Ms. N. May   

Physics       Week 1(Sem. 2)      Name____________________________ 

Chapter Summary 

Electricity­Electric Forces 

Basics of Electricity – All matter is composed of atoms which contain protons, neutrons and electrons.  Each of these subatomic particles has mass.  Protons and neutrons have a mass of 1.675x10‐27kg, while the electron has a mass of 9.11x10‐31 kg.  The neutron is neutral and therefore is said to have no charge.  Both the proton and the electron have been found to carry the same magnitude of charge.  The proton is +e and the electron is –e.  The SI unit for measuring charge is the coulomb(C), e=1.60x10‐19 C. 

Electric Charge 

The charge on an electron or a proton is the smallest amount of free charge that has been discovered.  Charges build up by the adding or removing of electrons.  Therefore all charges are integer multiples of e, the charge e is elementary and indivisible. 

The separation of charges, when two materials are brought into contact they can transfer charges.  For example, if a piece of plastic and a piece of fur were rubbed together some of the electrons will transfer from one object to the other.  This could cause the plastic rod to be negative and the fur to be positively charged.  The positive and negative charges that arise on these objects are due to the transfer of electrons.  There are no electrons or protons created or destroyed in the process.  Because electrons and protons have the magnitude of charges, the algebraic sum of the charges on the two objects will always equal zero. 

The Law of Conservation of Electric Charge 

The law of conservation of electric charge can be observed in the above mentioned situation as well as in chemical reactions, electric circuits and radioactive decay.  The law states that “during any process, the 

net electric charge of an isolated system remains constant (is conserved).” 

Electrostatic Force 

The electric force, electrostatic force, is the force that two electrically charged objects exert on one another.  If a negatively charged object were brought near a positively charged object there would be a force of attraction between them.  If two negatively charged objects, or two positively charged objects, were brought near each other there would be a repulsive force between them.  This force that is experienced between the objects can alter motion (cause acceleration).  As seen in Newton’s second law F=ma, if there is an external force on an object of mass m it will have an acceleration of a.  This concept is used in the painting of cars; they give the metal of the car on charge and the paint the opposite charge.  This difference in charge makes the paint attach to the cover and coat it evenly. 

Charging Objects 

There are two ways to electrically charge objects, by contact and by induction.  So by touching a negatively charged plastic rod to a conductive sphere, some of the excess negative charges (electrons) will transfer to the sphere.  Upon removal of the plastic rod from contact, the sphere would be left with a negative charge.  Similarly, if the plastic were to be positively charged the sphere would be left with a positive charge.  The process of giving an object a net electric charge by placing into contact with another object is called charging by contact. 

Charge by induction involves putting a charged object, let’s say a plastic rod, near an object.  As the charged rod approaches the object all of the like charges migrate toward the opposite side of the object, while all of the opposite charges tend to be attracted to the rod side of the object.  The migration, 

Ms. N. May   

or movement of charges within the object creates charged regions.  These regions were induced or persuaded by the presence of the charged plastic rod.  If the charged rod were to be removed all of the objects would migrate back to their original positions.  But if while the charged rod is close to the sphere there were a grounding wire attached to allow free electrons to flow into the ground (the Earth is a good conductor of electrons), then the object would be left with a net positive charge.  The ultimate result of bringing the negative plastic rod close to a grounded object would be a positively charged object.  If the sphere were an insulator no net charge would result.  Although the outside surface of the object would tend to be positive and the negatives would tend to move slightly away.  This is the cause of static cling in clothing. 

Coulomb’s Law 

The electrostatic force that charged objects exert on each other.  Experimentation has revealed that the higher the charge and the closer the charged objects are to each other the stronger the force between them, either repulsive or attractive force.  Assuming the charged objects are small enough to be considered points or point charges.  If their charge would be +q1 and ‐q2, then there would be a force (+F) that q1 exerts on q2.  And then charge q2 would exert a force of –F on q1.  Therefore Coulombs law is 

 

Where k=8.99x109 N.m2/C2, q1 is the charge of object 1 in Coulombs, q2 is the charge of object 2 in 

Coulombs, and r is the distance between the two charged objects. 

The constant k is also expressed in terms of εo, which is the permittivity of free space.  So k is equal to 1/4π εo, where εo is 8.85x10‐12 C2/(N m2).  Most charges are given in microCoulomb’s because they are typically very small. 

