Download Lecture 19-Wednesday March 11

Lecture 19-Wednesday March 11
Magnetic Forces on Moving Charges
Mass Spectrometers
Review of Monday
• Field of a long straight wire
• Field of a loop
• Field of a solenoid
Drawing Field Vectors and Field Lines of a
Current-Carrying Wire
0 I
B(r ) 
2 r
Slide 24-21
20%
20%
20%
E
D
A
B
C
D
E
A
1.
2.
3.
4.
5.
20%
C
20%
B
Point P is 5 cm above the wire as you look straight
down at it. In which direction is the magnetic field
at P?
Checking Understanding
The magnetic field at point P is zero. What are the magnitude and
direction of the current in the lower wire?
A.
B.
C.
D.
E.
F.
10 A to the right.
5 A to the right.
2.5 A to the right.
10 A to the left.
5 A to the left.
2.5 A to the left.
Slide 24-27
The Magnetic Field of a Current Loop
BCENTER OF LOOP 
0 I
2R
Slide 24-23
The Magnetic Field of a Solenoid
A short solenoid
A long solenoid
Slide 24-24
The Magnetic Field Inside a Solenoid
N
B  0 I
L
Magnetic field inside a solenoid
of length L with N turns.
Slide 24-31
Quiz #7
• What is the magnetic field strength and
direction at point A?
2 cm
A
2 cm
10 A
10 A
The Force on a Charged Particle Moving in a Magnetic
Field
Slide 24-32
The Right-Hand Rule for Forces
Slide 24-33
Slide 24-34
Paths of Charged Particles in Magnetic Fields
mv
r
qB
Slide 24-35
Charged particle moving perpendicular
to uniform magnetic field
• Magnetic force is perpendicular to velocity:
Fm  qvB sin   qvB
• What path if F ┴ v ?
mv 2
FCirc 
 q vB
r
mv
r
qB
The Mass Spectrometer
qBr
m
v
Slide 24-36
Next Monday
• Read: 24: 6—Force on a current carrying wire
• Problems: 24: CQ10, MC33, 22, 25, 26, 29, 31,
WB 24: 16-22