Ph272- General Physics II (Electricity and
Magnetsism)
Lecture 13: Ch. 28 - Magentic Field
Key Topics
Overview
- Magnets can attract or repel each other => (at least two
poles)
- Magnet can also exert a force on a moving charge
Magnets
- Magnetic materials: Iron, Nickel (more details in
Ch. 27)
- Electromagnet: Generate a magnet using electrical
current. (Ch. 25)
- No magnetic monopole (so far, none is found): Every
magnet has two poles. If you cut a magnet in two halves; each
piece has its own two poles. (Microscopically, you can imagine
that a big magnet made up of many "tiny" magnets stacking
on top of each other. These tiny magnets are infinitely small
and cannot be cut into two pieces.) This feature of magnet differs
from electric charges. We say that there is no magnetic charges
(or magnetic monopoles). You will see later that those "tiny"
magnets are really "moving" charges.
- Magneic dipole: A tiny magnet with a north and south
pole is a magnetic dipole.
Magnetic field lines
- Field lines convention: Magnetic field lines originates
from north poles and ends at south pole (outside the magnet),
reverse is true for inside the magnet. That is, magetic field
lines always form a closed loop (unlike those of a point electric
charge).
- Meaning of field line: Magneic dipoles lines up along
magnetic field lines similar to electric dipoles line up along
electric field lines.
- Note: Since there is no magnetic charges, the magnetic
field lines do not indicate the direction of force on a magnetic
charge.
Force on a moving charge.
- Experimental fact: A magnet exerts no force on a stationary
charge but it does exert a force on a moving charge.
- Force law: Given by Eq. 24-1.
- Mathematical technique: cross product and right-hand-rule.
Review
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