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Transcript of Induction
Induction - Spring 2006Induction - Spring 2006 11
InductionInduction
March 29, 2006March 29, 2006
Induction - Spring 2006Induction - Spring 2006 22
Calendar…Calendar…
Today we finish up some material from Today we finish up some material from the last chapter and begin the chapter on the last chapter and begin the chapter on induction.induction.
Friday – Quiz on Friday – Quiz on LASTLAST chapter (30) chapter (30) Next Friday … still likely date for the next Next Friday … still likely date for the next
exam.exam. If you have a problem with this date please If you have a problem with this date please
email me with reason and we will try to email me with reason and we will try to figure out how to deal with it.figure out how to deal with it.
Induction - Spring 2006Induction - Spring 2006 33
Let’s Finish Some Let’s Finish Some DetailsDetailsDisplacement Current Displacement Current
Induction - Spring 20064
Magnetic Flux
SurfaceOpen - Gauss Like AB d
For a CLOSED Surface we might expect this to be equal to some constant times the
enclosed poles … but there ain’t no such thing!
0 AB d
Induction - Spring 20065
Examples
S N
Induction - Spring 20066
Consider the poor little capacitor…
i i
CHARGING OR DISCHARGING …. HOW CAN CURRENTFLOW THROUGH THE GAP??
Induction - Spring 20067
Through Which Surface Do we measure the current for Ampere’s Law?
I=0
Induction - Spring 20068
In the gap… DISPLACEMENT CURRENT
dt
dI
ntDisplaceme
I
dt
dq
Let
dt
dq
dt
d
qEA
d
d
Current
1
S through FLUX The
0
0
0
2
Induction - Spring 2006 9
Induction - Spring 2006 10
From The Demo ..
Induction - Spring 2006 11
Faraday’s Experiments
??
Induction - Spring 2006 12
Insert Magnet into Coil
Induction - Spring 2006 13
Remove Coil from Field Region
Induction - Spring 2006 14
That’s Strange …..
These two coils are perpendicular to each otherThese two coils are perpendicular to each other
Induction - Spring 2006 15
Definition of TOTAL Definition of TOTAL ELECTRIC FLUX through a ELECTRIC FLUX through a surface:surface:
dA
is surface aLEAVING Field
Electric theofFlux Total
out
surfaced
nE
Induction - Spring 2006 16
Magnetic Flux:
THINK OFMAGNETIC FLUX
as the“AMOUNT of Magnetism”
passing through a surface.Don’t quote me on this!!!
Induction - Spring 2006 17
Consider a Loop Magnetic field passing
through the loop is CHANGING.
FLUX is changing. There is an emf
developed around the loop.
A current develops (as we saw in demo)
Work has to be done to move a charge completely around the loop.
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Induction - Spring 2006 18
Faraday’s Law (Michael Faraday)
For a current to flow around the circuit, there must be an emf.
(An emf is a voltage) The voltage is found to
increase as the rate of change of flux increases.
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Induction - Spring 2006 19
Faraday’s Law (Michael Faraday)
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demf
Law sFaraday'
We will get to the minus sign in a short time.
Induction - Spring 2006 20
Faraday’s Law (The Minus Sign)
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Using the right hand rule, wewould expect the directionof the current to be in thedirection of the arrow shown.
Induction - Spring 2006 21
Faraday’s Law (More on the Minus Sign)
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The minus sign means that the current goes the other way.
This current will produce a magnetic field that would be coming OUT of the page.
The Induced Current therefore creates a magnetic field that OPPOSES the attempt to INCREASE the magnetic field! This is referred to as Lenz’s Law.
Induction - Spring 2006 22
How much work?
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dt
ddVqW
sE/
ChargeWork/Unit
A magnetic field and an electric field areintimately connected.)
emf
Induction - Spring 2006 23
The Strange World of Dr. Lentz
Induction - Spring 2006 24
MAGNETIC FLUX
This is an integral over an OPENOPEN Surface. Magnetic Flux is a Scalar
The UNIT of FLUX is the weber 1 weber = 1 T-m2
AB dB
Induction - Spring 2006 25
We finally stated
dt
ddVemf
sE
FARADAY’s LAW
Induction - Spring 2006 26
From the equation
dt
ddVemf
sE
AB dB
LentzLentz
Induction - Spring 2006 27
Flux Can Change
If B changes If the AREA of the loop changes Changes cause emf s and currents and
consequently there are connections between E and B fields
These are expressed in Maxwells Equations
AB dB
Induction - Spring 2006 28
Maxwell’s Equations(Next Course .. Just a Preview!)
