sources of the magnetic field march 23, 2009 note – these slides will be updated for the actual...
TRANSCRIPT
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Sources of the Sources of the Magnetic FieldMagnetic Field
March 23, 2009March 23, 2009
Note – These slides will be Note – These slides will be updated for the actual updated for the actual
presentation.presentation.
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Remember the wire?
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Try to remember…
30
20 4
1
4
1
r
dq
r
dq
rd
rrE
VECTOR UNITr
r
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The “Coulomb’s Law” of Magnetism
A Vector Equation … duck
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For the Magnetic Field,current “elements” create the field.
TmATm
typermeabilir
id
r
id unit
770
30
20
1026.1/104
44
:field fashion tosimilar aIn
rsrs
B
E
This is the Law ofBiot-Savart
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Magnetic Field of a Straight Wire
•We intimated via magnets that the Magnetic field associated with a straight wire seemed to vary with 1/d.
•We can now PROVE this!
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From the Past
Using a Compass
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Right-hand rule: Grasp the element in your right hand with your extended thumb pointing in the direction of the current. Your fingers will then naturally curl around in the direction of the magnetic field lines due to that element.
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Let’s Calculate the FIELD
Note:
For ALL current elementsin the wire:
ds X r
is into the page
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The Details
02
0
20
)sin(
2B
it. DOUBLE and to0 from integrate
wesoamount equalan scontribute
wire theofportion Negative
)sin(
4
r
dsi
r
idsdB
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Moving right along
R
i
Rs
RdsiB
SoRs
R
Rsr
22
)sin(sin
0
02/322
0
22
22
1/d
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A bit more complicatedA finite wire
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P1)sin()sin(: NOTE
r
ds
2/122
20 )sin(
4
)sin(
)sin(
Rsr
r
dsidB
r
R
rdsrds
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More P1
R
i
whenRL
L
R
iB
and
Rs
dsiB
L
L
2B
,L 42
4
0
22
0
2/
2/2/322
0
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P2
22
0
0
2/322
0
4
4
Rs
L
R
iB
or
Rs
dsiRB
L
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APPLICATION: Find the magnetic field B at point P
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Center of a Circular Arc of a Wire carrying current
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More arc…
Cpoint at 4
44
44
0
0
0
02
0
20
20
R
iB
dR
i
R
iRddBB
R
iRd
R
idsdB
Rdds ds
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The overall field from a circular current loop
Sorta looks like a magnet!
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Iron
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Howya Do Dat??
0rsdNo Field at C
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Force Between Two Current Carrying Straight Parallel Conductors
Wire “a” createsa field at wire “b”
Current in wire “b” sees aforce because it is movingin the magnetic field of “a”.
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The Calculation
d
iLi
iFd
iB
ba
b
a
2F
angles...right at are and Since
2
:calculatedjust what weis a"" wire
todue b"" at wire FIELD The
0
b""on
0b""at
BL
BL
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Definition of the AmpereThe force acting between currents in parallel wires is the basis for the definition of the ampere, which is one of the seven SI base units. The definition, adopted in 1946, is this: The ampere is that constant current which, if maintained in two straight, parallel conductors of infinite length, of negligible circular cross section, and placed 1 m apart in vacuum, would produce on each of these conductors a force of magnitude 2 x 10-7 newton per meter of length.
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Ampere’s Law
The return of Gauss
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Remember GAUSS’S LAW??
0enclosedq
d AESurfaceIntegral
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Gauss’s Law
•Made calculations easier than integration over a charge distribution.
•Applied to situations of HIGH SYMMETRY.
•Gaussian SURFACE had to be defined which was consistent with the geometry.
•AMPERE’S Law is the Gauss’ Law of Magnetism! (Sorry)
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The next few slides have been lifted from Seb Oliveron the internet
Whoever he is!
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Biot-Savart•The “Coulombs Law of Magnetism”
20 ˆ
4 r
rdsB
id
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Invisible Summary
Biot-Savart Law (Field produced by wires)Centre of a wire loop radius RCentre of a tight Wire Coil with N turnsDistance a from long straight wire
Force between two wires Definition of Ampere
a
II
l
F
2
210
a
IB
2
0
R
IB
20
R
NIB
20
20 ˆ
4 r
rdsB
id
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Magnetic Field from a long wirer
IB
2
0
cosdsBdsB
I
B
r
ds1cos0 dsBdsB
dsr
I
2
0 dsB
Using Biot-Savart Law
Take a short vector on a circle, ds
Thus the dot product of B & the short vector ds is:
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Sum B.ds around a circular path
I
B
r
ds
dsr
I
2
0 dsB
dsB
Sum this around the whole ring
dsr
I
2
0 dsr
I
2
0
rds 2 Ιrr
I0
0 22
dsB
Circumference of circle
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Consider a different path
•Field goes as 1/r
•Path goes as r.•Integral
independent of r
0 dsB
i
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SO, AMPERE’S LAWby SUPERPOSITION:
We will do a LINE INTEGRATIONAround a closed path or LOOP.
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Ampere’s Law
enclosedid 0 sB
USE THE RIGHT HAND RULE IN THESE CALCULATIONS
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The Right Hand Rule .. AGAIN
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Another Right Hand Rule
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COMPARE
enclosedid 0 sB
0enclosedq
d AE
Line Integral
Surface Integral
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Simple Example
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Field Around a Long Straight Wire
enclosedid 0 sB
r
iB
irB
2
2
0
0
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Field INSIDE a WireCarrying UNIFORM Current
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The Calculation
rR
iB
andR
rii
irBdsBd
enclosed
enclosed
20
2
2
0
2
2
sB
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R r
B
R
i
2
0
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Procedure• Apply Ampere’s law only to highly
symmetrical situations.• Superposition works.
▫Two wires can be treated separately and the results added (VECTORIALLY!)
• The individual parts of the calculation can be handled (usually) without the use of vector calculations because of the symmetry.
• THIS IS SORT OF LIKE GAUSS’s LAW WITH AN ATTITUDE!
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The figure below shows a cross section of an infinite conducting sheet carrying a current per unit x-length of l; the current emerges perpendicularly out of the page. (a) Use the Biot–Savart law and symmetry to show that for all points P above the sheet, and all points P´ below it, the magnetic field B is parallel to the sheet and directed as shown. (b) Use Ampere's law to find B at all points P and P´.
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FIRST PART
Vertical ComponentsCancel
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Apply Ampere to Circuit
Infinite Extent
B
B
Li
: thereforeis loop theinsideCurrent
lengthunit per current
L
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The “Math”
Infinite Extent
B
B
20
0
0
B
LBLBL
id enclosedsB
Bds=0
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A Physical Solenoid
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Inside the Solenoid
For an “INFINITE” (long) solenoid the previous problem and SUPERPOSITION suggests that the field OUTSIDE this
solenoid is ZERO!
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More on Long Solenoid
Field is ZERO!
Field is ZERO
Field looks UNIFORM
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The real thing…..
Weak Field
Stronger - Leakage
Fairly Uniform field
Finite Length
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Another Way
0
:
0
0
0
niB
nihBhh
id
Ampere
enclosed
sB
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Application
•Creation of Uniform Magnetic Field Region•Minimal field outside
▫except at the ends!
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Two Coils
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“Real” Helmholtz Coils
Used for experiments.
Can be aligned to cancelout the Earth’s magneticfield for critical measurements.
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The Toroid
Slightly lessdense than
inner portion
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The Toroid
r
NiB
so
totalNirBd
Ampere
2
turns)# (N 2
:nintegratio ofpath the
in contained coil INNER about the
only worry need Weagain.
0
0
sB