Physics Ch 20 Equi Prob Notes

8/12/2019 Physics Ch 20 Equi Prob Notes

1/13

SomeExtra

Notes

to

Help

You

Know

How

toApproachaNewKindofCoulombs

LawProblem

(Toaccompanyworksheetproblems#10&11.)

EquilibriumProblems


2/13

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.)


3/13

YoushouldseethatF2,1isactingtotheleft,andthatF2,3isacting

totheright. Youshouldalsoseethatifq2wasplacedattheright

location,the

two

forces

F2,1andF2,3couldcanceleachotherout,

andthatq2wouldthereforeexperiencezerototalforce. Forthis

tohappenapproximatelywherewouldq2needtobeplaced?


TheBasic

Idea

(Whenyouthinkyouknowtheanswer,gotothenextpage.)


4/13

Youshouldseethatq2cantbeplaceddirectlyinthemiddle

betweenq1andq3. Thisisbecauseq3isalargerchargethanq1,

andwould

therefore

exert

astronger

force

if

q2wasatequal

distancesfrombothcharges. Butifq2wasshiftedalittlebitto

theleft(closertothesmallerchargedq1),thentheforcesfromq1

andq3could

be

equally

strong

and

perfectly

cancel

each

other.


TheBasic

Idea

Equilibriumproblems(liketheonesonyourworksheet)haveto

dowith

finding

the

exact

right

location

to

place

a3rd charge

for

allforcesactingonittocancelout,leavinganetforceofzero.


5/13

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.)


6/13

Example


7/13

Example


8/13

#10


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.


9/13

#10


10/13

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.)


OneMore

Idea


11/13

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.


OneMore

Idea


12/13

#11


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.)


13/13

#11

Physics Ch 20 Equi Prob Notes

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