Conceptual Questions:
1. Explain from an atomic point of view why charge is usually transfered by electrons.
2. Sketch the field lines near two neighboring positive charges, and those near neighboring positive and negative charges.
Numerical Problems: answers link to a help page.
1. An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be:
a. negative.
b. positive.
c. neutral.
d. attracted.
2. The number of electric field lines passing through a unit cross sectional area is indicative of:
a. field direction.
b. charge density.
c. field strength.
d. charge motion.
3. Which of the following best characterizes electrical conductors?
a. low mass density
b. high tensile strength
c. electric charges move freely
d. poor heat conductors
4. If the distance between two point charges is tripled, the mutual force between them will be changed by what factor?
5. The constant k which appears in Coulomb's law formula is equivalent dimensionally to which of the following?
a. N*m/Coul
b. N/Coul
c. N*m2/Coul2
d. N/Coul2
6. Two point charges, separated by 1.5 cm, have charge values of +2.0 and -4.0 µC, respectively. What is the value of the mutual force between them? (k = 9 x109)
7. An electron with a charge value of -1.6 x10-19 C, is in the presence of an electric field of 400 N/C. What force does the electron experience?
8. A ping pong ball covered with a conducting graphite coating has a mass of 0.005 kg and a charge of 4 µC. What electric field directed upward will exactly balance the weight of the ball? (g = 9.8 m/s2)
9. The average distance of the electron from the proton in the hydrogen atom is 0.51 x10-10m. What is the electric field from the proton's charge at the location of the electron? (k = 9 x109 N m2/C2, qe = 1.6 x10-19 C)
10. Two identical iron spheres have 1 mole (6 x1023) of iron atoms each, and equal positive charges. The force between them when they are 1 m apart is 1 N. If each atom provides one free electron to the metal, what percentage of the free electrons have been removed from each sphere? (k = 9 x109 N m2/C2, qe = 1.6 x10-19 C)
11. If a conductor is in electrostatic equilibrium near an electrical charge:
a. the total charge on the conductor must be zero.
b. the electric field inside the conductor must be zero.
c. any charges on the conductor must be uniformly distributed.
d. the sum of all forces between the conductor and the charge must be zero.
12. In X-ray machines, electrons are subjected to electric fields as great as 6 x105 N/C. Find an electron's acceleration in this field. (me = 9.1 x10-31 kg, qe = -1.6 x10-19 C)
13. In Millikan's oil drop experiment, if the electric field between two metal plates was of just the right magnitude, it would exactly balance the weight of the drop. Suppose a tiny spherical oil droplet of radius 1.6 x10-4 cm carries a single electric charge. What electric field is required to balance the weight? (The density of oil is 0.85 gram/cm3)