Chapter 12 Electrostatic Phenomena

21. Two equal charges repel one another with a force of 4.0 × 10^-4 N when they are 10 cm apart. If they are moved until the separation is 5.0 cm, the repulsive force will be A. 16.0 × 10-4N. B. 8.0 × 10-4N. C. 4.0 × 10-4N. D. 2.0 × 10-4N. E. 1.0 × 10-4N.

A. 16.0 × 10^-4N.

1. An electron situated near another electron would feel A. a repulsive electrical force. B. an attractive electrical force. C. a repulsive magnetic force. D. an attractive magnetic force. E. no forces from the other electron.

A. a repulsive electrical force.

5. Two equal positive charges are near each other. If we increase the amount of charge on just one of them, then A. both charges feel an increased electrical force. B. only smaller charge feels an increased electrical force. C. the electric field felt by both charges increases. D. the electric field felt by the smaller charge increases. E. Both answers A and D

A. both charges feel an increased electrical force. and D. the electric field felt by the smaller charge increases.

11. If the distance between two spherical charged objects increases by a factor of two, then the electrical force between the objects A. decreases by a factor of four. B. is half as big. C. does not change. D. doubles. E. increases by a factor of four.

A. decreases by a factor of four.

6. An atom has A. just as many electrons as protons. B. no neutrons in the nucleus. C. more protons than electrons. D. as many electrons as protons and neutrons combined. E. at least 1 neutron.

A. just as many electrons as protons.

4. The charge of a proton is due to A. physicists are not yet quite sure but explain charge as a property of the proton. B. periodic pulsations of the surface of the proton. C. the high density of the proton. D. rapid spinning of the proton.

A. physicists are not yet quite sure but explain charge as a property of the proton.

12. The electric field created by a moving negative charge A. points toward the charge. B. points away from the charge. C. describes concentric circles about the direction of motion. D. points in the direction of motion. E. points in the direction opposite to the velocity.

A. points toward the charge.

15. After a glass rod is rubbed with a nylon cloth A. the rod will have a positive charge and the cloth negative. B. both the rod and cloth will have a positive charge. C. the rod will have a negative charge and the cloth positive. D. both the rod and cloth will have a negative charge. E. the rod will have a negative charge and the cloth will be neutral.

A. the rod will have a positive charge and the cloth negative.

Unlike charges:

Attract each other

20. A 10 μC charge is at the origin. A -5 μC charge is on the x-axis 10 cm to the right of the origin. At what point other than at infinity can a 1 μC charge be placed so that there will be no net electrostatic force on it? A. At one point between the two charges. B. At one point on the x-axis to the right of the negative charge. C. At one point on the x-axis to the left of positive charge. D. At some point off the x-axis either above or below. E. At no point.

B. At one point on the x-axis to the right of the negative charge.

3. Compared to the magnitude of the charge on a proton, the magnitude of the electrical charge carried by an electron is A. about the same. B. exactly the same. C. is only 1⁄2 as large. D. is only 1⁄3 as large.

B. exactly the same.

17. A positively charged rod is brought close to an uncharged electroscope. While the rod is close, one's finger touches the far side of the metal ball on the electroscope. The finger is removed and then the rod is taken away. The electroscope is A. positively charged. B. negatively charged. C. uncharged.

B. negatively charged.

14. A metallic object has a net charge on it. For steady state conditions, the excess charge is located A. on the bottom of the object . B. on the surface of the object. C. near the side closest to the North magnetic pole. D. at the center of mass of the object. E. at the top of the object.

B. on the surface of the object.

16. A rubber rod is rubbed with fur, so that the fur becomes positive. The rod is then used to charge an electroscope. The charge on the electroscope will be A. positive if charged by contact. B. positive if charged by induction. C. neutral. D. negative if charged by induction.

B. positive if charged by induction.

10. Our model for explaining the electrical interaction between charged objects A. depends on the existence of three kinds of charge. B. postulates that an electric field surrounds each charged object. C. makes sense only at very high speeds. D. depends very strongly on the mass of the two objects.

B. postulates that an electric field surrounds each charged object.

7. A strip of fur is rubbed against a hard rubber rod. The rod thereby acquires a negative charge because A. frictional forces create electrons. B. rubbing caused electrons to move from the fur to the rod. C. friction caused protons to be removed from the rod. D. electrons from the air molecules are transferred to the rod.

B. rubbing caused electrons to move from the fur to the rod.

2. Consider a proton and an electron placed near one another with no other objects close by. They would A. accelerate away from each other. B. remain motionless. C. accelerate toward each other. D. be pulled together at constant speed. E. move away from each other at constant speed.

C. accelerate toward each other.

27. A capacitor consisting of two separated parallel horizontal plates has a uniform electric field directed upward. If a negative charge is placed exactly midway between the two plates, it will A. remain at rest. B. be accelerated upward. C. be accelerated downward. D. be accelerated to the right. E. be accelerated to the left.

