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Electric Charge and Force
Electric Charge
1. How many excess electrons are on a ball with a charge of - 4.00 x 10-17 C?
2. Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of
+12 x 10-9 C and the other is given a charge of -18 x 10-9 C. The spheres are connected by a
conducting wire. After equilibrium has occurred, find the charge on each sphere.
3. A strong lightning bolt transfers about 25 C to Earth.
a. How many electrons are transferred?
b. If each water molecule donates one electron, what mass of water lost an electron to the
lightning? One mole of water has a mass of 18 g.
Coulomb’s Law
4. Two electrons in an atom are separated by 1.5 x 10-10 m, the typical size of an atom. What is the force
between them?
5. A positive and a negative charge, each of magnitude 1.5 x 10-5 C, are separated by a distance of 15 cm.
Find the force on each of the particles.
6. Two negative charges of -3.0 x 10-6 C exert a repulsive force of 2.0 N on each other. By what distance
are the separated?
7. Three point charges, q1, q2, and q3, lie along the x-axis at x=0, x=3.0 cm, and x=5.0 cm, respectively.
Calculate the magnitude and the direction of the electric force on each of the three point charges when
q1 = +6.0 µC, q2=+1.5 µC, and q3 =-2.0 µC.
a. Draw the force diagram for this problem.
b. Find the net force on the middle particle.
b. Find the net force on the right particle.
8. Four charged particles are placed so that each particle is at the corner of a square. The sides of the
square are 15cm. The charge at the upper left corner is +3.0 µC, the charge at the upper right
corner is -6.0 µC, the charge at the lower left corner is -2.4 µC, and the charge at the lower right
corner is -9.0 µC. Find the net force on the charge at the upper left corner.
Draw Force Diagram here!!!
9. The two pith balls, shown below, each have a mass of 1.0 g and equal charges. One pith ball is
suspended by and insulating thread. The other is brought to 3.0 cm from the suspended ball. The
suspended ball is now hanging with the thread forming an angle of 30.0° with the vertical. The
ball is in equilibrium with FE, Fg, and T adding vectorially to yield zero. Draw a force diagram,
then calculate
a. Weight of each pith ball
30º
b. Electrostatic Force between the pith balls
3.0 cm
c. Charge on each pith ball.
10. A charge q1 of -5.00 x 10-9 C and a charge q2 of -2.00 x 10-9 C are separated by a distance of 40.0
cm. Find the equilibrium position for a third charge of +15.0 x 10-9 C.