Chapter 20 Review
20,1 I can define the basics of electrostatics,
conservation
and charge interactions
1. If an object is attracted by a positively charged rod, we can be sure that the body is:
a)
b)
c)
~
Electrical
negatively charged
losing electrons
either neutral or negative ~
2. As a positively charged object is brought steadily nearer to (but not touching) the knob of a
positively charged electroscope, the leaves of the electroscope:
3. Like charges repel, whereas opposite charges attract.
ca)15) True~
False
4. Electric charge is neither created nor destroyed; it is conserved.
(.1{AA..4! I~ (O"'S~LV'tO I
CJm~
IT CI'\I.J
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~--~
~~
1jE-
J) l>N ~
u..u..rrfJ
ott. IJIHrlUJ'tcI-(J. -l:>t".I.r"1ll~
@eoHu."""""'k,nc..r
P A-"'l~
1/oJ
5. Electric forces can be either repulsive or attractive, whereas gravitational force is always: c..H414r
ISN T
~:A:~uC~~~
~6
Su.U{
(1.l/~~
A-1J1(-CiLAuay
~S
CYcr)
((J't.H"~1fI
c. both a and b
d. neither a nor b
6. Metals contain __
electrons; rubber has __
electrons.
a. bound, free
CF)re-er,l:Jound
":)
c. excess, insufficient
7. Charge carriers in a metal are electron,:; rather than protons, because electrons are
a)-lighte .
) relatively far from a nucleus.
loosel bound
d) all of the above
e) none of the above
flte!
rl2..tr c! ~ L r!cnJ.orv 5
l605.CL. y.
D~
L
/)(jGLN
HOM
0
'f'tl.(
I\-t2..C
AND (L(Li\nilc!L'1
Ii.U(lW~,
20.2 Describe three methods
of charging
(friction,
conduction,
and induction)
8. A plastic ruler is rubbed with a towel and becomes charged. The towel is broUg~n
electroscope (like my pop can electropscope) and it is found to be charged. If I then bring my
ruler to the electroscope, it should be:
a) Neutral (not charged)
0PPOSI
__
e ycharged a5111elower~
(
-
i(!1...L \. V {J C Or:- CHA,1t C l
CA.iJI I:
Y
tr
AJ (?~Il - 1) u.r
'i"F Ll... I r 1:. c-l"tiiIlCi!/J
9. When a person standing on the ground touches a rod that is positively charged, the person:
•
c
tie-same charge as the t~
~
Ml.'''''<'lAJG.
\f () f.II
Cv+ N
gives the rod extra electrons
ives-theTod "extra protons
accepts the rods extra protons
accepts the rods extra electrons
10. Objects can be charged by
touching (conduction)
induction.
11. Bring a charged object near a conductor and then momentarily ground it by touching one part.
This demonstrates charge by
\t
/I( C' Nt.<.. r
lJC!it"- orl
F"ili
1",i"""
OdJl!c(
1:10 IN ~ T11C
I""
C.kAiJ.~
12. Pie tins fly off the Van de Graaff generator because the generator charges them and then repel
eGjber;)
. True
b. False
13.Ch~r
'ects are attracted to neutral objects because of~olarization
. True
.
a se
C,HA Jl uC
20.3 Draw the redistribution
of electric charges on a neutral object when a charged object is
brought near and explain why an attractive force results from bringing a charged object near a
neutral object.
14.A negatively charged balloon sticks to a wooden door. However, an uncharged balloon does
not stick to a wooden door. What is the nature of the charge on the wooden door?
1 Electrjcally
neutral
b) Positively c arged
c) Negatively charged
d) Cannot be determined
20.4 Calculate
the electric force between charged objects
7
15. What is the force between two small charged spheres that have charges of 2.0x1 0- C and
3 x 10-7 C and are placed 30 cm apart in air?
