Transcript From Last Time…

Exam 2 covers Ch. 28-33,
Lecture, Discussion, HW, Lab
Exam 2 is Tue. Oct. 28, 5:30-7 pm, 2103 Ch
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Chapter 28: Electric flux & Gauss’ law
Chapter 29: Electric potential & work
Chapter 30: Electric potential & field
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Chapter 31: Current & Conductivity
Chapter 32: Circuits
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(exclude 30.7)
(exclude 32.8)
Chapter 33: Magnetic fields & forces
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(exclude 33.3, 33.6, 33.10, Hall effect)
Tue. Oct. 28, 2008
Physics 208, Lecture 17
1
Electric current
produces magnetic field
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Current (flow of electric charges )
in wire produces magnetic field.
That magnetic field aligns compass needle
Current
Magnetic
field
Tue. Oct. 28, 2008
Physics 208, Lecture 17
2
Law of Biot-Savart
B out of page
r

I

Each element of current
produces a contribution
to the magnetic field.
ds
dB
dI
Tue. Oct. 28, 2008
r
o Ids  rˆ
dB 
4 r 2
Physics 208, Lecture 17
3
Magnetic field from
long straight wire:
Direction

What direction is 
the magnetic
field from an
infinitely-long
straight wire?
Tue. Oct. 28, 2008
Physics 208, Lecture 17
o Ids  rˆ
dB 
2
4 r
y
x
I
4
Current dependence
How does the magnitude of
the B-field change if the
current is doubled?
A) Is halved
o Ids  rˆ
dB 
2
4 r
y

B) Quadruples
x
C) Stays same
D) Doubles
I
E) Is quartered
Tue. Oct. 28, 2008
Physics 208, Lecture 17
5
ˆ

Ids

r
o
Distance dependence dB 
2
4 r
How does the magnitude of
the B-field at 2 compare
to that at 1?
A) B2=B1
2
y

1
B) B2=2B1
x
C) B2=B1/2
D) B2=4B1
I
E) B2=B1/4
Tue. Oct. 28, 2008
Physics 208, Lecture 17
6
Why?
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o Ids  rˆ
Biot-Savart says dB 
2
4 r
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Why B(r) 1/r instead of 1/r2 ?
Small contribution from
this current 
element.
Large contribution from
this current element.
~ independent of r
Decreases as 1/r2
r
Tue. Oct. 28, 2008

Physics 208, Lecture 17
I
7
Long straight wire
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All current elements produce B out of page

dB  o
4

 o
4

 o
4
Ids  rˆ
r2
I
sin 
2
r
I a o
a

I
r 2 r 4  x 2  a 2 3 / 2
Add them all up:
o Ia 
dx
o I
B


4   x 2  a 2 3 / 2 2a
r
a 
r  x 2  a2

 x
Tue. Oct. 28, 2008
Physics 208, Lecture 17
8
Field from a circular loop
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Each current element produce dB
All contributions add as vectors
Along axis, all
components cancel
except for x-comp
Tue. Oct. 28, 2008
Physics 208, Lecture 17
9
Magnetic field from loop
A.
Which of these graphs
best represents the
magnetic field on the
axis of the loop?
y
Bz
z
Bz
B.
x
z
Bz
z
C.
z
Bz
D.
z
Tue. Oct. 28, 2008
Physics 208, Lecture 17
10
Magnetic field from a current loop
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One loop:
field still loops around the wire.
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Tue. Oct. 28, 2008
Many loops: same effect
Physics 208, Lecture 17
11
Solenoid electromagnet
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Tue. Oct. 28, 2008
Sequence of
current loops can
produce strong
magnetic fields.
This is an
electromagnet
Physics 208, Lecture 17
12
Comparing Electric, Magnetic
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Biot-Savart:
calculate B-field from current distribution.
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Resulting B-field is a vector, and…
complication: current (source) is a vector!
Coulomb:
calculate E-field from charge distribution
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Resulting E is a vector
but charge (source) is not a vector
Tue. Oct. 28, 2008
Physics 208, Lecture 17
13
A shortcut: Ampere’s law
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Integral around closed
path proportional to
current passing through
any surface bounded
by path.
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Ampere’s law
 B ds   I
o
I

Right-hand ‘rule’:
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Tue. Oct. 28, 2008
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closed
path
surface
bounded by
path
Thumb in direction of
positive current
Curled fingers show
direction integration
Physics 208, Lecture 17
14
Ampere’s law
Sum up component of B around path
Equals current through surface.
Component of B
along path
 B ds   I
o
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Ampere’s law
B
I
closed
path

surface
bounded by path
Tue. Oct. 28, 2008
Physics 208, Lecture 17
15
‘Testing’ Ampere’s law
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oI
Long straight wire Br 
, Br
2 r
oI
oI
 B ds   2r ds  2r
oI
 ds  2r 2r  oI
B||ds

path length
= 2πr
Circular path
path has
constant r
r
B(r)
I
Surface bounded
by path
Tue. Oct. 28, 2008
Physics 208, Lecture 17
16
Using Ampere’s law
Could have used Ampere’s law to calculate B
oI
 B ds   Bds  B  ds  B2 r oI  B  2r

B||ds
path length
= 2πr
B constant
on path
Circular path
r
B(r)
I
Surface bounded
by path
Tue. Oct. 28, 2008
Physics 208, Lecture 17
17
Quick Quiz

Suppose the wire has uniform current density. How
does the magnetic field change inside the wire?
A. Increases with r
r
B
B. Decreases with r
C. Independent of r
D. None of the above
 B  ds  2 rB   I
o cut
Tue. Oct. 28, 2008
 r2
o I
 o I
 Br 
r
2
2
R
2 R
Physics 208, Lecture 17
18