Download Magnetic

Physics 102: Lecture 08
Today’s Lecture: Magnetism
•
•
Magnetic fields
Magnetic force on charged
particles
Physics 102: Lecture 8, Slide 1
Magnets
• North Pole and South Pole
-
– Opposites Attract
– Likes Repel
+
• Magnetic Field Lines
– Arrows give direction
– Density gives strength
– Looks like dipole!
S
N
S
Physics
102: Lecture 8, Slide 2
15
N
S
N
The Earth is a Magnet
Physics 102: Lecture 8, Slide 3
Physics 102: Lecture 8, Slide 4
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 5
Magnetic Fields and Forces
• Magnetic fields exert forces on moving charge
• We will discuss the magnitude of the force shortly
• Direction of force is given by a “right-hand rule”
Physics 102: Lecture 8, Slide 6
Direction of Magnetic Force on
Moving Charges
• Magnetic force is perpendicular to both B and v
• Right-hand rule:
– Point thumb of right hand in v direction
– Point fingers of right hand in B direction
– Then out-of-palm points
• in the direction of F for + charge
• opposite to F for – charge
• Note:
different versions by different
books etc.
Physics 102: Lecture 8, Slide 7
Physics 102: Lecture 8, Slide 8
Direction of Magnetic Force on
Moving Charges
Velocity
out of page
out of page
B
right
left
out of page
up
Force
up
down
ACT:
1) Up
2) Down
Physics 102: Lecture 8, Slide 9
3) Right
4) Left
5) Zero
Direction of Magnetic Force on
Moving Charges
Velocity
out of page
out of page
out of page
out of page
Physics 102: Lecture 8, Slide 10
B
right
left
up
down
Force
up
down
left
right
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
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
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?
A) down
B) left
C) right
D) into page
E) out of page
Physics 102: Lecture 8, Slide 14
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
– Note if v is parallel to B then F=0
Physics 102: Lecture 8, Slide 15
V
q
B
Physics 102: Lecture 8, Slide 16
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?
A) 1
B) 2
C) 3
D) 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.
A) True
B) False
B
3
2
1
Physics 102: Lecture 8, Slide 17
Comparison
Electric vs. Magnetic
Source:
Act on:
Magnitude:
Direction:
Physics 102: Lecture 8, Slide 18
Electric
Magnetic
Charges
Charges
F = Eq
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.
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.
A) Into Page
Physics 102: Lecture 8, Slide 19
B) Out of page
C) Left
D) Right
Physics 102: Lecture 8, Slide 20
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
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
41
Physics 102: Lecture 8, Slide 21
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
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
Physics 102: Lecture 8, Slide 22
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 23
Physics 102: Lecture 8, Slide 24
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 25
ACT
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
48
Physics 102: Lecture 8, Slide 26
extra questions to try
B
1.
2.
3.
Which angle do you use to determine the
magnitude of the force on the charged
particle? (q1, q2 , either one).
Below I have drawn the trajectory for two
charged particles traveling through a
magnetic field. Is particle 1 positive or
negatively charged?
If particles 1 and 2 have the same mass and
velocity, which has the largest charge?
Physics 102: Lecture 8, Slide 27
V
q1
q2
x
x x x 1x x x
x
x x x
x x x
x
x x x
x x x
x
x x x
x x x
2
Summary
• We learned about magnetic fields B
• We learned about magnetic forces on
moving charged particles
Physics 102: Lecture 8, Slide 28