2 .Oli{(/V
'(.Olt';' N'"
1A ~
r _I' 'l,,0&
r •. -I...~
c
k~
-7
('<0.30",
it's"f,({)
-zr-
{o X(O~J1J
20.5 Determine the new electric force on charged objects after they touch and are then
separated.
17. One metal sphere has a charge of +1 OC and another has a charge of +8C. They are allowed
...-to !ollcR-elld ~ separated to a distance of one meter. What is the force between them?
11
(g~:..29 x 10
.
~"t'(
b) 7.29
c) 7.29 x 106N
\)Jl~'-
C" ~ ,,-p "Jlr'f
t,,: r c 'i.e.::- 'Ie.. r = t....
F> k 2.f -= (7.C, XIO ';t)(
A
:"
20.6 Predict electrostatic
variables
04-.'1" cltnUt
force and strength
,
lL ~ /.OX/O
~'//>\)r~ j\
-t-lC
-t
1 ",>
t
(
/01(..
~llq lJo
of electric field based on relationship
'(i,Z?1tO'
between
18. Particle A has twice as much charge as particle B. Compared to the force on particle A, the
force on particle B is
two times as much.
four times as much.
the same.
a as much.
e) none of the above
/1.J
19.__
vary/varies inversely as the square of the distance between two point objects.
a. Gravitational force
b. Electric force
c. Nei ee a nOJ:.!:>
~th a and b)
F ~C
j
lit" 1'1"
F. .= k
c-
r "2If • q n
~ r"1..
20.lf the distance between two charges becomes 1/6 as much, what is the new force between
them?
/,
1~6
~
Fe. ~ f-.
0 A QA
~'(~
6 times the original force
F6-L
•
f:l
1. Electrons carry a
p.
k 't..•• 1A
36 ~mes as much as the origi,rce
31.
ltJ
.
l
-vz.
\
t" !J
rl.
lO£tLV! khgrge.
Negative charge
c. ne
ge.
d. variable charge.
~
Answer: B
Diff: 1
3) Since there is a universal Law of Conservation of Charge
A) charges can be annihilated but not created.
B charges-cannot be"createa or aestroye
C) charges can be created and destroyed.
- ~
'-l.'t
4) Which is true?
A) Unlike charges repel.
B) AI barge.s attract.
Unlike charges attr~
D) Like charges a rac!.
Answer: C
Oijf: 1
<ill
C~
I/.J ~AI~
eBb
Answer: C
Oiff:2
5) A neutral atom has
A) equal numbers
B) equal numbers
C)-equaLnumber.s
equal numbers
E) zero neutrons.
MoW-,
of
of
of
of
electrons, protons, and neutrons.
protons and neutrons.
electrons and neutrons.
electrons and p~
Answer: 0
Oijf: 1
7) Charge is
A) invariant.
B) conserved
Q gUantized.
~ant,
conserved, and quantized
Answer: 0
Diff:1
10) The two types of charge (plus and minus) were named by
A) Lord Rutherfo .
&J13rBiam
n r~;k~
C) A €"ffEtnstet~
D) Richard Feynman.
E) Newton.
Answer: B
Diff:1
11) Can you get an electric shock by touching a charged insulator?
.f
Byes
no
~
you are insulated
Answer: B
Diff:1
1) A glass rod is rubbed with a piece of silk. During the process the glass rod acquires a positive
charge and the silk
Answer: C
Oiff:/
2) Explain how objects can get charged by FRICTION.
Answer: When two dissimilar materials are rubbed together, one literally rubs or tears electrons off
the one type of molecule, leaving it positively charged, and the other material acquires the extra
electrons, thereby becoming negative.
Oiff: 2
3) The charge carried by one electron is e = -1.6 x 10.19 C. The number of excess electrons
necessary to produce a charge of 1.0 C is
e: I, b X ( (). "C
A}6.3 x 1 18
.6 X 1019.
C) 6.3 X 109.
Cs'
D) 6.3 X 106.
