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MAGNETISM
•Normally denoted by B
•Magnetic forces affect moving charged particles*
•Forces are perpendicular to both magnetic field and direction
of motion
*Or objects made of charged particles
Magnetic Field B
velocity v
charge q
Force F

F  q vB

Magnetic units
•Magnetic field units: Ns/Cm
•Called a Tesla (T)
•A Tesla is a big magnetic field
•MRI magnet -> 1-4 Tesla
N s
N
1 T 1
1
Cm
Am
Electric Field Lines
•Graphical Illustration of Electrical Fields
•Lines start on positive charges and end on negative
•Number of lines from/to a charge is proportional to that charge
•Density of lines tells strength of field.
+
-
+
-
Magnetic Field Lines
•Graphical Illustration of Magnetic Fields
•Lines start on north pole and end on south pole
•Opposite poles attract, like poles reply
•Density of lines tells strength of field.
•Key differences between magnetic fields and electric fields:
•All magnets have a north and south pole! No such thing as
an isolated north or south pole. (have magnetic dipoles, not
monopoles)
•Different force laws!
Vector Product
Vector product -> two vectors make a vector
  
A B  C
Geometric
C has magnitude ABsinq. Direction perpendicular
to the plane containing A and B.
Algebraic
 
A  B  (a y bz  by az )i  (az bx  bz ax ) j  (axby  bx a y )k
The right hand rule
Force F
velocity v
Magnetic
Field B

F  q vB

F  qvB sin q
Magnetic Force
•Can only affect moving particles!
•Force depends on charge just like electric fields
•Force is maximum when the velocity and field are
perpendicular, and zero when they are parallel
•When the velocity and field are neither perpendicular nor
parallel, the force still exists!

F  q vB

3-d directions of vectors
into the plane
velocity v
out of the plane
The two vectors shown represent the velocity and the
Magnetic
magnetic field. For a negatively charged particle, which Field B
way will the particle accelerate?
A)
C)
B)
D)