C. be accelerated downward.

22. A positive charge is fixed at the origin. A negative charge is moved along the x-axis from x = 50 cm to x = 10 cm. The potential energy of the negative charge A. does not change. B. increases. C. decreases.

C. decreases.

18. A negatively charged rod is brought close to an uncharged electroscope. While the rod is close one's finger touches the far side of the metal ball on the electroscope. After the charged rod has been removed the finger is removed. The electroscope is A. positively charged. B. negatively charged. C. uncharged.

C. uncharged.

Charge in units:

Coulombs (C)

25. A uniform electric field has a magnitude of 10 N/C and is directed upward. A charge brought into the field experiences a force of 5.0 N downward. The charge must be A. +50 C. B. -50 C. C. +0.5 C. D. -0.5 C. E. -2.0 C.

D. -0.5 C.

26. A capacitor consisting of two parallel plates separated by 2.0 cm has a potential of 100 V on the top plate and a potential of 0 V on the bottom plate. The value of the electric field in the middle is A. 5.0 N/C. B. 50 N/C. C. 500 N/C. D. 5000 N/C. E. 200 N/C.

D. 5000 N/C.

13. If the potential at point A is one volt it means that A. an object at A has charge equal to one coulomb. B. an electron at A can perform one joule of work. C. there is a small battery present. D. a charge at A has potential energy one joule for each coulomb of charge.

D. a charge at A has potential energy one joule for each coulomb of charge.

24. Three equal negative charges are placed at three of the four corners of a square. The direction of the electric field at the remaining corner of the square is A. along a side of the square toward one of the charges. B. along a side of the square away from one of the charges. C. along the diagonal connecting this corner and another charge, away from the other charge. D. along the diagonal connecting this corner and another charge, toward the other charge. E. no direction, zero field.

D. along the diagonal connecting this corner and another charge, toward the other charge.

19. Comparing the electrostatic force and the gravitational force we can say that A. both have the same dependence on distance, both involve attraction and repulsion but the gravitational force is stronger. B. both have the same dependence on distance, both involve attraction and repulsion but the electrostatic force is stronger. C. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only repulsive, and the electrostatic force is weaker. D. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only attractive, and the electrostatic force is stronger. E. the electrostatic force falls off more rapidly with distance, the electrostatic force can be either attractive or repulsive while gravitation is only attractive and the electrostatic force is stronger.

D. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only attractive, and the electrostatic force is stronger.

9. The electrical force between any two charged objects is A. always attractive. B. always repulsive. C. always balanced by the action-reaction forces. D. dependent on the sign of the charge on each object.

D. dependent on the sign of the charge on each object.

8. The electrical force between two charged objects depends on A. the mass of the objects. B. the color of the objects. C. the amount of combined charge of both objects. D. the amount of charge on each of the two objects. E. which object has the greatest charge.

D. the amount of charge on each of the two objects.

33. A negative charge released between the parallel plates of a capacitor moves downward. The potential energy of the negative charge _________________________ (indicate change).

Decreases

28. The unit for electric field, N/C can be written equivalently as A. V. B. m/V. C. V/C. D. C/V. E. V/m.

E. V/m.

23. Imagine four equal positive charges, q, placed on points of a circle of radius r at the 3, 6, 9 and 12 o'clock positions. The magnitude of the electric field at the center of the circle is given by A. 4kq/r. B. 4kq/r2. C. kq/r. D. kq/r2. E. zero.

E. zero.

31. A positive test charge is moved closer to another positive charge. The potential of the test charge _____________________ (indicate change).

Increases

Semiconductors

Materials that are intermediate between conductors and insulators EX: silicon, wooden rod

Insulators

Materials that do not ordinarily permit the flow of charge EX: plastic, glass

Conductors

Materials through which charge can flow readily EX: metal rods, our bodies

29. A plastic rod is rubber with fur. The rod is then touched to the metal knob of an electroscope. The sign of the charge on the electroscope after contact is ___________________.

Negative

30. A glass rod is rubbed with nylon. The rod is then brought close to the metal knob of an electroscope. A person's finger briefly touches the knob on the far side and is then removed. After the rod is removed the charge on the electroscope will be _________.

Negative

36. A small bit of paper is electrically neutral. A charged rod can attract the bit of paper by the process of __________________.

Polarization

Like charges:

Repel each other

34. In a certain region the electric lines of force get farther apart as one moves to the right. The force on a positive charge will decrease as the charge is moved to the ____________________.

Right

32. It is observed that a positive charge released between the parallel plates of a capacitor moves upward. The direction of the electric field between the capacitor plates is __________________.

Upward

35. Four equal positive charges are placed at the corners of a square. The force on a +1 C charge placed at the center due to any single charge is 10 N. The value of the electric field at the center due to all charges is ____________________ N/C.

Zero