Review Chapter 17 & 18Pre AP 2012
1. What sentence best characterizes electron conductors?
a. They have low mass density
b. They have high tensile strength
c. They have electric charges that move freely
d. They are poor heat conductors
2. What happens when a rubber rod is rubbed with a piece of fur, giving it a negative charge?
a. Protons are removed from the rod
b. The fur is also negatively charged
c. The fur is left neutral
d. Electrons are added to the rod
3. An attracting force occurs between two charged objects when
a. charges are of unlike signs
b. charges are of like signs
c. charges are of equal magnitude
d. charges are of unequal magnitude
4. A repelling force occurs between two charged objects when
a. charges are of unlike signs
b. charges are of like signs
c. charges are of equal magnitude
d. charges are of unequal magnitude
5. Which sentence best characterizes electric insulators?
a. Charges on their surface do not move
b. they have high tensile strength
c. Electric charges move freely on them
d. They are good heat conductors
6. When a glass rod is rubbed with silk and becomes positively charged,
a. electrons are removed from the rod
b. protons are added to the silk
c. protons are removed from the silk
d. the silk remains neutral
7. Which of the following transfers charge most easily?
a. nonconductors
b. conductors
c. semiconductors
d. insulators
8. If a positively charged glass rod is used to charge a metal bar by induction,
a. the charge on the bar will be equal in magnitude to the charge on the glass rod
b. the charge on the bar must be negative
c. the charge on the bar must be positive
d. the charge on the bar will be greater in magnitude than the charge on the glass rod
9. The process of charging a conductor by bringing it near another charged object and then grounding the conductor is called
a. charging by conduction
b. charging by polarization
c. neutralization
d. charging by induction
10. Electric charge is
a. found only in a conductor
b. conserved
c. found only in insulators
d. not conserved
12. A surface charge can be produced on insulators by
a. grounding
b. induction
c. polarization
d. contact
13. When a charged body is brought close to an uncharged body without touching it, a(n) _____ charge may result on the uncharged body. When a charged body is brought into contact with an uncharged body and then is removed, a(n) _____ charge may result on the uncharged body.
a. negative, positive
b. positive, negative
c. induced, residual
d. residual, induced
14. A balloon has a negative charge of 3.5 μC. How many excess electrons are on the balloon?
a. 3.5 x 10-6
b. 2.2 x 1013
c. 3.9 x 10-16
d. 3.8 x 1024
15. What determines the direction of the force between two charges?
a. the signs of the charges
b. the magnitude of the charges
c. the distance between the charges
d. Coulomb’s law
16. Mark the following statements concerning electric field lines True or False.
T a. Electric field lines cannot cross each other.
Tb. Electric field lines may begin at a positive charge and end at a negative charge.
Tc. Electric field lines are always tangent to the direction of the electric field.
T d. The density of Electric field lines shows the strength of the electric field.
T e. The field lines from a charge may extend out to or originate at infinity.
17. Two parallel plates are oppositely charged. The left plate is negative and the right plate is positive. In which direction does the electric field point?
a. Right
b. Left
c. Cannot be determined
Problems:
- What is the electric field half way between a proton and an alpha particle (charge +2e) that are separated by a distance of 3.0 x 10-6 m?
r1 = r/2
E = E1 + -E2 Kq1/r12 - KKq2/r12 = K/r12(e-2e) = [9.0e9/(1.5e-6)]2(1.6e-19-3.2e-19) = -640 N/C
- Find the electric force on a 60 nC charge located at (3,0) ) if there are two other charges of +40.0 nC, one located at (-2,0) and the other at (0,0).
F3 = F31 + F32 = Kq3{q2/22 + q3/52} = (9.0e9)(60e-9)[(40e-9)/9+(40e-9)/25] =
3.3 x10-6 NIn the positive x-direction
- A +50µC and a + 10 µC charge are located on the Y-axis 4 m apart.
- Where between the two charges would a third charge be at equilibrium?
F31 = F32 k q3q1 /L2 = k q3q2/(4-L)2 10 (4-L)2 = 50 L2 L = 4/(1+√5)= 1.236 m from the 10 µC charge
- What is the electric field strength at that point?
zero
- Is there another point along the Y axis but not between the charges where the third charge could be at equilibrium? NO
- Calculate the net charge on a substance consisting of a combination of 7.0 x 1013 protons and 4.0 x 1013 electrons.
7.0 – 4.0 = 3 x 1013 protons If 1 proton = 1.6 x 10-19 C then 3 x 1013 * 1.6 x 10-19 C = 4.8 C
- A Van de Graaff generator is charged so that the electric field at its surface is 3.0 x 104 N/C.
- What is the magnitude of the electric force on a proton released at the surface of the generator?
- What is the magnitude of the electric force on an electron at the same location?
Same as 6a
- If free to move, would the proton or electron experience greater acceleration? Why? The electron as a = F/m and an electron has smaller mass
than a proton.
8. Two metal spheres that are initially uncharged are mounted on insulating stands as shown. Sphere X is touching sphere Y. A negatively charged metal rod is brought close to (but not touching) sphere X. While the rod is present, sphere Y is removed some distance away. The rod is then withdrawn. After the process,
- What are the changes on sphere X and Sphere Y? X is positive and Y is negative.
- How does the quantity of charge on X compare to that on Y? Same
- What is the overall charge of the X & Y system? Zero
- While still avoiding contact with the rod, how could you modify the process to get the same charge on both X and Y after separation?
Ground sphere y while it is still in contact with sphere x.
C> D= E> B= G> A= F
For C: both forces to left, distance between charges is minimum
D & E are same but in opposite directions, force due to both charges is in same direction for each case.
B & G are same but in opposite directions, forces due to charges are in opposite directions
A & F are both zero