AP Physics Chp11 : Electricity
Lesson 1: Coulomb’s Law
1. What is Coulomb’s Law? (with absolute values)
2. What does this tell us?
3. This equation has absolute values. This means the force will always be positive. We need to analyze the situation
in order to tell the _____________________________ of the force.
4. What is the charge of a proton?__________________________ of an electron?_______________________
5. What is the difference between Q and q?
6. What is the equation that relates Q and q? (You should be able to get this yourself)
7. Charge A (+4 microcoulomb) is 25 cm from Charge B (-8 microcoulombs).
a. What is the force of A on B?
b. What is the force of B on A?
8. On a number line, Charge A (+1C) is at the origin. Charge B (+2C) is at +3 m. Charge C (-2) is at 6 m. What is the
direction of the net force on each?
9. On a number line, Charge A (+3 C) is at x=3 m. Charge B (+5 C) is at +5 m. Where can a +4 C charge be placed so
that Charge A is in equilibrium?
10. Charge A is at the origin. It is a -1 nanocoulomb charge. Charge B (-1 nC) is at (0,-2). Charge C (-2 nC) is at (3,0).
Where do you put Charge D (-4 nC) so that Charge A is in equilibrium?
11. Charge A (6C) is at (2,0). Charge B (8C) is at (-4,0). Charge C (2C) is at (0,-6).Find the net force on charge C.
12. Charge A has a magnitude of Q. It is located at the origin. Charge B has a magnitude of –Q. it is located at point
(d,0). Charge C has a magnitude of q. It is located at (3d,0). Find the total force on charge C.
13. A triangle has sides measuring 5m, 12m, and 13m. The 13m side is on top and horizontal. The 5m side is on the
left. Charge Q2 is on the bottom corner. It is positive. Q3 is on the left corner. Q1 is on the right corner. The net
force on Q2 is horizontal and to the right
a) What are the signs of the Q3 and Q1?
b) Draw and label the individual forces between Q3-Q2 and between Q1-Q2.
c) if Q1=5 C, Q2 = 4 C, and Q3 = 6C, find the net force on Q2
d) Where would you put Q4=7C so that the Q3 is in equilibrium?
For the next 4 problems, draw force diagrams for a 9C charge and a -7C
charge in these positions:
14) the 9C is at the right end of a horizontal spring lying on the ground. The left end is attached to a wall. The -7C
charge is hanging from a string above and to the right of the spring.
15) The 9C and -7C charges are at the ends of 2 strings. The 2 strings are attached to the ceiling at different
spots. The two strings angle toward each other.
16) The same two charges are between 2 horizontal plates. The top plate is positively charged. The bottom plate
is negatively charged.
17) A spring is hung from the ceiling. The 9C charge is at the bottom of the spring. The 7C charge is on the floor
below the spring.
18. Two charges are suspended by 2 strings. The 2 strings are attached to the ceiling at the same spot. Charge 1
is +2q. Charge 2 is +3q. They repel each other so that the strings angle away from the vertical with angles θ1 and
θ2. Which is true:
a) θ1 < θ2
b) θ1 = θ2
c) θ1 > θ2
19) Write two expressions involving θ1 and the force of tension for charge 1 in the previous problem. Do not
actually solve for either variable.
20) Two spheres are attached to each other by a string. The left sphere is 4 microcoulombs and 2 kg. The right
sphere is 5 microcoulombs and 3 kg. The string is 6m long.
a) Find the tension
b) Find the acceleration of the left sphere after the string is cut.
c) Find the acceleration of the right sphere after the string is cut.
d) Sketch the acceleration vs. distance graph for the spheres (qualitative shape only)
e) Sketch the speed vs. time for the spheres (qualitative shape only)
21) A 0.2 kg sphere is tied to the end of a 0.6 m string. The string is attached to a vertical plate. The plate has a
uniform negative charge. The sphere has a -160 microcoulomb charge.
a) Draw a free-body diagram
b) Find the tension
c) Find the distance between the wall and the ball.
d) Find the acceleration when the string is cut.
e) Describe the shape of the path of the ball after the string is cut.
22) Two spheres, both with mass 14g and charge q, are hung from 2 equal strings with length 10.1 cm. Both strings
make an 80 degree angle with the ceiling. Find the size of charge q.
Do problems 62, 63, 64, 68, and 69 on page 673.