3. In what ways is Coulomb’s Law like Newton’s Law of Gravity? In what ways is it different?
(We are not concerned here with the size of the force, only the general behavior.)
4. Consider the situation below where we have one point charge (A) of +4.0 C and another point
charge (B) of +1.0 C. Draw this figure on your paper and add force arrows on each point for two
electrostatic forces: the force on A from B and the force on B from A. Make the lengths of the
arrows proportional to their magnitudes. What can you say about the magnitudes and directions
of these two forces? Is Newton’s Third Law supported or contradicted by Coulomb’s Law?
Point A
Point B
+4.0 C
+1.0 C
5. We now place another point charge between A and B, call it C, as shown below. Assume
that C is –100.0 C. What effect does C have on the force on A from B (only from B, not total)
and the force on B from A (only)? Have your TA or instructor check your answers for 4 and 5.
Rev. 18-Oct-06 Bedrosian
Point A
Point C
Point B
+4.0 C
-100.0 C
+1.0 C
Exercise: Superposition of Forces
6. We want to find the total force on q1 from q2 and q3. To do that, we will calculate
separately the force on q1 from q2 and on q1 from q3, then add them as vectors. Note that you
will need to calculate X and Y components in order to add the vectors. Take the +X axis to the
right on the page and the +Y axis up on the page. Express the total force as magnitude and angle
(in degrees). Use the convention that a positive angle is counter-clockwise from the +X axis and
a negative angle is clockwise from the +X axis. Use k = 9.0 x 10+9 N m2/C2 and calculate your
answer to two significant digits. Have your TA or instructor check your answer.
Rev. 18-Oct-06 Bedrosian