Ch. 24 Magnetic Fields

J. A. Gonzalez SEM Pre
-
AP Physics Notes Page No.
1

Ch.

2
4

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Magnetic Fields



2
4
.1
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Magnets: Permanent and Temporary


General Properties of Magnets




Magnets have two sides called poles, one north
-
seeking (north pole) and one south
-
seeking
(south pole).


Polarized
: The quality of having two opposite magne
tic poles, one north
-
seeking and one south
-
seeking.




Experimentally, it can be observed that



Like poles repel each other



Unlike poles attract each other




Magnets always have two opposite magnetic poles.


Magnetism


Magnetic Force
: Force associated with th
e motion of electric charges.


Magnetic Field
: Disturbance in the region of space surrounding objects in which electric charges are
moving. It is a vector quantity that relates the force exerted on a magnet or a current
-
carrying wire to
the strength of t
he magnet or the size of the current.


Magnetic Flux
: The number of magnetic field lines passing through a given surface area.


First Right
-
Hand Rule
: Method used to determine the direction of the magnetic field around a
current carrying wire.


Magnetic
field near a coil


Solenoid
: A coil of wire with many loops that acts like a magnet when a current is allowed to flow
through it.


Electromagnet
: A current
-
carrying coil with a north and a south pole that is iteslf a magnet.


Second Right
-
Hand Rule
: Met
hod used to determine the direction of the field produced by an
electromagnet.


Ferromagnetic Materials




When you place a piece of iron, nickel or cobalt near a permanent magnet, they become
magnetic.


Ferromagnetism
: The property of a substance by which
it is strongly attracted by a magnet. [It
involves electron spin and alignment of magnetic moments in domains.]

J. A. Gonzalez SEM Pre
-
AP Physics Notes Page No.
2

Magnetic Domains




Each electron in an atom acts like a tiny electromagnet.




The magnetic fields of the electrons in a group of neighboring ato
ms combine together to form
“domains.”


Domain
: A small group, usually 10 to 1000 microns, that is formed when the magnetic fields of the
electrons in a group of neighboring atoms combine together.




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4
.2

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Forces Caused by Magnetic Fields

.

Forces on
Currents in Magnetic Fields




If you move a current through a wire in a magnetic field, the wire will experience an external
force.


Third Right
-
Hand Rule
: Method used to determine the direction of the force on a current
-
carrying
wire in a magnetic field.


Force on a Current Carrying Wire in a Magnetic Field
:
F = BIL


Galvanometer
: Device used to measure very small currents.


Electric Motor
: Device that converts electrical energy into kinetic energy.


Forces on a Single Charged Particle


Cathode Ray Tube
:

Electron tube that converts electrical signals into a pattern on a screen. It
consists of an electron gun which produces an electron beam that is focused onto and moved across a
luminescent screen by magnetic and/or electric fields.




It forms the basis
of the oscilloscope and TV/computer monitors.