Gauss
Faraday
Induction - Spring 2006 29
Another View Of That damned minus sign again …..SUPPOSE that B begins to INCREASE its MAGNITUDE INTO THE PAGE
The Flux into the page begins to increase.
An emf is induced around a loop
A current will flow That current will create a
new magnetic field. THAT new field will change
the magnetic flux.
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Induction - Spring 2006 30
Lenz’s Law
Induced Magnetic Fields always FIGHT to stop what you are trying to do!i.e... Murphy’s Law for Magnets
Induction - Spring 2006 31
Example of Nasty Lenz
The induced magnetic field opposes thefield that does the inducing!
Induction - Spring 2006 32
Induction - Spring 2006 33
Don’t Hurt Yourself!
The current i induced in the loop has the directionsuch that the current’s magnetic field Bi opposes thechange in the magnetic field B inducing the current.
Induction - Spring 2006 34
Let’s do theLentz Warp
again !
Induction - Spring 2006 35
Lenz’s Law
An induced current has a directionsuch that the magnetic field due tothe current opposes the change in the magnetic flux that induces thecurrent. (The result of the negative sign!) …
OR
The toast will always fall buttered side down!
Induction - Spring 2006 36
An Example
The field in the diagramcreates a flux given byB=6t2+7t in milliWebersand t is in seconds.
(a)What is the emf whent=2 seconds?
(b) What is the directionof the current in the resistor R?
Induction - Spring 2006 37
This is an easy one …
mVemf
tdt
demf
ttB
31724
seconds 2at t
712
76 2
Direction? B is out of the screen and increasing.Current will produce a field INTO the paper (LENZ). Therefore current goes clockwise and R to left in the resistor.
Induction - Spring 2006 38
Figure 31-36 shows two parallel loops of wire having a common axis. The smaller loop (radius r) is above the larger loop (radius R) by a distance x >> R. Consequently, the magnetic field due to the current i in the larger loop is nearly constant throughout the smaller loop. Suppose that x is increasing at the constant rate of dx/dt = v. (a) Determine the magnetic flux through the area bounded by the smaller loop as a function of x. (Hint: See Eq. 30-29.) In the smaller loop, find (b) the induced emf and (c) the direction of the induced current.
v
Induction - Spring 2006 39
B is assumed to be constant through the center of the small loop and caused by the large one.
Induction - Spring 2006 40
The calculation of Bz
2/322
20
2/122220
2/122
220
2
4
cos
4coscos
xR
iRB
Rdds
xR
R
xR
idsdB
xR
R
xR
idsdBdB
z
z
z
Induction - Spring 2006 41
More Work
In the small loop:
Vx
iRr
dt
demf
x
iRr
xR
iRrBrAB zz
4
20
2
3
20
2
2/322
20
22
2
3
2
)prescribed asAway (Far RFor x
2
dx/dt=v
Induction - Spring 2006 42
Which Way is Current in small loop expected to flow??
Induction - Spring 2006 43
What Happens Here?
Begin to move handle as shown.
Flux through the loop decreases.
Current is induced which opposed this decrease – current tries to re-establish the B field.
Induction - Spring 2006 44
moving the bar
R
BLv
R
emfi
BLvdt
dxBL
dt
demf
BLxBAFlux
sign... minus theDropping
Induction - Spring 2006 45
Moving the Bar takes work
v
R
vLBP
vR
vLBP
FvFxdt
d
dt
dWPOWER
R
vLBF
orR
BLvBLBiLF
222
22
22
Induction - Spring 2006 46
What about a SOLID loop??
METAL Pull
Energy is LOSTBRAKING SYSTEM
Induction - Spring 2006 47
Back to Circuits for a bit ….
Induction - Spring 2006 48
Definition
Current in loop produces a magnetic fieldin the coil and consequently a magnetic flux.
If we attempt to change the current, an emfwill be induced in the loops which will tend tooppose the change in current.
This this acts like a “resistor” for changes in current!
Induction - Spring 2006 49
Remember Faraday’s Law
dt
ddVemf
sE
Lentz
Induction - Spring 2006 50
Look at the following circuit:
Switch is open NO current flows in the circuit. All is at peace!
Induction - Spring 2006 51
Close the circuit…
After the circuit has been close for a long time, the current settles down.
Since the current is constant, the flux through
the coil is constant and there is no Emf. Current is simply E/R (Ohm’s Law)
Induction - Spring 2006 52
Close the circuit…
When switch is first closed, current begins to flow rapidly.