It is interesting to note the similarity of Coulomb’s law to Newton’s law of Gravitation.  They are both inversely related to the inverse of the distance between two objects.  The Forces are also a function of the two objects, mass or electric charge.  The one major difference is that Newton’s law is always attractive and Coulomb’s law can be either repulsive or attractive depending on the charges of the objects. 

Force by Multiple Charges 

Coulomb’s law applies to two charges interacting, as well as multiple charges interacting.  For example, if a charged object q1 experienced a force from a charge q2 and another charge q3.  Coulomb’s Law can be applied to the force experienced by q1 from q2.  Then the force experienced by q1 from q3 can be calculated separately, ignoring q2 completely.  Finally, the net electrostatic force experienced by the charge q1 will be the sum of the forces of q2 and q3 on q1.  If the forces are not in the same direction then the components of forces (in the x and y direction) can be added.  The net force experienced by that point charge would then be determined by taking the resultant forces and using Pythagorean’s theorem. 

 

         

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Name: ____________________________________________

1) Neutral atoms always have equal numbers ofA) protons and positronsB) protons and electronsC) electrons and neutronsD) protons and neutrons

2) When a rod is brought near a neutral electroscope, theleaves diverge. Which statement best describes the chargeon the rod?A) It may be neutral.B) It must be negative.C) It may be positive or negative.D) It must be positive.

3) Which diagram best represents the charge distribution on aneutral electroscope when a negatively charged rod is heldnear it?

A)

B)

C)

D)

4) A body will maintain a constant negative electrostaticcharge if the bodyA) maintains the same excess of protonsB) maintains the same excess of electronsC) continuously receives more protons than it losesD) continuously receives more electrons than it loses

5) The diagrams below represent cross sections of negativelycharged rods. Which diagram best illustrates the electricfield around the negatively charged rod?

A)

B)

C)

D)

6) A neutral rubber rod is rubbed with fur and acquires acharge of -2 x 10-6 coulomb. The charge on the fur is

A) -2 x 10-6 CB) -1 x 10-6 C

C) +1 x 10-6 CD) +2 x 10-6 C

7) Which electric charge is possible?

A) 8.0 x 10-20 coulombB) 6.32 x 10-18 coulombC) 3.2 x 10-19 coulombD) 2.4 x 10-19 coulomb

8) Metal sphere A has a charge of -2 units and an identicalsphere B has a charge of -4 units. If the two spheres arebrought together and then separated, the charge onsphere A will beA) +4 unitsB) -2 units

C) -3 unitsD) 0 units

9) Sphere A has a charge of +2 x 10-6 coulomb and is broughtinto contact with a similar sphere, B, which has a charge of-4 x 10-6 coulomb. After it is separated from sphere B,sphere A will have a charge of

A) -2 x 10-6 CB) -1 x 10-6 C

C) +6 x 10-6 CD) +2 x 10-6 C

10) When two neutral materials are rubbed together, there is atransfer of electrical charge from one material to the other.The total electrical charge for the systemA) remains constant as protons are transferredB) remains constant as electrons are transferredC) increases as protons are transferredD) increases as electrons are transferred

11) One of two identical metal spheres has a charge of +q, andthe other sphere has a charge of -q. The spheres arebrought together and then separated. Compared to the totalcharge on the two spheres before contact, the total chargeon the two spheres after contact isA) greaterB) lessC) the same

12) What is the charge of a proton?

A) 6.3 x 1018 CB) 1.6 x 10-19 C

C) 1.7 x 10-27 CD) 9.1 x 10-31 C

13) The coulomb is a unit of electricalA) resistanceB) charge

C) potentialD) current

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Questions 29 and 30 refer to the following:

The diagram below represents two small, charged conducting spheres, identical in size, located 2.00 meters apart.

14) A conductor is connected between spheres A and B andthen removed after charge has transferred. During theperiod the conductor was attached, what was the directionof the flow of charged particles?A) Protons flowed from B to A as electrons flowed from A

to B.B) Protons flowed from A to B as electrons flowed from B

to A.C) Electrons flowed from B to A, only.D) Protons flowed from A to B, only.

15) What is the net combined charge on both spheres?

