Transcript Magnetism

Physics 102: Lecture 08
Magnetism
• Today’s lecture will cover Textbook
Sections 19.1- 4
Physics 102: Lecture 8, Slide 1
Magnets are Cool!
• North Pole and South Pole
-
– Opposites Attract
– Likes Repel
+
• Magnetic Field Lines
– Arrows give direction
– Density gives strength
– Looks like dipole!
Lets Break it!
Physics 102: Lecture 8, Slide 2
S
N
S
N
S
N
Field Lines of Bar Magnet
S
N
Magnetic field lines don’t start or stop.
There are no magnetic charges (monopoles)
Physics 102: Lecture 8, Slide 3
Preflight 8.1
1
Which drawing shows the correct
field lines for a bar magnet?
(1)
(2)
S
N
S
N
S
N
2
(3)
3
Physics 102: Lecture 8, Slide 4
Preflight 8.1
1
Which drawing shows the correct
field lines for a bar magnet?
(1)
(2)
S
N
S
N
S
N
2
(3)
• Magnetic field lines are continuous
• Arrows go from N to S outside the
magnet (S to N inside).
Physics 102: Lecture 8, Slide 5
3
Comparison:
Electric Field Lines vs. Magnetic Field Lines
• Similarities
– Density gives strength
– Arrow gives direction
• Leave +, North
• Enter -, South
• Differences
– Start/Stop on electric charge
– No Magnetic Charge, lines are continuous!
• FYI
– x x x x x x x INTO Page
– • • • • • • • • • OUT of Page
Physics 102: Lecture 8, Slide 6
No Magnetic Charges
• Magnetic Fields are created by
moving electric charge!
• Where is the moving charge?
Orbits of electrons about nuclei
Intrinsic “spin” of
electrons (more
important effect)
Physics 102: Lecture 8, Slide 7
Direction of Magnetic Force on
Moving Charges
Velocity
out of page
out of page
B
right
left
out of page
out of page
up
down
Force
F  qv  B
Right Hand Rule
• Thumb ___ , Fingers ___ , Palm ___
Physics 102: Lecture 8, Slide 8
B
F
v
Direction of Magnetic Force on
Moving Charges
1) Up
Velocity
out of page
out of page
B
right
left
out of page
up
2) Down
3) Right
Force
up
down
4) Left
5) Zero
B
Right Hand Rule
• Thumb v, Fingers B, palm F
Physics 102: Lecture 8, Slide 9
F
v
Direction of Magnetic Force on
Moving Charges
Velocity
out of page
out of page
out of page
out of page
B
right
left
up
down
Force
up
down
left
right
B
Right Hand Rule
• Thumb v, Fingers B, palm F
• Negative charge has opposite F!
Physics 102: Lecture 8, Slide 10
F
v
Preflight 8.3
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the force on the particle just as it
enters region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 11
Preflight 8.3
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the force on the particle just as it
enters region 1?
1) up
Particle is moving straight upwards
2) down
then veers to the right.
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 12
Preflight 8.4
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 1?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 13
Preflight 8.4
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 1?
_
1) up
2) down v (thumb) points up, F(palm) points
right: so B(fingers) must point out.
3) left
_
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 14
ACT: 2 Chambers
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 2?
1) up
2) down
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 15
ACT: 2 Chambers
Each chamber has a unique magnetic
field. A positively charged particle
enters chamber 1 with velocity 75 m/s
up, and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the direction of the magnetic field in region 2?
1) up
2) down v (thumb) points right, F(palm) points
up, B(fingers) point in.
3) left
4) right
5) into page
6) out of page
Physics 102: Lecture 8, Slide 16
Magnitude of Magnetic Force on
Moving Charges
• The magnetic force on a charge depends on
the magnitude of the charge, its velocity,
and the magnetic field.
F = q v B sin(q)
– Direction from RHR
• Thumb (__), fingers (__), palm (__)
– Note if v is parallel to B then F = 0
Physics 102: Lecture 8, Slide 17
V
q
B
ACT: Moving Charges
The three charges below have equal charge and speed, but are traveling in
different directions in a uniform magnetic field.
1) Which particle experiences the greatest magnetic force?
1) 1
2) 2
)3
4) All Same
2) The force on particle 3 is in the same direction as the force on particle 1.
