Major Concepts of Physics PHY102
Lecture #2
Magnetic fields and forces
January 25th, 2016
Prof. Liviu Movileanu
http://movileanulab.syr.edu/MajorConceptsPhysics2016.html
lmovilea@syr.edu
Room 211, Physics Bldg., 443-8078
Major Concepts of Physics PHY102 – Lecture #2
2016Syracuse University
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Lecture objectives
• Magnetic Fields
• Magnetic Force on a Point Charge
• Motion of a Charged Particle in a Magnetic
Field
• Lecture demonstrations
• Conceptual Examples
• Planning/Announcements/Assignments
Major Concepts of Physics PHY102 – Lecture #2
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§19.1 Magnetic Fields
 All magnets have at
 Magnetic Dipole least one north pole and
one south pole.
 Field lines emerge from north poles
and enter through south poles.
Major Concepts of Physics PHY102 – Lecture #2
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Field lines inside and outside the
magnet
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Magnetic forces can be both attractive and repulsive
 Magnets exert forces on one another.
 Opposite magnetic poles attract and like magnetic poles repel.
 Magnetic field lines are closed loops. There is no (known!) source of magnetic
field lines. (No magnetic monopoles)
 If a magnet is broken in half, then you just end up with two magnets.
Major Concepts of Physics PHY102 – Lecture #2
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Types of waves
 Near the surface of
the Earth, the magnetic
field is that of a dipole.
 Note the orientation of the
magnetic poles!
 Away from the Earth, the magnetic
field is distorted by the solar wind.
Evidence for magnetic pole reversals has been found on the ocean floor. The
iron bearing minerals in the rock contain a record of the Earth’s magnetic field.
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Magnetotactic bacteria
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2016Syracuse University
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§19.2 Magnetic Force on a Point
Charge
The magnitude of FB is:
FB  qBv sin  
where vsin is the component of the velocity perpendicular to the direction of the
magnetic field.  represents the angle between vectors v and B.
v

B
Draw the vectors tail-to-tail to
determine .
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Fig. 19.06
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What is vectorial or cross product?
For a general cross product:
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Ex. 19.01
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Conceptual example 19.1 - continuation
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Ex. 19.02
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Conceptual example 19.2 - Continuation
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Announcements
1. This week we will have the first workshop (Workshop #1),
titled “Electric Motor”
1. Read textbook materials, pp. 707-716.
2. Sections
19.1 Magnetic fields pp. 707-711
19.2 Magnetic force pp. 711-716
Conceptual examples 19.1, 19.2
1. Wednesday, January 27th, you will get your first HW assignments,
which will be due on the lab session on the next week.
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