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Physics 132 Quiz # 18
1.1. Ampère’s
Ampère’slaw
lawgives
givesthe
themagnetic
magneticfield
fieldproduced
producedby
byaa
distribution
distributionof
ofcurrents.
currents.Which
Whichcondition(s)
condition(s)must
mustbe
be
satisfied?
satisfied?
A.
A.The
Thedistribution
distributionof
ofcurrents
currentsmust
mustbe
besteady.
steady.
B.
B.In
Inorder
ordertotosolve,
solve,the
thedistribution
distributionmust
musthave
have
sufficient
sufficientsymmetry.
symmetry.
C.
C.both
bothof
ofthe
theabove
above
D.
D.neither
neitherof
ofthe
theabove
above
2.2. Gauss’s
Gauss’slaw
lawfor
formagnetism
magnetismstates
statesthat
thatthe
thenet
netflux
fluxof
of
the
thefield
fieldthrough
throughany
anyclosed
closedsurface
surfaceisis
A.
A.proportional
proportionaltotothe
thecurrents
currentspenetrating
penetratingthe
thesurface
surface
B.
B.proportional
proportionaltotothe
thecurrents
currentsof
ofclosed
closedloops
loopsfully
fully
enclosed
enclosedby
bythe
thesurface
surface
C.
C.zero
zero
LECTURE 20
Concepts
Concepts and
and Topics
Topics
••
••
••
••
••
Field
Field created
created by
by currents
currents
Interaction
Interaction between
between currents
currents
Ampere’s
Ampere’s law
law
ANNOUNCEMENTS
ANNOUNCEMENTS
Pick
Pick up
up your
your graded
graded
assignments
assignments
Review
Review session
session on
on
Sunday,
Sunday, March
March 3rd
3rd
at
at 7pm
7pm ,, NSH
NSH 123
123
Magnetic fields caused by currents
µ0 2 I
B=
4π R
N
S
N
S
BEarth
Concept
ConceptTest:
Test:After
Afterclosing
closingthe
theswitch
switchin
inthe
the
above
aboveexperiment,
experiment,the
thecurrent
currentflows
flows
a.)
a.)from
fromthe
theswitch
switchtowards
towardsthe
thecompass
compass
b.)
b.)from
fromthe
thecompass
compasstowards
towardsthe
theswitch
switch
c.)
c.)not
notenough
enoughinformation
informationto
todecide
decide
Concept
Concept Test
Test::AAbattery
batteryestablishes
establishesaasteady
steady
current
currentaround
aroundthe
thecircuit
circuitbelow.
below.AAcompass
compassneedle
needle
isisplaced
placedsuccessively
successivelyatatpoints
pointsP,Q,
P,Q,and
andR.
R.The
The
Relative
Relativedeflection
deflectionof
ofthe
theneedle,
needle,in
indescending
descending
order,is
order,is
P
1.
2.
3.
4.
5.
P, Q, R
Q, R, P
R, Q, P
P, R, Q
Q, P, R
Q
R
Concept
Concept Test
Test::Two
Twovery
verylong
longfixed
fixedwires
wirescross
cross
each
eachother
otherperpendicularly
perpendicularly(see
(seefigure).
figure).They
Theydo
do
not
nottouch,
touch,but
butare
arevery
veryclose
closeto
toeach
eachother,
other,as
as
shown.
shown.Indicate
Indicateall
allthe
thepoints
pointswhere
wherethe
thenet
net
magnetic
magneticfield
fieldisiszero
zero
I
I
Force between two parallel wires
µ 0 I1
B1 =
2π R
µ 0 I1 I 2
F
= I 2 B1 =
2π R
l
I
Repulsive
I
Attractive
I
Concept
Concept Test:
Test:
Two
Twovery
verylong,
long,parallel
parallelconducting
conductingwires
wirescarry
carryequal
equal
currents
currentsininopposite
oppositedirections.
directions. The
Thenumbered
numbered
diagrams
diagramsshow
showend
endviews
viewsof
ofthe
thewires
wiresand
andthe
the
resultant
resultantforce
forcevectors
vectorsdue
duetotocurrent
currentflow
flowinineach
each
wire.
wire. Which
Whichdiagram
diagrambest
bestrepresents
representsthe
thedirection
directionof
of
the
theforces?
forces?
1)
1)
2)
2)
3)
3)
4)
4)
5)
5)
11
22
33
44
55
Problem
ProblemAArectangular
rectangularloop
loopisisplaced
placednext
nextto
toaa
straight
straightwire
wireas
asshown
shownin
inthe
thefigure.
figure.What
Whatisisthe
the
direction
directionand
andthe
themagnitude
magnitudeof
ofthe
thenet
netmagnetic
magnetic
force
forceacting
actingon
onthe
theloop?
loop?
I
d
I
w
l
Ampere’s Law
r r
∫ B ⋅ dl = µ0 I C
Example: Infinitely long wire
B( 2πr ) = µ0 I C
µ0 I
B=
2π r
Ampere’s Law: Applications
Toroid
Toroidwith
withNNturns:
turns:
Calculate
Calculateand
andsketch
sketchthe
the
magnetic
magneticfield
fieldaround
aroundthe
the
toroid
toroidas
asaa function
functionof
of
the
theradial
radialdistance
distancerrfrom
from
its
itscenter.
center.
B(r)
a
b
r