Solution Manual For
Physics for Scientists and Engineers
Authors: Raymond A. Serway
Volume II 5th Edition
,CHAPTER 21: Electric Charge And Electric Field
Responses To Questions
1. Suspend A Plastic Ruler By A Thread And Then Rub It With A Cloth. As Shown In Fig. 21–2a,
The Ruler Is Negatively Charged. Now Bring The Charged Comb Close To The Ruler. If The
Ruler Is Repelled By The Comb, Then The Comb Is Negatively Charged. If The Ruler Is Attracted
By The Comb, Then The Comb Is Positively Charged.
2. The Shirt Or Blouse Becomes Charged As A Result Of Being Tossed About In The Dryer
And Rubbing Against The Dryer Sides And Other Clothes. When You Put On The Charged
Object (Shirt), It Causes Charge Separation Within The Molecules Of Your Skin (See Fig. 21–
9), Which Results In Attraction Between The Shirt And Your Skin.
3. Fog Or Rain Droplets Tend To Form Around Ions Because Water Is A Polar Molecule, With A
Positive Region And A Negative Region. The Charge Centers On The Water Molecule Will Be
Attracted To The Ions Or Electrons, Since Opposite Charges Attract.
4. See Also Fig. 21–9 In The Text. The
Negatively Charged Electrons In The Paper
Are Attracted To The Positively Charged Rod
And Move Towards It Within Their
Molecules. The Attraction Occurs Because
The Negative Charges In The Paper Are
Closer To The Positive Rod Than Are The
Positive Charges In The Paper, And Therefore
The Attraction Between The Unlike Charges
Is
Greater Than The Repulsion Between The Like Charges.
5. A Plastic Ruler That Has Been Rubbed With A Cloth Is Charged. When Brought Near Small
Pieces Of Paper, It Will Cause Separation Of Charge (Polarization) In The Bits Of Paper, Which
Will Cause The Paper To Be Attracted To The Ruler. A Small Amount Of Charge Is Able To
Create Enough Electric Force To Be Stronger Than Gravity. Thus The Paper Can Be Lifted.
On A Humid Day This Is More Difficult Because The Water Molecules In The Air Are Polar. Those
Polar Water Molecules Will Be Attracted To The Ruler And To The Separated Charge On The Bits
Of Paper, Partially Neutralizing The Charges And Thus Reducing The Attraction.
6. The Net Charge On A Conductor Is The Sum Of All Of The Positive And Negative Charges In The
Conductor. If A Neutral Conductor Has Extra Electrons Added To It, Then The Net Charge Is
Negative. If A Neutral Conductor Has Electrons Removed From It, Then The Net Charge Is
Positive. If A Neutral Conductor Has The Same Amount Of Positive And Negative Charge, Then
The Net Charge Is Zero.
The “Free Charges” In A Conductor Are Electrons That Can Move About Freely Within The
Material Because They Are Only Loosely Bound To Their Atoms. The “Free Electrons” Are
Also Referred To As “Conduction Electrons.” A Conductor May Have A Zero Net Charge
But Still Have Substantial Free Charges.
,Chapter 21 Electric Charge and Electric Field
7. Most Of The Electrons Are Strongly Bound To Nuclei In The Metal Ions. Only A Few Electrons
Per Atom (Usually One Or Two) Are Free To Move About Throughout The Metal. These Are
Called The “Conduction Electrons.” The Rest Are Bound More Tightly To The Nucleus And Are
Not Free To Move. Furthermore, In The Cases Shown In Figs. 21–7 And 21–8, Not All Of The
Conduction Electrons Will Move. In Fig. 21–7, Electrons Will Move Until The Attractive Force On
The Remaining Conduction Electrons Due To The Incoming Charged Rod Is Balanced By The
Repulsive Force From Electrons That Have Already Gathered At The Left End Of The Neutral Rod.
In Fig. 21–8, Conduction Electrons Will Be Repelled By The Incoming Rod And Will Leave The
Stationary Rod Through The Ground Connection Until The Repulsive Force On The Remaining
Conduction Electrons Due To The Incoming Charged Rod Is Balanced By The Attractive Force
From The Net Positive Charge On The Stationary Rod.
8. The Electroscope Leaves Are Connected Together At The Top. That Connection
Can Be Modeled As A Tension Force. The Horizontal Component Of This
Tension Force Balances The Electric Force Of Repulsion. The Vertical
Component Of The Tension Force Balances The Weight Of The Leaves.
9. The Balloon Has Been Charged. The Excess Charge On The Balloon Is Able To Polarize The
Water Molecules In The Stream Of Water, Similar To Fig. 21–9. This Polarization Results In A Net
Attraction Of The Water Towards The Balloon, So The Water Stream Curves Towards The
Balloon.
10. (A) When The Leaves Are Charged By Induction, No Additional Charge Is Added To The Leaves.
