Transcript Magnetism_ppt_RevW10

Physics 106 Lesson #19
Magnetism
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review: Engineering
Trade Studies
• When a number of possible engineering
solutions exist to meet a given need or
goal, engineers perform a “trade study”.
• This is a useful approach whenever a
choice is to be made between a number
of possible options.
• The benefits, disadvantages and costs
are quantified for each option and
decisions are then made by “trading off”
advantages against disadvantages for
the various options based on a goal
such as minimum cost, safety, or
maximum reliability.
Magnets
• All magnets have two
poles: North and
South
• Like poles repel each
other; Unlike poles
attract each other
• Magnetic poles are
always found in pairs
• Isolated magnetic
poles have never
been found
Magnetic Forces
Electricity and Magnetism
• Originally thought to be
independent phenomena
• Like gravity, both have actionat-a-distance force laws
• Faraday and others believed
in the unity of nature
• Maxwell proved that they were
both aspects of a single
phenomenon:
Michael Faraday
Electromagnetism
James Clerk Maxwell
The Magnetic Field
The magnetic field is a
vector and denoted by
the letter B
B
• The north pole of a
compass needle is a
magnetic north pole
• The direction of the
magnetic field at
any point in space is
the direction
indicated by the
north pole of a small
compass needle
(test magnet) placed
at that point
Magnetic Field Lines
• Magnetic field lines
surround magnets
• The magnetic field is
always tangent to the
magnetic field line
• The number of lines per
unit area (density) is
proportional to the
magnetic field strength
• Outside of the magnet,
magnetic field lines point
away from north poles
and toward south poles
B
Iron Filings: Visualizing
Magnetic Field Lines
The magnetic field lines inside a bar
magnet run from south pole to north pole.
The Earth’s Magnetic Field
• The Earth's magnetic
field is similar to that of
a bar magnet tilted 11
degrees from the spin
axis of the Earth
• Caused by the “dynamo
effect”, the rotation of a
partially ionized outer
core → internal electric
current!
• The north pole of a
compass needle is
attracted to the
geographic North Pole,
which is therefore a
magnetic south pole
South magnetic pole
Magnetic Field Produced
by a Long, Straight Wire
Demonstration: Oersted Experiment
Field Direction for a Straight Wire
• The magnetic field
due to the current
in a long straight
wire has circular
field lines around
the wire
• The direction of the
field is given by a
right hand rule
Electromagnets
• Current flowing in a loop of wire creates a
magnetic field
• The Current loop can be visualized as a
“phantom bar magnet”
N
S
=
N
S
Which End is the North Pole?
• Another Right Hand Rule!
N
S
The Field of a Solenoid
Current Into Page
B
Current Out of Page
Inside a solenoid
(away from the ends)
the magnetic field is
constant.