The flux through the inductor changes rapidly. An emf is created in the coil that opposes the
increase in current. The net potential difference across the resistor is the
battery emf opposed by the emf of the coil.
Induction - Spring 2006 53
Close the circuit…
dt
demf
0
)(
dt
diRV
notationVEbattery
Induction - Spring 2006 54
Moving right along …
0
solonoid, aFor
N. turns,ofnumber the toas wellas
current the toalproportion isflux The
0
)(
dt
diLiRV
dt
diL
dt
d
NLii
dt
diRV
notationVE
B
battery
Induction - Spring 2006 55
Definition of Inductance L
i
NL B
UNIT of Inductance = 1 henry = 1 T- m2/A
is the flux near the center of one of the coilsmaking the inductor
Induction - Spring 2006 56
Consider a Solenoid
n turns per unit lengthniB
or
nliBl
id enclosed
0
0
0
sBl
Induction - Spring 2006 57
So….
AnlL
or
AlnL
ori
niAnl
i
nlBA
i
NL B
2
20
0
lengthunit
inductance/
Depends only on geometry just like C andis independent of current.
Induction - Spring 2006 58
Inductive Circuit
Switch to “a”. Inductor seems like a
short so current rises quickly.
Field increases in L and reverse emf is generated.
Eventually, i maxes out and back emf ceases.
Steady State Current after this.
i
Induction - Spring 2006 59
THE BIG INDUCTION
As we begin to increase the current in the coil The current in the first coil produces a
magnetic field in the second coil Which tries to create a current which will
reduce the field it is experiences And so resists the increase in current.
Induction - Spring 2006 60
Back to the real world…
i
0
equationcapacitor
theas form same
0
:0 drops voltageof sum
dt
dqR
C
qE
dt
diLiRE
Switch to “a”
Induction - Spring 2006 61
Solution
R
L
eR
Ei LRt
constant time
)1( /
Induction - Spring 2006 62
Switch position “b”
/
0
0
teR
Ei
iRdt
diL
E
Induction - Spring 2006 63
Max Current Rate ofincrease = max emfVR=iR
~current
Induction - Spring 2006 64
constant) (time
)1( /
R
L
eR
Ei LRt
Solve the lo
op equation.
Induction - Spring 2006 65
IMPORTANT QUESTION
Switch closes. No emf Current flows for a
while It flows through R Energy is conserved
(i2R)
WHERE DOES THE ENERGY COME FROM??
Induction - Spring 2006 66
For an answerReturn to the Big C
We move a charge dq from the (-) plate to the (+) one.
The (-) plate becomes more (-)
The (+) plate becomes more (+).
dW=Fd=dq x E x d+q -q
E=0A/d
+dq
Induction - Spring 2006 67
The calc
2
0
2
020
2
00
22
0
2
00
00
2
1
eunit volum
energy
2
1
2
1
2
1)(
2
2
)()()(
E
E
u
AdAdAd
AA
dW
or
q
A
dqdq
A
dW
dA
qdqddqEddqdW
The energy is inthe FIELD !!!
Induction - Spring 2006 68
What about POWER??
Ridt
diLiiE
i
iRdt
diLE
2
:
powerto
circuit
powerdissipatedby resistor
Must be dWL/dt
Induction - Spring 2006 69
So
2
2
2
12
1
CVW
LiidiLW
dt
diLi
dt
dW
C
L
L
Energystoredin the
Capacitor
Induction - Spring 2006 70
WHERE is the energy??
l
Al
NiBA
l
Ni
niB
nilBll
id enclosed
0
0
0
0
0
B
or
0
sB
Induction - Spring 2006 71
Remember the Inductor??
turn.onegh flux throu MagneticΦ
current.
inductorin turnsofNumber
i
Ni
NL
?????????????
Induction - Spring 2006 72
So …
l
AiN
l
NiANiW
l
NiA
iNi
NiLiW
L
Ni
i
NL
2220
0
0
0
22
2
1
2
1
2
1
2
1
2
1
Induction - Spring 2006 73
2
0
2
0
22
0
0
2220
0
2
1
or
(volume) 2
1
2
1
B
:before From
2
1
BV
Wu
VBl
AlBW
l
Ni
l
AiNW
ENERGY IN THEFIELD TOO!
Induction - Spring 2006 74
IMPORTANT CONCLUSION
A region of space that contains either a magnetic or an electric field contains electromagnetic energy.
The energy density of either is proportional to the square of the field strength.
Induction - Spring 2006 75