Tuesday, March 23, 1999
Announcements: Mean Score on the exam:  64.7

Lecture notes:
Summary for the rest of the semester:
1.) Effect of Magnetic Field (B) on moving charges. (Chapter 29)
2.) Where does a magnetic field come from? (Chapter 30)
3.) What are the effects if B changes with time? (Chapter 31)
4.) Read Chapter 32 on your own.
5.) AC current (Chapter 33)
6.) What if charges are accelerated? (Chapter 34).

• If we bring two magnets close to each other, they will experience forces that act on each other. The forces can be either attractive or repulsive.

• An important difference between magnets and electric charges is that while there can be isolated charges, there cannot be isolated magnetic poles. Magnets always come in North and South poles.
• In our discussion of the forces between charges, it is convenient to introduce the concept of an electric field, E. Similarly, it is convenient to talk about a magnetic field, B, due to a magnet, or to several magnets or to moving charges.
• How does one "map" out the magnetic field of a magnet?
• It is sometimes helpful to use a 'test magnet' (i.e. a compass) or "induced" small magnets (iron filings)
• See figure 29-4 on page 704 of the text.
• Field lines form closed loops. By convention, they leave from the North pole of the magnet and enters the South pole of the magnet.
• Density of field lines scales with the strength of B; direction of B is indicated by direction of field lines.
• Other names of B
• Magnetic Field
• Magnetic Induction
• Magnetic Field Strength
• Magnetic Flux Density...... B (dot product) A = magnetic flux = F_B

• F_B a q,       F_B a v,.... if v = 0 then  F_B = 0
• Direction of F_B is such that it is perpendicular to v and to B; if v and B are parallel, then  F_B = 0.
• To sum up,  F_B = qv (Cross product) B
• This is comparable to F_e = qE
• It will be necessary to use the right hand rule!
• If q is negative, F_B is in the opposite direction.
 
 
• Right Hand Rule
• Remember that i (cross) j = k; j (cross) k = i; k (cross) i = j
• F = qv (cross product) B
• 
• Absolute value of F = qvB sin q
• Direction? Line up right hand palm with v, bend fingers along direction of B; Direction of F given by thumb.
• Units of B= weber/m² = Tesla
• Another commonly used unit: 1 gauss = 10^-4 Tesla
• See problem 29-7 from page 722 of the text.