Lecture Presentation - Calhoun Physics · Slide 20-55 20.3: Coulomb’s Law 12) Two tiny beads are 25 cm apart with no other charges or fields present. Bead A carries 10 µC of charge

Chapter 20

Lecture Presentation

Electric Fields and Forces

© 2015 Pearson Education, Inc.

Slide 20-2

The Reference Table


Slide 20-3

Improvised

𝑞 = 𝑛𝑒


20.1 Charges and Forces

20.2 Charges, Atoms, and Molecules


Slide 20-5

Charges, Atoms, and Molecules

• Dense, positively

charged nucleus.

• Positive Protons

• Neutral Neutrons

• Electron Cloud

• Much-less-massive

orbiting negatively

charged electrons


Slide 20-6

An Atomic View of Charging

• The Elementary (most basic) Charge

• Protons (+)

• Electrons ( - )

• An object is …

• Positive

• More protons than electrons

• Negative

• More electrons that protons

• Neutral

• Even number of protons and electrons

• Only Electrons Move!!!


Slide 20-7

An Atomic View of Charging

𝑞 = 𝑛𝑒

q = charge on an object ( C )

n = the number of excess protons or electrons

e = the elementary charge (Front of Reference Table)


Slide 20-8

Excess diagrams

+ +

+

q = -4.8 x 10-19 C

Slide 20-9

Excess diagrams

+ +

+

+ +

q = +6.4 x 10-19 C

Slide 20-10

Excess diagrams

q = -3.2 x 10-19 C

Slide 20-11

Can this charge exist?

•𝑞 = −1.7 × 10−20C?

•𝑞 = +2.4 × 10−18C?

•𝑞 = −1.28 × 10−18C?

•𝑞 = +2.816 × 10−18C?

•Can’t have half a proton or electron

No

No

Yes

Yes

METER STICK & 2x4

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

- + - + - + - + - + - + - + - + - + - + - + - + - + - + -

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

- + - + - + - + - + - + - + - + - + - + - + - + - + - + -

- - - - - - - - - - - - - - - - - + - + - + - + - + - + - + - +

METER STICK & 2x4

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

- + - + - + - + - + - + - + - + - + - + - + - + - + - + -

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - +

- + - + - + - + - + - + - + - + - + - + - + - + - + - + -

- - - - - - - - - - - - - - - - - + - + - + - + - + - + - + - +

WATER STREAM

+ -

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WATER STREAM

PITH BALL

+ - + - + - + -

- + - + - + - +

+ - + - + - + -

PITH BALL

+ - + - + - + -

- + - + - + - +

+ - + - + - + -

PITH BALL

+ - + - + - + -

- + - + - + - +

+ - + - + - + -

Slide 20-19

Polarization

•Neutral objects become attractive

•Object’s total charge remains neutral

•Electrons move to one side

•Making that side negative

•Leave behind protons

•Making that side positive

Slide 20-20

Polarization

• The polarization force arises because the charges in a

neutral object are slightly separated, not because the

objects are oppositely charged.

• The polarization force between a charged object and a

neutral one is always attractive.


Slide 20-21

The Demonstrations

Balloon

balloons-and-static-electricity_en.html

Slide 20-22

Van de Graaff Generator

•Pith Ball

•Dots

•Fur

•Hair

•Neutral objects become charged and repel

Slide 20-23

Visualizing Charge

• Law of Conservation of Charge

• neither created nor destroyed

• only transferred

• Charge can be transferred when objects touch

• Discharging is removing a charge from an object

• Conductors

• through which charge easily moves

• metal

• Insulators

• charge is immobile

• glass and plastics


Slide 20-24

QuickCheck 20.1

Charged glass and plastic rods hang by threads. An object

attracts the glass rod. If this object is then held near the

plastic rod, it will

A. Attract the plastic rod.

B. Repel the plastic rod.

C. Not affect the plastic rod.

D. Either A or B. There’s not enough information to tell.


Slide 20-25

QuickCheck 20.2

A rod attracts a positively charged hanging ball.

The rod is

A. Positive.

B. Negative.

C. Neutral.

D. Either A or C.

E. Either B or C.


Slide 20-26

QuickCheck 20.3

Consider two objects A and B. Object A has a net charge

while B is uncharged. Based on this information, it must be

true that

A. A is a conductor, B is an insulator.

B. A is an insulator, B is a conductor.

C. A and B are both insulators.

D. A and B are both conductor.

E. There’s not enough information to tell.


Slide 20-27

QuickCheck 20.4

Metal spheres 1 and 2 are touching. Both are initially neutral.

a. The charged rod is brought near.

b. The charged rod is then removed.

c. The spheres are separated.