E) 1.6 x 1018
.
Answer: A
Diff:2
4) When an electron is removed from a neutral atom, it becomes
'A) a POSIIve Ion.
ga Ive Ion.
C) heavier.
D) a bipolar atom.
E) more massive.
Answer: A
Diff:/
15.3 Electric Force
1) The Coulomb constant has which of the following units?
Answer: (b) N_m2/C2
Oiff 1
I
2) Consider the x-axis with 4.0 IJC at x = 1.0 meter and -2.0 IJC at the origin. Where should 6.0 IJC be
pI ~
~a
:Jet
force. on it is :ero?
[". 2,O'it()
-2.4 meter
./
'c
\)\ITt(M,(...f'
r. -
. .
Olff. 2
l4"
'Ill 0 •t.
t,.: ~KIO -fe
A",;
13.
0~
4
r
f..c
(£J
".
.s-r'I
,J
~
~
1."''1.(
-
ll1K)
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L
a. \
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,
<-
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_U t t
< (I.
'I" •...
r-1 -
4,. +'- ~ 0
4) Consider a container of 2.0 grams of hydrogen (one gram mole). Suppose you removed all the
23
electrons and moved them to the other side of the E rth (Earth diameter 12740. km). (12.04 x 10
7
t,,\l140,"Q"...
electrons)
= I.'iXIas-e.
fl\=(J2,OYX'O~~fl.(dX(()''f~t-
(a) How much charge is left behind?
LI!>~ I, f XI O~"
(b) What is the attractive force between the protons here and the electrons at the other side 0
fe ~ r. t
:4
A
Earth?
~,e-y1O!;-,,"l1(I,9K{OS<:.)j.7Qd"c:)
':'
r'
5
Answer: (a) 1.9 x 10 Coulombs
(b) 2.1 x 106 Newtons
2.1 x 102 tons!
=
)
/.
V '2., (((0,000 rl1 ) z
[::--2-, O-X-(-o-''f}''''''-'
Oiff3
e
5) There is a 5.0 J..lCcharge at each of 3 corners of a square (each side 70. mm long). What is the
force on +6.0 J..lCplaced at the center of the square?.s ~{i
CI "I
r" f til) ~
f'.blb
A~swer~
\lOI-J
Newtons toward the empty corner
Olff: 2
7
l(1e,
,;"
th;7
.••
t' M
,,;)titL
r.
"G
"~
=-
f. oj f. t'" -?
~
r0 ~ f •.~t:ll.. Jl.or/o' ~•.~;~~~~
• I[!\CU7.-.1 "0
V
"-til 'S,/f
•.
I
('
\'
6) Ben Franklin named charges "plus" and "minus". Suppose'he ha gotten it reversed and had
called plus charges "minus" and called minus "plus", what would have been the consequences?
r
9) Only gravitation determines the planetary orbits. Why aren't the other fundamental forces (and
electromagnetism in particular) important for the motions?
Answer: Only gravity and electromagnetism are long range, however the planets and satellites are
electrically neutral. The planets do not have "negative mass" to neutralize the positive mass so
gravity predominates.
Oif!: 2
10) The force which binds, or holds, atoms together to form molecules is
A) gravitational.
VflLI!Ni,
1()r'lIC y4.Wf:) j-IYl::JtlOCCIII t!PAliJ~
B) magnetic.
C) friction.
A\'" L-- /J (,lIZ 'fr1 & L~<..
i'tiJ (/If-( •.
0) nuclear.
E) elec rica
AU
F'at2
'-C
Answer: E
Diff: 1
11) How far apart should two protons be if the electrical force of repulsion is equal to the weight
(attraction to the Earth) of one of the protons?
Answer: 12. em
Oif!:2
. I b"
'l"-,
-I'/.
I() c.. ....(i b
I-
~r= 1"7y()-l7~
j~
1.8~2
r.J '" F.~
'lA 'l. 4
_ I.