A) -1.00 x 10-6 coulombB) -9.00 x 10-6 coulombC) +1.00 x 10-6 coulombD) +9.00 x 10-6 coulomb

16) Two charges that are 2 meters apart repel each other with aforce of 2 x 10-5 newton. If the distance between thecharges is decreased to 1 meter, the force of repulsion willbe

A) 1 x 10-5 NB) 8 x 10-5 N

C) 4 x 10-5 ND) 5 x 10-6 N

17) If the charge on each of two small spheres a fixed distanceapart is doubled, the force of attraction between the sphereswill beA) quadrupledB) halved

C) quarteredD) doubled

18) Electrostatic force F exists between two point charges witha separation distance d. Which graph best represents therelationship between F and d?

A)

B)

C)

D)

19) The electrical force of attraction between two point chargesis F. The charge on one of the objects is quadrupled andthe charge on the other object is doubled. The new forcebetween objects isA) 6FB) 8F

C) \FD) 2F

20) As the distance between two point charges is tripled, theelectrostatic force between the charges will become

A) as great

B) 9 times as great

C) as great

D) 3 times as great

Questions 19 and 20 refer to the following:

The diagram below represents a system consisting of two chargedmetal spheres with equal radii.

21) What is the magnitude of the electrostatic force exerted onsphere A?

A) 1.1 x 10-9 NB) 120 N

C) 10. ND) 1.3 x 10-8 N

22) If the two spheres were touched together and thenseparated, the charge on sphere A would be

A) -8.0 x 10-4 CB) -3.0 x 10-4 C

C) 2.0 x 10-4 CD) -6.0 x 10-4 C

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23) The diagram below represents two charged spheres, X and Y.

The magnitude of the force between the two spheres is

A) 3.0 x 103 N B) 1.0 x 10-3 N C) 3.0 x 10-3 N D) 1.0 x 103 N

24) An inflated balloon which has been rubbed against aperson's hair is touched to a neutral wall and remainsattracted to it. Which diagram best represents the chargedistribution on the balloon and wall?

A)

B)

C)

D)

25) The diagram below shows the arrangement of three chargedhollow metal spheres, A, B, and C. The arrows indicate thedirection of the electric forces acting between the spheres.At least two of the spheres are positively charged.

Which sphere, if any, could be negatively charged?A) no sphereB) sphere C

C) sphere BD) sphere A

26) The diagram below shows the initial charge and position ofthree identical metal spheres, X, Y, and Z, which have beenplaced on insulating stands.

All three spheres are simultaneously brought into contactwith each other and then returned to their original positions.Which statement best describes the charge of the spheresafter this procedure is completed?A) All the spheres are neutral.B) Sphere Y has a greater charge than spheres X or Z.C) Each sphere has a net charge of +4 x 10-6 coulomb.D) Each sphere retains the same charge that it had

originally.

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27) The diagram below shows four charged metal spheressuspended by strings. The charge of each sphere isindicated.

If spheres A, B, C, and D simultaneously come into contact,the net charge on the four spheres will beA) +1 CB) +2 C

C) +3 CD) +4 C

28) Two electrically neutral metal spheres, A and B, oninsulating stands are placed in contact with each other. Anegatively charged rod is brought near, but does not touchthe spheres, as shown in the diagram below.

How are the spheres now charged?A) A is positive and B is negative.B) A is negative and B is negative.C) A is positive and B is positive.D) A is negative and B is positive.

29) Two similar metal spheres possessing +1.0 coulomb ofcharge and -1.0 coulomb of charge, respectively, arebrought toward each other. Which graph best representsthe relationship between the magnitude of the electric forcebetween the spheres and the distance between them?

A)

B)

C)

D)

30) In the diagram below, a positive test charge is locatedbetween two charged spheres, A and B. Sphere A has acharge of +2q and is located 0.2 meter from the test charge.Sphere B has a charge of -2q and is located 0.1 meter fromthe test charge.

If the magnitude of the force on the test charge due tosphere A is F, what is the magnitude of the force on the testcharge due to sphere B?

A) 2F

B)

C)

D) 4F

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31) The diagram below shows two identical metal spheres, Aand B, separated by distance d. Each sphere has mass mand possesses charge q.

Which diagram best represents the electrostatic force Feand the gravitational force Fg acting on sphere B due tosphere A?

A)

B)

C)

D)

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