1) True
2) False
B
3
2
1
Physics 102: Lecture 8, Slide 18
ACT: Moving Charges
The three charges below have equal charge and speed, but are traveling in
different directions in a uniform magnetic field.
1) Which particle experiences the greatest magnetic force?
1) 1
2) 2
3) 3
4) All Same
F = q v B sin(q)
2) The force on particle 3 is in the same direction as the force on particle 1.
1) True
2) False
B
Thumb (v), fingers (B), palm (F)
3
2
1
Physics 102: Lecture 8, Slide 19
into page!
Comparison
Electric vs. Magnetic
Source:
Act on:
Magnitude:
Direction:
Physics 102: Lecture 8, Slide 20
Electric
Magnetic
Charges
Charges
F = qE
Parallel to E
Moving Charges
Moving Charges
F = q v B sin(q)
Perpendicular to v,B
Velocity Selector
Determine magnitude and direction of
magnetic field such that a positively
charged particle with initial velocity v
travels straight through and exits the
other side.
E
v
What direction should B point if you want to select
negative charges?
1) Into Page
Physics 102: Lecture 8, Slide 21
2) Out of page
3) Left
4) Right
Velocity Selector
Determine magnitude and direction of
magnetic field such that a positively
charged particle with initial velocity v
travels straight through and exits the
other side.
FB
E
v
FE
Electric force is down, so need magnetic force up.
By RHR, B must be into page
For straight line, need |FE |= |FB |
q E= q v B sin(90)
B = E/v
What direction should B point if you want to select
negative charges? FE would be up so FB must be down.
1) Into Page
Physics 102: Lecture 8, Slide 22
2) Out of page
3) Left
4) Right
Motion of Q in uniform B field
• Force is perpendicular to B,v
– B does no work! (W=F d cos q ) x x x x x x x
– Speed is constant (W=D K.E. ) x x x x x x x
x x x x x x x
– Circular motion
• Solve for R:
x
x x x x x x
x
x x x x x x
x x x x x x x
Uniform B into page
Physics 102: Lecture 8, Slide 23
Motion of Q in uniform B field
• Force is perpendicular to B,v
– B does no work! (W=F d cos q ) x x x
– Speed is constant (W=D K.E. ) x x x
x x x
v
– Circular motion
v2
F m
R
v2
qvBsin(q )  m
R
Physics 102: Lecture 8, Slide 24
F
x x x x
x x x x x x
x
x x x x x x
F
x
v
• Solve for R:
x x x x
R
x x x x
x x x x x x x
Uniform B into page
mv
R
qB
Preflight 8.6
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the speed of the particle when it leaves
chamber 2?
1) v2 < v1
2) v2 = v1
3) v2 > v1
Physics 102: Lecture 8, Slide 25
Preflight 8.6
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
What is the speed of the particle when it leaves
chamber 2?
1) v2 < v1
2) v2 = v1
3) v2 > v1
Magnetic force is always perpendicular to velocity,
43
so it changes direction, not speed of particle.
Physics 102: Lecture 8, Slide 26
Preflight 8.8
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
Compare the magnitude of the magnetic field in
chambers 1 and 2
1) B1 > B2
2) B1 = B2.
3) B1 < B2
Physics 102: Lecture 8, Slide 27
Preflight 8.8
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = +25 mC
Compare the magnitude of the magnetic field in
chambers 1 and 2
1) B1 > B2
2) B1 = B2.
3) B1 < B2
Physics 102: Lecture 8, Slide 28
mv
R
qB
Larger B, greater force, smaller R
Preflight 8.9
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = ?? mC
A second particle with mass 2m enters the chamber and
follows the same path as the particle with mass m and
charge q=25 mC. What is its charge?
1) Q = 12.5 mC
2) Q = 25 mC
3) Q = 50 mC
Physics 102: Lecture 8, Slide 29
Preflight 8.9
Each chamber has a unique magnetic
field. A positively charged particle enters
chamber 1 with velocity v1= 75 m/s up,
and follows the dashed trajectory.
2
1
v = 75 m/s
q = ?? mC
A second particle with mass 2m enters the chamber and
follows the same path as the particle with mass m and
charge q=25 mC. What is its charge?
1) Q = 12.5 mC
2) Q = 25 mC
3) Q = 50 mC
Physics 102: Lecture 8, Slide 30
mv
R
qB
See you later!
• Read Sections 19.6-8
Physics 102: Lecture 8, Slide 31