AP Physics B Chapter 20 Magnetism 20-1

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Transcript AP Physics B Chapter 20 Magnetism 20-1

AP Physics B Chapter 20
Magnetism 20-1
•All magnets have 2 poles called north and
•Materials that show strong magnetic effects
are said to be ferromagnetic.
•A magnetic field surrounds any magnet or
•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
• 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
20-2 Electric Currents that Produce
• 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
• 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
• See pg 594 and pg 595 for conventions of
representing 3d fields in 2d.
• See Examples 20-3 and 20-4 p595 and
• P616 1-12 due BOP Monday