AP Physics B Chapter 20
Magnetism 20-1
•All magnets have 2 poles called north and
south.
•Materials that show strong magnetic effects
are said to be ferromagnetic.
•A magnetic field surrounds any magnet or
conductor.
•Magnetic field lines point the same way a
north magnetic compass would point at that
location. (N to S) See pg. 589 diagrams.
20-1 Magnets and Magnetism
(cont’d)
• We difine the magnetic field, at any point
as a vector, represented by the symbol B,
pointing to the south pole.
• The angular difference between true north
and magnetic north, on Earth, is called
magnetic declination.
• The angle of dip is the result of the
magnetic field not being tangent at all
points.
20-2 Electric Currents that Produce
Magnetism
• Hans Christian Oersted found that when a
compass needle is placed near an electric
wire, it shows a magnetic field when
current is flowing.
• We use the right-hand-rule to determine
the direction of current or magnetic field.
• See pg 591 for diagrams.
20-3 Force on an Electric Current
in a Magnetic Field; Definition of B
• Magnets exert a force on a current carrying
conductor.
• Use another right hand rule for the force with
moving electrons.
• 1. Orient your rt. hand to point fingers with the
conventional current. (+)
• 2. …from this position, bend your fingers so they
point in the direction of the field-lines.
• 3. The extended thumb is the direction of the
force on a wire.
20-3 Force on an Electric Current
in a Magnetic Field; Definition of B
• F=IlBsinq
• If the current is perpendicular to the field q =90,
then sinq=1
• The SI unit for magnetic field is the tesla (T).
• Another common unit is the gauss (G)
• G=10-4 T.
• The earth’s magnetic field is about .5x10-4T.
• See Example 20-1 and Example 20-2 p593/p594.
20-4 Force on an Electric Charge
Moving in a Magnetic Field
• F=qvBsinq
• Obviously Fmax =qvB
• The force is zero if the particle moves
parallel, it is maximum when it moves
perpandicular.
• See pg 594 and pg 595 for conventions of
representing 3d fields in 2d.
• See Examples 20-3 and 20-4 p595 and
p596
• P616 1-12 due BOP Monday