If The Charged Rod Is Near The Top Of The Electroscope It Repels Charge Onto The Leaves
Causing Them To Separate As In Fig. 21–11(A). When The Rod Is Removed The Charge
Returns To Its Initial Equilibrium Position And The Leaves Come Back Together.
(b) When The Leaves Are Charged By Conduction, Positive Charge Is Placed Onto The
Electroscope From The Rod Causing The Leaves To Separate. When The Rod Is Removed,
The Charge Remains On The Electroscope And The Leaves Remain Separated. If Not All Of
The Excess Charge Leaves The Rod, Then When The Rod Is Removed, The Leaves Might
Come Back Together Slightly From Their Maximum Deflection.
(c) Yes. The Electroscope Has A Negative Charge On The Top Sphere And On The Leaves.
Therefore The Electroscope Has A Total Net Negative Charge, So It Must Have Been Charged
By Conduction.
11. Coulomb’s Law And Newton’s Law Are Very Similar In Form. When Expressed In SI Units, The
Magnitude Of The Constant In Newton’s Law Is Very Small, While The Magnitude Of The
Constant In Coulomb’s Law Is Quite Large. Newton’s Law Says The Gravitational Force Is
Proportional To The Product Of The Two Masses, While Coulomb’s Law Says The Electrical Force
Is Proportional To The Product Of The Two Charges. Newton’s Law Only Produces Attractive
Forces, Since There Is Only One Kind Of Gravitational Mass. Coulomb’s Law Produces Both
Attractive And Repulsive Forces, Since There Are Two Kinds Of Electrical Charge.
12. The Gravitational Force Between Everyday Objects On The Surface Of The Earth Is Extremely Small.
−11
(Recall The Value Of G: 6.67 10 Nm Kg . ) Consider Two Objects Sitting On The Floor Near Each
2 2
Other. They Are Attracted To Each Other, But The Maximum Force Of Static Friction For Each Is
Much Greater Than The Gravitational Force Each Experiences From The Other, And So They Don’t
Move. Even In An Absolutely Frictionless Environment, The Acceleration Resulting From The
Gravitational Force Would Be So Small That It Would Not Be Noticeable In A Short Time Frame.
We Are Aware Of The Gravitational Force Between Objects If At Least One Of Them Is Very
, Physics for Scientists & Engineers with Modern Physics, 5e, Global Edition Instructor Solutions Manual
Massive, As In The Case Of The Earth And Satellites Or The Earth And You.
Physics for Scientists and Engineers
Authors: Raymond A. Serway
Volume II 5th Edition
,CHAPTER 21: Electric Charge And Electric Field
Responses To Questions
1. Suspend A Plastic Ruler By A Thread And Then Rub It With A Cloth. As Shown In Fig. 21–2a,
The Ruler Is Negatively Charged. Now Bring The Charged Comb Close To The Ruler. If The
Ruler Is Repelled By The Comb, Then The Comb Is Negatively Charged. If The Ruler Is Attracted
By The Comb, Then The Comb Is Positively Charged.
2. The Shirt Or Blouse Becomes Charged As A Result Of Being Tossed About In The Dryer
And Rubbing Against The Dryer Sides And Other Clothes. When You Put On The Charged
Object (Shirt), It Causes Charge Separation Within The Molecules Of Your Skin (See Fig. 21–
9), Which Results In Attraction Between The Shirt And Your Skin.
3. Fog Or Rain Droplets Tend To Form Around Ions Because Water Is A Polar Molecule, With A
Positive Region And A Negative Region. The Charge Centers On The Water Molecule Will Be
Attracted To The Ions Or Electrons, Since Opposite Charges Attract.
4. See Also Fig. 21–9 In The Text. The
Negatively Charged Electrons In The Paper
Are Attracted To The Positively Charged Rod
And Move Towards It Within Their
Molecules. The Attraction Occurs Because
The Negative Charges In The Paper Are
Closer To The Positive Rod Than Are The
Positive Charges In The Paper, And Therefore
The Attraction Between The Unlike Charges
Is
Greater Than The Repulsion Between The Like Charges.
5. A Plastic Ruler That Has Been Rubbed With A Cloth Is Charged. When Brought Near Small
Pieces Of Paper, It Will Cause Separation Of Charge (Polarization) In The Bits Of Paper, Which
Will Cause The Paper To Be Attracted To The Ruler. A Small Amount Of Charge Is Able To
Create Enough Electric Force To Be Stronger Than Gravity. Thus The Paper Can Be Lifted.
On A Humid Day This Is More Difficult Because The Water Molecules In The Air Are Polar. Those
Polar Water Molecules Will Be Attracted To The Ruler And To The Separated Charge On The Bits
Of Paper, Partially Neutralizing The Charges And Thus Reducing The Attraction.
