Transcript Magnetism – Part 3

MAGNETISM – PART 3
CALENDAR
Magnetism
Monday – Recorded Lecture.
 Today – Brief review of the material + a few
problems.
 Today, Friday we will continue with chapter 20
materials.
 There will be a Quiz on Magnetism on Friday.
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ABOUT THOSE EXAMS -
Magnetism
Grades look pretty bad. I will review on Friday after I
have a look at the papers.
 Each exam (both sections) had similar problems.
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A Kirchoff Law Problem – simple
 A combine either capacitors or resistors and calculate what
was happening at one of them.
 A problem involving polarization – a thinker.
 A question on how much energy was required to bring three
charges together.
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THESE WERE THE HINTS I GAVE YOU!
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Magnetism
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Anything in the three chapters is fair game.
Read the sections on charge and charge effects very carefully. We
didn’t cover some of this in class. (Problem 2)
Know the difference between Potential and Potential energy.
Know how much work it takes to create a charge distribution. We
did it in class. (Problem 1)
Know how to add capacitors and resistors and how to solve simple
circuit problems. (Problem 3)
There WILL be a Kirchhoff's Law problem. (Problem 4)
Coulomb’s Law and the addition of forces. Calculation of the
potential (scalar)
Be sure to understand all of the HW problems that were assigned –
or not assigned!
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Magnetism
The force is perpendicular
to the direction of motion.
The force has a constant
magnitude = Bqv
This will produce circular
motion as in PHY2053.
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Magnetism
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LOOK AT THE DIRECTION OF THE FORCE
AND THE VELOCITY
Magnetism
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RECALL
Magnetism
v2
Centripeta l Accelerati on 
r
mv 2
Centripeta l Force 
r
The magnetic force is qvB
mv 2
qvB 
r
mv
r
Bq
Recall : v  r or r  v/ 
v Bq

r m
This is called the cyclotron

angular frequency
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Magnetism
Bq

  2f
m
mv T  period 
r
Bq
1
f
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OFF ANGLE
Magnetism
PITCH
P  v parallelT
P
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PROBLEM
(a) the magnitude and direction of the magnetic field that will cause
the electron to follow the semicircular path from A to B and
Magnetism
An electron at point A in the figure has a speed v0 of 1.4 x 106 m/s.
Find
(b) the time required for the electron to move from A to B.
(c) What magnetic field would be needed if the particle were a proton
instead of an electron?
m=9.1E-31 Kg
e=1.6E-19 C
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FORCE ON A WIRE CARRYING A CURRENT IN A B
FIELD
F  Bil
Magnetism
A straight vertical wire carries a
current of 1.20 A downward in a
region between the poles of a large
electromagnet where the field
strength is 0.588 T and is horizontal.
What are the magnitude and
direction of the magnetic force on a
1.00 cm section of this wire if the
magnetic-field direction is
(a) toward the east,
(b) toward the south
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• Novel applications have been devised to make use of the force that a magnetic
field exerts on a conductor carrying current.
Magnetism
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CURRENT LOOP
Loop will tend to rotate due to the torque the field applies to the loop.
Magnetism
What is force
on the ends??
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THE LOOP (FROM THE TOP)
OBSERVATION
Force on Side 2 is out
of the paper and that on
the opposite side is into
the paper. No net force
tending to rotate the loop
due to either of these forces.
The net force on the loop is
also zero,
pivot
Magnetism
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THE OTHER SIDES
t1=F1 (b/2)Sin(q)
=(B i a) x
(b/2)Sin(q)
total torque on
the loop is: 2t1
Total torque:
t=(iaB) bSin(q)
=iABSin(q)
(A=Area)
Magnetism
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APPLICATION: THE MOTOR
If the conductor is a loop, the torque can
create an electric motor.
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Magnetism
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Magnetism
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Magnetism
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Magnetism
A circular coil of wire 8.6 cm in
diameter has 15 turns and
carries a current of 2.7 A. The
coil is in a region where the
magnetic field is 0.56 T.
What orientation of the coil
gives the maximum torque on
the coil.
What is this maximum torque?
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ANOTHER APPLICATION
THE GALVANOMETER
Magnetism
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Magnetism
CURRENTS CAUSE MAGNETIC
FIELDS
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MAGNETIC FIELD OF LONG STRAIGHT CONDUCTOR –
Magnetism
• Placed over a compass,
the wire would cause the
compass needle to deflect.
This was the classic
demonstration done by
Oersted as he
demonstrated the effect.
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RESULT
Magnetism
0 I
B
2r
r
Tm
 0  4 10
(exact)
A
7
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FORCE BETWEEN TWO CURRENT CARRYING
CONDUCTORS
Magnetism
First wire produces a magnetic field at the
second wire position.
The second wire therefore feels a force = Bil
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TWO WIRES
Magnetism
B From First Wire
 0 I1
B
2r
 0 I1
F  BI 2l 
I 2l
2r
F  0 I1 I 2

l
2r
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CURRENTS IN A LOOP –
Magnetism
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FIELD OF A CURRENT LOOP
Magnetism
B
 0 NI
2R
N turns of wire
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Magnetism
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B   0 nI
SOLONOID
Magnetism
Total number
N of Turns
n 
L
Length
B=~0 outside
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THE SOLENOID –
Magnetism
 0 NI
B
2r
B=0 outside
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Magnetism

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