Afterward, the charges on the sphere are:

A. Q1 is + and Q2 is +

B. Q1 is + and Q2 is –

C. Q1 is – and Q2 is +

D. Q1 is – and Q2 is –

E. Q1 is 0 and Q2 is 0


Slide 20-28

QuickCheck 20.5

Metal spheres 1 and 2 are touching. Both are initially neutral.

a. The charged rod is brought near.

b. The spheres are separated.

c. The charged rod is then removed.

Afterward, the charges on the sphere are:

A. Q1 is + and Q2 is +

B. Q1 is + and Q2 is –

C. Q1 is – and Q2 is +

D. Q1 is – and Q2 is –

E. Q1 is 0 and Q2 is 0


Slide 20-29

QuickCheck 20.7

Identical metal spheres are initially charged as shown.

Spheres P and Q are touched together and then separated.

Then spheres Q and R are touched together and separated.

Afterward the charge on sphere R is

A. –1 nC or less

B. –0.5 nC

C. 0 nC

D. +0.5 nC

E. +1.0 nC or more


Slide 20-30

20.1: Charges and Forces 20.2: Charges, Atoms, and Molecules

2) If two objects are electrically attracted to each other,

A) both objects must be negatively charged.

B) both objects must be positively charged.

C) one object must be negatively charged and the other

object must be positively charged.

D) the objects could be electrically neutral.

E) None of the above statements are absolutely true.


Slide 20-31


6) Two uncharged metal spheres, #1 and #2, are mounted on insulating

support rods. A third metal sphere, carrying a negative charge, is then

placed near #2. Now a copper wire is momentarily connected between

#1 and #2 and then removed. Finally, sphere #3 is removed. In this

final state

A) spheres #1 and #2 are still uncharged.

B) sphere #1 carries positive charge and #2 carries negative charge.

C) sphere #1 carries negative charge and #2 carries positive charge.

D) spheres #1 and #2 both carry positive charge.

E) spheres #1 and #2 both carry negative charge.


Slide 20-32


9) A piece of plastic has a net charge of +2.00 μC. How

many more protons than electrons does this piece of plastic

have? (e = 1.60 × 10-19 C)

10) What is the charge on 2.0 kg of protons?(e = 1.60 × 10-

19 C, mproton = 1.67 × 10-27 kg)


Electroscope Lab


Slide 20-34

The Electroscope

knob

stem

leaves

Slide 20-35

The Electroscope

•A device used to determine the

charge of an object

•Leaves indicate the presence of a

charge

Slide 20-36

Charging

•Conduction – charge through contact

•Negative charge touches an object

the object becomes Negative

•Positive charge touches an object the

object becomes Positive

•How?

Slide 20-37

Charging

• Induction – indirect charging

•Negative charge comes near an object the object becomes Polarized

•Positive charge comes near an object the object becomes Polarized

•How?

•Which?

Slide 20-38

Charging


Slide 20-39

Electroscope Practice

Slide 20-40

Electroscope Practice

Section 20.3 Coulomb’s Law


The Electrostatic Force

electric-hockey_en (2).jar

Slide 20-43

Coulomb’s Law – The Reference Table

𝐹 𝑒 = 𝑘𝑞1𝑞2

𝑟2

Fe = electrostatic force (n)

k = electrostatic constant (on front of ref. table)

q = charge (C)

r = distance between objects (m)


Slide 20-44

Coulomb’s Law


Slide 20-45

Coulomb’s Law


Slide 20-46

What does Coulomb’s Law remind you of?

𝐹 𝑒 = 𝑘𝑞1𝑞2

𝑟2

𝐹𝑔 =𝐺𝑚1𝑚2

𝑟2


Attractive or Repulsive

Attractive

Slide 20-47

QuickCheck 20.8

The charge of sphere 2 is twice that of sphere 1. Which

vector below shows the force of 2 on 1?

A.

B.

C.

D.

E.


Slide 20-48

QuickCheck 20.9

The charge of sphere 2 is twice that of sphere 1. Which

vector below shows the force of 1 on 2 if the distance

between the spheres is reduced to r/2?

A.

B.

C.

D. None of the above.


Slide 20-49

QuickCheck 20.10

Which of the three right-hand charges experiences the

largest force?

A. q

B. 2q

C. 4q

D. q and 2q are tied

E. q and 4q are tied


Slide 20-50

QuickCheck 20.11

In each of the following cases, an identical small, positive

charge is placed at the black dot. In which case is the force

on the small charge the largest?


Slide 20-51

QuickCheck 20.12

In each of the following cases, an identical small, positive

charge is placed at the black dot. In which case is the force

on the small charge the largest? (All charges shown are of

equal magnitude.)