M( - 1'_
~
of'r:
('L
fi¥
'l
1.1
r:: ~{I(5~
Answer: Like charges would still repel, unlike charges would attract. Coulomb's Law would still
describe the electric force.
Oiff: 3
Co
(407
12) A charged rod will attract a thin stream of water falling from a faucet even though the water is
neutral, it has no net charge. How can the rod attract the water?
Answer: The Coulomb force of the rod polarizes each molecule of water (one sign of charge is
attracted and the opposite sign is repelled). The charge that is attracted feels a slightly larger
attraction than the opposite charge is repelled because it is at a slightly different distance from the
rod.
Oif!:2
13) Two charged objects attract each other with a force F. What happens to the force between them if
one charge is doubled, the other charge is tripled, and the separation distance between their centers
is reduced to one-fourth its original value? The force is now equal to
A) 24 F.
B) 6/16 F.
~
~
~(2i4 X~{d)
~1
cf/
-
Answer: 0
Diff:2
14) A point charge of +Q is placed at the centroid of an equilateral triangle. When a second charge of
+Q is placed at one of the triangle's vertices, an electrostatic force of 4 N acts on it.
.0
G
I
it
'",
#
••
I
#
•
•
I....
+a&----.-----.----- ...~
I
#
#I
0
•
-"
·
••
,
•
/'
What is the magnitude of the force that acts on the center charge when a third charge of +Q is placed
at one of the other vertices?
~ tJ Q.
60"
41J e IlC. ::. ~!J(!,fo()
~
L~
A~Q) V ~m(iL~
0) 16 N
Answer: B
Diff: 3
L\~
15) Two charged objects attract each other with a certain force. If the charges on both objects are
doubled with no change in separation, the force between)them
r
r.
t (z. '!.'I)(2
(j)
6
U
A) becomes zero.
~:
2..
B) doubles.
Y'..,
halves.
eI
A
OJ quadruples.
K. ~
E) Increases:O t we can't say how much without knowing the distance between them.
k.
l.
rr
(
Answer: D
Diff:2
18) Consider point charges of +Q and +4Q which are separated by 3 m. At what point, on a line
between the two charges, would it be possible to place a charge of -Q such that the electrostatic force
acting on it would be zero?
3",
C\
r
p(
r -=
A
W
3/5
m from the +Q charge Gl.
B) 1m fromtlie +Q charge
~
m e +4Q c arge
D) 3 m from the +4Q charge
E) There is no such point possible.
C
----
4 1.
v
1
1-,.,
e.3,
o S.,l,
to
.;
L
/()
"
J3 _,. \
_/(
(r)C" tf
Q
'fa
O-rJol
t. Q:"
~>-
')1r+ 3
k.
=t.~.,...
~.,.~f
~
.de,
~..
c-)
{)
-IA
AG
i/ QQ.
r
-"';)
)
~
~.
in
Answer: B c
f."
Oift: 2
T -6., (=
~I
'( ': 3
0('"
f
.(.•..~
~
~
(3 -I".)
-
...••.
~'("
•
\
\
~L
20) Two charges are separated by 5.0 meters and attract one a tn . one charge is halved, the
other tripled, and the distance quartered, by what factor does the attraction change?
A) 48
B) 3
.91...
~E)
Pc
).
r:=
'"
I c.
16
k. C:! f: 'l.3t.)
----( '(' '2
_
( .I2.G\~
/ k ['" 11
.::.;:t-
-=r::-
~~ )
1'/
k _'LAt••
/(,
("to
k ev 111
-
Answer: D
Oift 2
(
22) Sphere A carries a net charge and sphere B is neutral. They are placed near each other on an
insulated table. Which statement best describes the electrostatic force betwee them?
(1)
A) There is no force between them since one is neutral.
Nw.~11(
A-rr-12..4..q....
I?) There is a force of rep'ulsion between them.
