Download Lecture #3 Magnetic fields and forces Major Concepts of Physics

Major Concepts of Physics PHY102
Lecture #3
Magnetic fields and forces
January 25th, 2017
Prof. Liviu Movileanu
http://movileanulab.syr.edu/MajorConceptsPhysics.html
[email protected]
Room 211, Physics Bldg., 443-8078
Major Concepts of Physics PHY102 – Lecture #3
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Lecture objectives
• Motion of a Charged Particle in a Magnetic
Field – Continuation
• Magnetic force on a current-carrying wire
• Lecture demonstrations
• Conceptual Examples
• Announcements/Homework Assignment
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Conceptual example 19.2 - Continuation
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Why do we have a tilt of
the magnetic field?
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§19.3 Charged Particle Moving
Perpendicular to a Uniform B-field
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v
A positively charged particle has a
velocity v (orange arrow) as shown. The
magnetic field is into the page.
The magnetic force, at this instant, is shown in blue. In this region of space, this
positive charge will move in a circular path.
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§19.3 Charged Particle Moving
Perpendicular to a Uniform B-field
Applying Newton’s 2nd Law to the charge:
åF = F
B
= mar
v2
qvB = m
r
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Mass Spectrometer
A charged particle is shot into
a region of known magnetic
field.
B
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Detector
V
2
Here,
v
qvB = m
r
or qBr = mv = p
Particles of different mass will travel
different distances before striking the
detector. (v, B, and q can be controlled.)
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Other devices that use magnetic fields to bend particle paths are cyclotrons
and synchrotrons.
Synchrotrons are being tested for use in treating tumors.
Ernest Lawrence
1929 - Cyclotron
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Proton-beam radiosurgery
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Magnetic force on a current carrying wire
The magnitude of force is
F = I (L ´ B)
is
and its direction is given by the right-hand rule.
Where F is magnetic force, L is a length vector, and B is a magnetic field vector.
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Conceptual example 19.50
Example (text problem 19.50): A 20.0 cm by 30.0 cm loop of wire carries 1.0
A of current clockwise.
(a) Find the magnetic force on each side of the loop if the magnetic field is 2.5
T to the left.
I = 1.0 A
Left: F out of page
L1
Top: no force
B
Right: F into page
Bottom: no force
L2
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Conceptual example 19.2 - Continuation
Example continued:
The magnitudes of the nonzero forces are:
F = ILB sin q
= (1.0 A )(0.20 m )(2.5 T )sin 90°
= 0.50 N
(b) What is the net force on the loop?
Fnet = 0
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Announcements
1. This week we will have the first workshop (Workshop #1)
2. Read textbook materials, pp. 707-717.
3. Sections
19.3 Charged particle moving perpendicularly to a
uniform magnetic field pp. 717-718, 720-721
19.6 Magnetic force on a current-carrying wire
pp. 727-728
Conceptual examples 19.2
3. Homework assignment HW #1 is due on next week’s lab sessions.
This is Problems 1-8 on pages 745-746 of the text (Please do not
look at multiple-choice questions, which are on the same).
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