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1. Two isolated spheres X and Y of unknown materials are touching one another as shown below.

sphere X,earthed

sphere Y, negatively charged

Sphere Y is negatively charged and sphere X is earthed. The earth connection is removed from sphere X and then the spheres are separated as shown below.

sphere X,positively charged

sphere Y, negatively charged

Sphere X is found to be positively charged and sphere Y remains negatively charged.

Which of the following describes the nature of the materials from which the spheres are made?

Sphere X Sphere Y

A. Insulator Insulator

B. Insulator Conductor

C. Conductor Insulator

D. Conductor Conductor

(1)

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2. The diagram below shows a positively charged rod brought near an isolated uncharged metal plate.

+

+

+

+

+

+

++

rod

plate

As a result of bringing the rod near to the plate,

A. the metal plate will gain a charge dependent on the separation of the rod and the plate.

B. the metal plate will remain uncharged.

C. the metal plate will gain a negative charge.

D. the metal plate will gain a positive charge. (1)

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3. The diagram below shows a charged rod R suspended by insulating strings. When a stationary rod S is placed nearby, rod R is attracted towards it.

rod R rod S

insulated stand

Consider the following statements regarding the possible nature of the rod S.

I. Rod S is charged

II. Rod S is an uncharged insulator.

III. Rod S is an uncharged conductor.

Which statement(s) can explain the attraction of rod R to rod S?

A. I only

B. II only

C. III only

D. I and III only (1)

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4. A positively charged rod is brought close to an earthed sphere S, as shown below.

S

The earth connection to the sphere is removed and then the charged rod is removed. The sphere S is found to be negatively charged. Which one of the following describes the material of S and the movement of charge between S and earth?

Material of S Movement of charge

A. conductor negative charge moves from earth to S

B. insulator negative charge moves from earth to S

C. conductor positive charge moves from S to earth

D. insulator positive charge moves from S to earth

(1)

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5. An electrically neutral conducting sphere is suspended vertically from an insulating thread.

Q

thread

sphere

A point charge of magnitude Q is brought near the sphere. The electric force between the point charge and the sphere

A. depends on whether Q is positive or negative.

B. is always zero.

C. is always repulsive.

D. is always attractive. (1)

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6. An isolated, uncharged metal conductor is brought close to a positively charged

insulator.

+ + + +

+ + + +insulating handle

conductor

insulator

The conductor is earthed (grounded) for a short time and then the insulator is removed.

Which of the following best represents the charge distribution on the surface of the conductor as a result of these actions?

—

—— —

— —— —

——————

——A.

D.C.

B.

(1)

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7. What is the force between a 3 C charge and a 2 C charge separated by a distance of 5 m?

8. What is the magnitude and direction of the electric force on the electron of a hydrogen atom exerted by the single proton (Q2 = +e) that is the atom's nucleus? Assume the average distance between the revolving electron and the proton is 0.53 x 10-10 m. (8.20E-08 N)

9. Two positive point charges, Q1 = 50 µC and Q2 = 1µC, are separated by a distance L. Which is larger in magnitude, the force that Q1 exerts on Q2 or the force that Q2 exerts on Q1?

10. The force between a pair of 0.001 C charges is 200 N. What is the distance between them?

REM: Convention. F32: force on 3 by 2.

11. Three point charges in line. Net electrostatic force on particle 3? (1.5 N left)

12. In above figure, net force on 1?

REM: Vector addition: graphical, calculation

13. Three charges at each apex of a right angled triangle. What is the net electrostatic force on 3? (282 N, 650 to horizontal)

14. Net force on 1? (273 N, 1650 anticlockwise to +ve horizon)

15. In problem 13, where to put a fourth, equal, charge so as to make net force on 3 zero? (Equal charge @ 0.37 m)

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16. Two identical, small spheres of mass 2.0 g are fastened to the ends of a 0.60 m long light, flexible, insulating fish line. The fish line is suspended by a hook in the ceiling at its exact center. The spheres are each given an identical electric charge. They are in

static equilibrium, with an angle of 30between the string halves, as shown. Calculate the magnitude of the charge on each sphere. g = 9.8 m/s2. (1.20E-07 C)

