physics ch 20 equi prob notes
TRANSCRIPT
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8/12/2019 Physics Ch 20 Equi Prob Notes
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SomeExtra
Notes
to
Help
You
Know
How
toApproachaNewKindofCoulombs
LawProblem
(Toaccompanyworksheetproblems#10&11.)
EquilibriumProblems
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Startbythinkingforamomentaboutthedirectionsofsome
forcesactingbetweenchargesinthediagrambelow. (The
unknowndistances
shouldnt
matter
for
these
directions.)
Inwhatdirection istheforceonq2becauseofq1(F2,1)acting?
Inwhatdirection istheforceonq2becauseofq
3(F
2,3)acting?
EquilibriumProblems:
TheBasic
Idea
(Whenyouthinkyouknowtheanswers,gotothenextpage.)
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YoushouldseethatF2,1isactingtotheleft,andthatF2,3isacting
totheright. Youshouldalsoseethatifq2wasplacedattheright
location,the
two
forces
F2,1andF2,3couldcanceleachotherout,
andthatq2wouldthereforeexperiencezerototalforce. Forthis
tohappenapproximatelywherewouldq2needtobeplaced?
EquilibriumProblems:
TheBasic
Idea
(Whenyouthinkyouknowtheanswer,gotothenextpage.)
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Youshouldseethatq2cantbeplaceddirectlyinthemiddle
betweenq1andq3. Thisisbecauseq3isalargerchargethanq1,
andwould
therefore
exert
astronger
force
if
q2wasatequal
distancesfrombothcharges. Butifq2wasshiftedalittlebitto
theleft(closertothesmallerchargedq1),thentheforcesfromq1
andq3could
be
equally
strong
and
perfectly
cancel
each
other.
EquilibriumProblems:
TheBasic
Idea
Equilibriumproblems(liketheonesonyourworksheet)haveto
dowith
finding
the
exact
right
location
to
place
a3rd charge
for
allforcesactingonittocancelout,leavinganetforceofzero.
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Example Thefollowingproblemisanexampleofastandard
equilibriumproblem. Thesolutionisonthenext
page,so
after
looking
at
the
question,
look
at
the
solution
and
makesureyouunderstandthestepsinvolvedwithsolvingit. (You
dontneedtodoanythingelsewiththisproblem,though.)
Twochargesarearrangedasinthefigure. Findthepointinspace
betweenthetwochargeswhereathirdchargeq3=+2Ccanbe
placed,so
that
the
net
force
acting
on
q3will
be
zero.
(Give
your
answerintermsofthedistancefromq1toq3.)
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Example
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Example
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#10
Twochargesarearrangedasinthefigure. Findthepointinspace
betweenthetwochargeswhereathirdchargeq3=+4Ccanbeplaced,
so
that
the
net
force
acting
on
q3will
be
zero.
(Give
your
answerintermsofthedistancefromq1toq3.)
Thisproblemisonyourworksheet. Trytodotheproblem
yourself,basedontheexampleyoujustsaw. Thencheck
youranswer/work
with
the
next
page
once
you
think
you
know
howtodoit.
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#10
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Ifyoureallythinkaboutit,youshouldunderstandthatthezero
forcelocationfora3rd charge(whichcanalsobecalledthe
equilibriumposition)
will
NOT
necessarily
be
between
the
other
twocharges. Forinstance,lookattheexamplebelowandtryto
understandwhya3rd charge(picturethe3rd chargeasbeing
positive,though
it
doesnt
technically
matter),
placed
anywhere
betweentheothertwochargeswouldNEVERbeinequilibrium.
(Whenyouthinkyouunderstand,gotothenextpage.)
EquilibriumProblems:
OneMore
Idea
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Fortheexamplebelow,athirdchargecouldntgobetweenthe
twocharges,becauseBOTHCHARGESWOULDBETRYINGTO
PUSHTHE
3RD CHARGE
IN
THE
SAME
DIRECTION,
which
means
theywouldntbeabletocanceleachother. Whenthishappens
inaproblem,the3rd chargecouldtheoreticallybeplacedallthe
wayto
the
left
of
both
charges,
or
all
the
way
to
the
right
of
both
charges,inordertohavethetwoforcesactinginopposite
directions. Sotodecidewheretoplacethe3rd charge,ALWAYS
PLACE
IT
CLOSER
TO
THE
SMALLER
OF
THE
2
CHARGES.
EquilibriumProblems:
OneMore
Idea
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#11
Twochargesarearrangedasinthefigure. Findthepointinspace
whereathirdchargecanbeplaced,sothatthenetforceacting
onq3will
be
zero.
(Give
your
answer
in
terms
of
the
distance
fromq1toq3.)
Thisproblemisonyourworksheet. Trytodotheproblem
yourself,basedontheexampleyoujustsaw. Thencheck
youranswer/work
with
the
next
page
once
you
think
you
know
howtodoit. (Butbecarefulyouthinkaboutwhatwasdiscussed
onthelastcoupleofpagesofthesenotes.)
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#11