~
C) There is a force of attraction between the
/-J ~tI2.A" 1\ 1( l A q"
)
e orce is attractive If A IS cffilTged ositively and repulsive if A is mrrged negatively.
•
Answer: C
Dift 2
23) Charge +2q is placed at the origin and charge -q is placed at x = 2a. Where can a third positive
charge +q be placed so that the force on it is zero?
2. or 'l. ~ 2.4.'t LA" ,f" '\"
Q
k
;;:. rC
'2<t
ie-c.
~~t.
'>@
t:::
Answer: x = 6.83a
Dift: 3 f
•._tAt.
6)
_
cz.qt~" -
'7
_
7-
•
2..
~1!I.) =1 J!)
(2.4 fi \\
)
Choose from the following:
(a) C2/N_m2
(d) N/C
(b) N_m2/C2
(e imeASi less
c) coulomb
25) Charge has which of the units shown above?
Answer: (c) coulomb
Dift: 1
26) An electron and a proton are separated by a distance of 1 m. What happens to the size of the
force on the proton if the electron is moved 0.5 m closer to the proton?
A) It increases
t decreases
C) It increases
D) It decreases
E) It increases
Answer: A
Diff:2
to 4 times its ori inal val
to one-half its original value.
to 2 times its original value.
to one-fourth its original value.
to 8 times its original value.
~l
----'
f~{
\
\
i:i
Advanced
16) One kilogram is hung from a string and it stretches 4.0 em. A charge of 3.0 ~C is placed on the
kilogram. How much charge must be placed 4.0 em below the kg (before it moves) so that the kg
moves up by 1.0 em to a new equilibrium position?
rr.' 4,0,.. o,o.SiJ... r-~ (J.dI ••.. f~r.3x'a-'C
f:Q ~)
'f.8,J
e. ~ (:
'-1.; (f{)f
Answer: 0.23 ~C J
r.•..
r.:=,~':-
'!!.k
F :.-b = '
Diff: 2
f {'2.I(S
h. -- £...
- r.e!' _':\< • ~Ou
;..
"/~.()'~=1.l{!{tJ) -=.
.IS c' "
4 b ~.
fi.'
f.-
~c
_f
,r. .•.
f-"~NJ
N ().•()~...
r'"
-"" {Ml
t 9..~
I. ,"
~)(I
19) A point charge of +Q is placed at the center of a square, and a second point c arge of -Q is
placed at the upper-left comer. It is observed that an electrostatic force of 2 N acts on the positiv
charge at the center.
+2--;:2IJ
~
7
t"fa ::
-0«-..--_._-:
I~'
I
I
+
Fl-
<'
2 ~ ~ 13-S""~
_a~_.f~••••.l
L
2.
_ 2.Bz~
'2-z.~"""
()
et::w:~
reo'
What is the magnitude of the force that acts on the center charge if a third charge of -Q is placed at
the lower-left comer?
A) zero
,-BfNzv
-q-4N
D)ZV3N
Answer:
Oiff: 2
r/
I"
\j
21) Two volleyballs, each of mass 0.300 kg, are charged by an electrostatic generator. Each is
attached to an identical string and suspended from the same point. They repel each other and hang
with separation 50.0cm. The length of the string from the point of support to the center of a ball is
=
~250. em.
2.F 0
t::s t
0
DetJrmln~ the charge on each ball.
n tf. =
~ ~ =-
0 =- ~
+-
r
r.:~','
""'~tmPoJ
r~-;::
:ffr
f
t
-e.~~~)1.1'J
t- fT
FT ~ 2.7S'tJ
Answer: 2.86 ~C
Diff: 2
-
~
~r~~ t- c.
r:(o.n.rJ j
(s;~5y
•
;~'y("-zC)~
-I.I.{IY -j,'ir«
r ~N!.
I
-',
/, I.( f'(
t,
:I
I
)
.0
I
_
if:::.
1/,