ELECTRIC FIELD

1. Calculate magnitude/direction of electric field at pt P 30 cm to right of a pt charge -3.00E-06 C. [3.00E05 N/C]

2. Two point charges 10 cm apart. Charge 1: -25 µC. Charge 2: +50 µC. Determine mag/dir of E at pt P between the two charges 2 cm from -ve charge. [6.33E08 N/C]

3. In Q2, if an electron (mass 9.11E-31 kg) placed at P and released, its initial accel--mag/dir? [1.11E20 m/s2]

4. Calculate the total electric field at pt A--mag/dir. [4.5E06 N/C, 760]

5. Calculate the total electric field at pt B. [3.6E06 N/C, 00]

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*F MORE BASICS PRACTICE 2) Is it possible for two negative charges to attract each other? A) Yes, they always attract. B) Yes, they will attract if they are close enough. C) Yes, they will attract if one carries a larger charge than the other. D) No, they will never attract. Diff: 1 Type: BI Var: 2 Page Ref: Sec. 16.1-16.4 3) Is it possible for a positive and a negative charge to attract each other? A) Yes, they always attract. B) Yes, they will attract if they are close enough. C) Yes, they will attract if one carries a larger charge than the other. D) No, they will never attract. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 4) A glass rod is rubbed with a piece of silk. During the process the glass rod acquires a positive charge and the silk A) acquires a positive charge also. B) acquires a negative charge. C) remains neutral. D) could either be positively charged or negatively charged. It depends on how hard the rod was rubbed. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 7) A neutral atom always has A) more neutrons than protons. B) more protons than electrons. C) the same number of neutrons as protons. D) the same number of protons as electrons. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 8) An atom has more electrons than protons. The atom is A) a positive ion. B) a negative ion. C) a superconductor. D) impossible. iff: 1 Type: BI Var: 2 Page Ref: Sec. 16.1-16.4 9) Materials in which the electrons are bound very tightly to the nuclei are referred to as A) insulators. B) conductors. C) semiconductors. D) superconductors. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 10) Materials in which the electrons are bound very loosely to the nuclei and can move about freely within the

material are referred to as A) insulators. B) conductors. C) semiconductors. D) superconductors. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 11) A negatively charged rod is brought near one end of an uncharged metal bar. The end of the metal bar farthest

from the charged rod will be charged A) positive. B) negative. C) neutral. D) none of the given answers Diff: 2 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 12) Sphere A carries a net positive charge, and sphere B is neutral. They are placed near each other on an insulated

table. Sphere B is briefly touched with a wire that is grounded. Which statement is correct?

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A) Sphere B remains neutral. B) Sphere B is now positively charged. C) Sphere B is now negatively charged, D) The charge on sphere B cannot be determined without additional information. Diff: 2 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 13) How can a negatively charged rod charge an electroscope positively? A) by conduction B) by induction C) by deduction D) It cannot. Diff: 1 Type: BI Var: 2 Page Ref: Sec. 16.1-16.4 14) An originally neutral electroscope is briefly touched with a positively charged glass rod. The electroscope A) remains neutral. B) becomes negatively charged. C) becomes positively charged. D) could become either positively or negatively charged, depending on the time of contact. Diff: 1 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 15) An originally neutral electroscope is grounded briefly while a positively charged glass rod is held near it. After

the glass rod is removed, the electroscope A) remains neutral. B) is negatively charged. C) is positively charged. D) could be either positively or negatively charged, depending on how long the contact with ground lasted. Diff: 2 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4 16) A positive object touches a neutral electroscope, and the leaves separate. Then a negative object is brought near

the electroscope, but does not touch it. What happens to the leaves? A) They separate further. B) They move closer together. C) They are unaffected. D) cannot be determined without further information Diff: 2 Type: BI Var: 1 Page Ref: Sec. 16.1-16.4