6. The Net Charge On A Conductor Is The Sum Of All Of The Positive And Negative Charges In The
Conductor. If A Neutral Conductor Has Extra Electrons Added To It, Then The Net Charge Is
Negative. If A Neutral Conductor Has Electrons Removed From It, Then The Net Charge Is
Positive. If A Neutral Conductor Has The Same Amount Of Positive And Negative Charge, Then
The Net Charge Is Zero.
The “Free Charges” In A Conductor Are Electrons That Can Move About Freely Within The
Material Because They Are Only Loosely Bound To Their Atoms. The “Free Electrons” Are
Also Referred To As “Conduction Electrons.” A Conductor May Have A Zero Net Charge
But Still Have Substantial Free Charges.
,Chapter 21 Electric Charge and Electric Field
7. Most Of The Electrons Are Strongly Bound To Nuclei In The Metal Ions. Only A Few Electrons
Per Atom (Usually One Or Two) Are Free To Move About Throughout The Metal. These Are
Called The “Conduction Electrons.” The Rest Are Bound More Tightly To The Nucleus And Are
Not Free To Move. Furthermore, In The Cases Shown In Figs. 21–7 And 21–8, Not All Of The
Conduction Electrons Will Move. In Fig. 21–7, Electrons Will Move Until The Attractive Force On
The Remaining Conduction Electrons Due To The Incoming Charged Rod Is Balanced By The
Repulsive Force From Electrons That Have Already Gathered At The Left End Of The Neutral Rod.
In Fig. 21–8, Conduction Electrons Will Be Repelled By The Incoming Rod And Will Leave The
Stationary Rod Through The Ground Connection Until The Repulsive Force On The Remaining
Conduction Electrons Due To The Incoming Charged Rod Is Balanced By The Attractive Force
From The Net Positive Charge On The Stationary Rod.
8. The Electroscope Leaves Are Connected Together At The Top. That Connection
Can Be Modeled As A Tension Force. The Horizontal Component Of This
Tension Force Balances The Electric Force Of Repulsion. The Vertical
Component Of The Tension Force Balances The Weight Of The Leaves.
9. The Balloon Has Been Charged. The Excess Charge On The Balloon Is Able To Polarize The
Water Molecules In The Stream Of Water, Similar To Fig. 21–9. This Polarization Results In A Net
Attraction Of The Water Towards The Balloon, So The Water Stream Curves Towards The
Balloon.
10. (A) When The Leaves Are Charged By Induction, No Additional Charge Is Added To The Leaves.
If The Charged Rod Is Near The Top Of The Electroscope It Repels Charge Onto The Leaves
Causing Them To Separate As In Fig. 21–11(A). When The Rod Is Removed The Charge
Returns To Its Initial Equilibrium Position And The Leaves Come Back Together.
(b) When The Leaves Are Charged By Conduction, Positive Charge Is Placed Onto The
Electroscope From The Rod Causing The Leaves To Separate. When The Rod Is Removed,
The Charge Remains On The Electroscope And The Leaves Remain Separated. If Not All Of
The Excess Charge Leaves The Rod, Then When The Rod Is Removed, The Leaves Might
Come Back Together Slightly From Their Maximum Deflection.
(c) Yes. The Electroscope Has A Negative Charge On The Top Sphere And On The Leaves.
Therefore The Electroscope Has A Total Net Negative Charge, So It Must Have Been Charged
By Conduction.
11. Coulomb’s Law And Newton’s Law Are Very Similar In Form. When Expressed In SI Units, The
Magnitude Of The Constant In Newton’s Law Is Very Small, While The Magnitude Of The
Constant In Coulomb’s Law Is Quite Large. Newton’s Law Says The Gravitational Force Is
Proportional To The Product Of The Two Masses, While Coulomb’s Law Says The Electrical Force
Is Proportional To The Product Of The Two Charges. Newton’s Law Only Produces Attractive
Forces, Since There Is Only One Kind Of Gravitational Mass. Coulomb’s Law Produces Both
Attractive And Repulsive Forces, Since There Are Two Kinds Of Electrical Charge.
12. The Gravitational Force Between Everyday Objects On The Surface Of The Earth Is Extremely Small.
−11
(Recall The Value Of G: 6.67 10 Nm Kg . ) Consider Two Objects Sitting On The Floor Near Each
2 2
Other. They Are Attracted To Each Other, But The Maximum Force Of Static Friction For Each Is
Much Greater Than The Gravitational Force Each Experiences From The Other, And So They Don’t
Move. Even In An Absolutely Frictionless Environment, The Acceleration Resulting From The
Gravitational Force Would Be So Small That It Would Not Be Noticeable In A Short Time Frame.
We Are Aware Of The Gravitational Force Between Objects If At Least One Of Them Is Very
, Physics for Scientists & Engineers with Modern Physics, 5e, Global Edition Instructor Solutions Manual
Massive, As In The Case Of The Earth And Satellites Or The Earth And You.
