Download Magnetism Part I

Magnetism
2/27/2017
Magnetism
https://twistedsifter.files.wordpress.com/2012/05/visualizing-magnetic-fields-with-iron-filings-2.jpg?w=800&h=600
Magnets

Are dipoles as opposed to monopoles
+ 


+
Referred to as “North” and “South” pole
Like poles repel; unlike poles attract
Have a potential field that extends into space

Local field is characterized by
Magnitude
 Direction
 Depicted as a vector

Familiar Magnets
 Permanent
magnets
 Electromagnets
http://science.howstuffworks.com/electromagnet3.htm
Familiar Magnets
Similarities between gravity
and magnetic methods
 Both
based on potential fields
 Similar math
 Similar acquisition, reduction and
interpretation strategies
http://www.gemsys.ca/site-characterization-for-using-overhauser-magnetometer/
Differences between gravity
and magnetic methods


Magnetic properties of
rocks vary a lot and
aren’t tied as closely to
rock type
Magnetic fields can be
attractive or repulsive
(imagine if gravity were repulsive!)

Magnetic field
fluctuates a lot on all
time scales
http://www.solarsystemcentral.com/solar_storm_page.html
Deriving the force field around
a magnet
 Start
with the field around a magnetic
monopole (which don’t actually exist)
 Add together fields for a positive
monopole and a negative monopole
 Result is the dipole field
Monopole Field
𝐹𝑚
=
1 𝑝1 𝑝2
𝜇 𝑟2
− force applied by one monopole
on another
 𝑝1 , 𝑝2 - “value” of monopoles
 𝑟 - distance between monpoles
 𝜇 – magnetic permeability
 𝐹𝑚
Monopole Field
𝐹𝑚
=
 What
1 𝑝1 𝑝2
𝜇 𝑟2
would 𝑝2 ‘feel’ in the field
produced by 𝑝1 =1 if 𝑝2 = 0?
 What is shown is the field detected
by 𝑝2 as it is moved around 𝑝1
Monopole Field
Dipole Field
Dipole field



Typically depicted
with field lines
Spacing of field lines
depicts intensity
Strength is referred to
as magnetic
moment =

the torque the
magnet will
experience in an
external magnetic
field.
Units
 𝐹𝑚
=
1 𝑝1 𝑝2
𝜇 𝑟2
- Newtons (N) or
kg.m/s2
 𝜇 – has no units
 𝑟 – meters (m)
 𝑝1 - ampere x m
 𝐹𝑚
 Ampere
– a unit of
electrical current

Amp = Coulomb/s
 Coulomb
– a unit
of electrical
charge
Quick Aside:
Relationship between electricity and magnetism
 Changing
electric fields
produce magnetic fields
and vice versa


Flowing current (e.g.
electromagnet)
Orbiting & spinning electron
Units
 𝐹𝑚
=
1 𝑝1 𝑝2
𝜇 𝑟2
- Newtons (N) or
kg.m/s2
 𝜇 – has no units
 𝑟 – meters (m)
 𝑝1 - ampere x m



 𝐹𝑚



How do we characterize
𝐹𝑚 in a useful way?
𝐹𝑚
𝑝2
=
1 𝑝1
𝜇 𝑟2
=H
allows us to think about
the field around 𝑝1
without worrying about
𝑝2
H units of N/(Amp x m)
1 Tesla = 1 N/(Amp x m)
For earth:


nanoTesla (nT)
1 nT = 1 gamma