*BLab, Electroscope Lab 1. Charges and Electrostatics INTRODUCTION: Most modern applications of electricity involve moving electric charges or current electricity. Historically, however, the first studies of electricity involved static charges, or electrostatics. You certainly feel the effects of electrostatic charges every time you touch a doorknob in the wintertime and get zapped. In this lab you will be introduced to an instrument called an electroscope, which is used to measure electric charge. By the end of this lab you should understand how the electroscope physically works, you should know how to use it to measure electric charge, and you should be able to use it as a tool to discover some electrical properties of various plastic materials. THE ELECTROSCOPE: The instrument that you will use in this lab is called an electroscope. The electroscope consists of a metal ball connected by a metal rod (the stem) to a very thin leaf of gold foil. The fragile part of the instrument is enclosed in a protective case. Electrons can flow freely within the ball, stem, and gold leaf. The leaf is very

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lightweight and floppy, so that an electrical repulsive force acting to lift it up can more easily overcome the downward force of gravity. PRECAUTIONS & NOTES: • Never tilt the electroscope or turn it upside down. This can tear the foil off. Just leave it on the table. • When you bring a charged object near the electroscope, do so from the top. Do not bring any charged object near the glass sides, as this can twist the gold foil and rip it off. • In order to ground the electroscope (that is, remove all the net charge from it so it’s neutral), touch the metal grounding plate to the metal ball on top of the electroscope. Touching your hand to the top of the electroscope can sometimes work, if you do not have any charge on your hands. • Before charging and testing the plastic rods, make sure that they are neutral first. You can ground the plastic rods by covering the metal grounding plate with a damp towel and wipe the rods across the cloth. Always test the rod with the electroscope to make sure it is neutral first. • If you rub two objects together to test the charge on each, make sure you hold the side that was rubbed near to the metal ball on the electroscope. USING THE ELECTROSCOPE The simplest way to obtain an electric charge is to take almost any two materials and rub them together. Often, one of the materials becomes negatively charged and the other positively. For example, if you scuff the rubber soles of your shoes along a carpet, you will acquire some of the electrons from the rug. Your body will then have an excess of electrons – to get rid of these, just touch the nearest doorknob or metal bookcase. A similar effect occurs if you rub a teflon rod with a woolen cloth; the teflon rod acquires a negative charge and the wool acquires an equal amount of positive charge. We know that all matter contains positive and negative charges. The positive charge is deep inside each atom on the atomic nucleus, while the negative charge is on the electrons that orbit outside the nucleus. In normal, neutral matter the amounts of positive and negative charge are equal, and we are unaware of either. Given any two different materials, it usually happens that one has a greater attraction, or affinity, for electrons than the other; when the two are rubbed together, the material with higher affinity captures some electrons from the material with lower affinity. For example, teflon has higher electron affinity than wool; when they are rubbed together the teflon captures a few electrons from the wool; the teflon therefore acquires an excess of electrons and is negatively charged, while the wool is left with a deficit of electrons and is therefore positively charged. It is important to note here the difference between insulators and conductors: electrons can move freely within conductors (such as metal), but don’t move very easily on insulators (like cloth or plastic rods). The preceding paragraph applies to insulators in this lab.

1) Make sure there is no charge on the electroscope so that its leaf hangs straight down. Now rub the teflon rod with the wool and bring the rod close to (but not touching) the ball of the electroscope, causing the leaf to rise. Using a diagram, explain what happens.

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2) Take away the teflon rod, and bring the wool cloth close to (but not touching) the ball. Draw a diagram and explain what happens.

3) *CUsing just a neutral electroscope as you did here, can you tell the sign of the charge which is close to the ball? If so, how?

4) Take a charged rod and touch it to the metal ball on the electroscope, then take the rod away. Using one or more diagrams, explain what happens.

5) *D Charge the electroscope negatively by touching the ball with the negatively charged rod. Find out and explain how you can increase the amount of charge left on the electroscope. How can you tell that more charge has been left?

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POTENTIAL

1. How much work must be done to bring a 4.0 µC charged object to within 1.0 m of a 6.0 µC charged

object from infinity? (At infinity, Ep = 0.) [0.216 J]

2. How much work must be done to bring a -7.0 µC charged object to within 0.5 m of a 5.0 µC charged

object from infinity? [-0.63 J]

3. Calculate the potential at P in the diagram. [694,286 V]

4. What is the pd between A and B due to the charge shown? [72k V]

5.

[3.60E-06 J]

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6. How much work must be done to take a -7.0 µC charge which is 0.5 m from a 5.0 µC charge to

infinity?