Slide 20-52

QuickCheck 20.15

The direction of the force on charge –q is

A. Up

B. Down

C. Left

D. Right

E. The force on –q is zero


Slide 20-53

QuickCheck 20.13

Which is the direction of the net force on the

charge at the lower left?

E. None of these.


Slide 20-54

QuickCheck 20.14

Which is the direction of the net

force on the charge at the top?

E. None of these.


Slide 20-55

20.3: Coulomb’s Law

12) Two tiny beads are 25 cm apart with no other charges or

fields present. Bead A carries 10 µC of charge and bead B

carries 1 µC. Which one of the following statements is true

about the magnitudes of the electric forces on these beads?

A) The force on A is 10 times the force on B.

B) The force on B is 10 times the force on A.

C) The force on A is exactly equal to the force on B.

D) The force on A is 100 times the force on B.

E) The force on B is 100 times the force on A.


Slide 20-56


14) In outer space, a positive charge q is released near a

positive fixed charge Q, as shown in the figure. As q moves

away from Q, what is true about the motion of q? (There

may be more than one correct choice.)

A) It will move with decreasing speed.

B) It will move with constant acceleration.

C) It will move with increasing acceleration.

D) It will move with decreasing acceleration.

E) It will move with increasing speed.


Slide 20-57


20) Three equal charges +Q are at three of the corners of a

square of side d. A fourth charge +Q of equal magnitude is

at the center of the square as shown in the figure Which one

of the arrows shown represents the net force acting on the

charge at the center of the square?

A) A

B) B

C) C

D) D


Slide 20-58


25) An electron and a proton are released simultaneously from rest

and start moving toward each other due to their electrostatic attraction,

with no other forces present. Which of the following statements are

true just before they are about to collide? (There could be more than

one correct choice.)

A) They are closer to the to the initial position of the electron that to

the initial position of the proton.

B) They are closer to the to the initial position of the proton that to the

initial position of the electron.

C) They are at the midpoint of their initial separation.

D) They both have the same speed.

E) The electrostatic force on the proton is greater than the electrostatic

force on the electron.


Slide 20-59

20.3: Coulomb’s Law - Students

29) Two electrons are 20mm apart at closest approach. What

is the magnitude of the maximum electric force that they

exert on each other? (e = 1.60 × 10-19 C, k = 1/4πε0 = 9.0

109 N ∙ m2/C2)

30) The force of attraction that a -40.0 μC point charge

exerts on a +108 μC point charge has magnitude 4.00 N.

How far apart are these two charges? (k = 1/4πε0 = 9.0 109

N ∙ m2/C2)


Slide 20-60

20.3: Coulomb’s Law - Students

31) When 1.0-µC point charge is 15 m from a second point

charge, the force each one experiences a force of 1.0 µN.

What is the magnitude of the second charge? (k = 1/4πε0 =

9.0 109 N ∙ m2/C2)

32) Two 10¢ coins (dimes) carrying identical charges are

lying 2.5 m apart on a table. If each of these coins

experiences an electrostatic force of magnitude 2.0 N due to

the other coin, how large is the charge on each coin? (k =

1/4πε0 = 9.0 109 N ∙ m2/C2)


Slide 20-61


28) An asteroid of mass 58,000 kg carrying a negative

charge of is from a second asteroid of mass carrying a

negative charge of What is the magnitude of the net force

the asteroids exert upon each other, assuming we can treat

them as point particles? (G = 6.67 × 10-11 N ∙ m2/kg2, k =

1/4πε0 = 9.0 109 N ∙ m2/C2)

38) Suppose you wanted to hold up an electron against the

force of gravity by the attraction of a fixed proton some

distance above it. How far above the electron would the

proton have to be? (k = 1/4πε0 = 9.0 109 N ∙ m2/C2, mproton =

1.67 × 10-27 kg, melectron = 9.11 × 10-31 kg)


Slide 20-62


47) As shown in the figure, three charges are at the vertices

of an equilateral triangle. The charge Q is 6.7 nC, and all

the other quantities are accurate to two significant figures.

What is the magnitude of the net electric force on the charge

Q due to the other two charges? (k = 1/4πε0 = 9.0 109 N ∙

m2/C2)


Slide 20-63


49) As shown in the figure, the charge Q is midway between

two other charges. If what must be the charge q1 so that

charge q2 remains stationary as Q and q1 are held in place?


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Lecture Presentation - Calhoun Physics · Slide 20-55 20.3: Coulomb’s Law 12) Two tiny beads are 25 cm apart with no other charges or fields present. Bead A carries 10 µC of charge