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Magnetic Fields and Forces
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
Learning Objectives:
 Investigate basic magnetic phenomena
 Develop a dipole model of magnetism
 Explore magnetic forces on moving charges
 Predict the motion of charged particles in
magnetic fields
 Discuss a simple atomic-level model of
ferromagnetism
© 2010 Pearson Education, Inc.
Experiments
 Observing to Find a Pattern
 Visualizing the Region Surrounding a
Magnet
 The Magnetic Field: Definition of the magnitude
of the Magnetic Field
B = F/qv sin θ
SI Units: 1 tesla = 1 T = 1 N / (A∙m)
OR
1 gauss = 1 G = 10-4 T
(Small magnetic fields are measured in the gauss G unit )
© 2010 Pearson Education, Inc.
Magnetic Fields Around Us
© 2010 Pearson Education, Inc.
Predict: What would happen if you placed the
compass over the wire instead of under the wire?
1. Construct a simple circuit. Again, place a compass under
one wire. Open and close the circuit several times.
Record your observation.
2. Now, place the compass over the wire. Open and
close the circuit several times. Record your observation.
3. Reverse the battery and repeat steps 1 and 2.
Suggest an explanation for these observations.
© 2010 Pearson Education, Inc.
Electric Currents Also Create Magnetic Fields
A long, straight
wire
A current loop
A solenoid
Recall the lab when you used a compass to detect
something happening in the wires of a circuit.
© 2010 Pearson Education, Inc.
The Magnetic Field of a Straight Current-Carrying
Wire
Magnetic field lines form circles around the wire!
© 2010 Pearson Education, Inc.
Direction of the Field in a Current-Carrying Wire
© 2010 Pearson Education, Inc.
Representing Vectors and Currents That Are
Perpendicular to the Page
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Drawing Field Vectors and Field Lines of a
Current-Carrying Wire
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The Magnitude of the Field Due to a Long,
Straight, Current-Carrying Wire
0  permeability constant  1.257  10 6 T m/A
© 2010 Pearson Education, Inc.
Summary
MAGNETIC FIELD LINES
 Field is stronger near poles, weakens with distance
 Spacing of lines indicates magnitude of vector B
 Field lines continue within the body of a magnet; always
form closed loops (so never cross)
 Direction of magnetic field vector at any point on the line is
tangent to the line
 Direction: leaves magnet at N pole and enters at S pole
© 2010 Pearson Education, Inc.
MAGNETISM
 Magnetism has no fundamental magnetic charge (as
electrical q does have)!
 Magnetic fields are created by currents in wires and
permanent magnets; these fields in turn exert forces
on currents in wires, permanent magnets!!
 Hans Christian Oersted (Danish, 1820) discovered the
connection between electricity and magnetism.
© 2010 Pearson Education, Inc.
Summary
© 2010 Pearson Education, Inc.