That Natural Attraction
Inductance and Magnetic Storage
A Learning Summary
• A circular aperture of diameter d

sin   1.22 (1st minimum)
d
• Capacitors store charge, thereby storing electric field
and maintaining a potential difference
• Capacitors can be used to store binary info
• Capacitance is found in many different aspects of
integrated circuits: memory (where it’s desirable),
interconnects (where it slows stuff down), and
transistors (ditto)
Magnetic Fields
• Magnetic fields are created two ways:
– By moving charges (currents)
– Intrinsic property of elementary particles
• In most matter, the intrinsic magnetic fields
of nuclei and electrons each cancel
• In ferromagnetic materials, the intrinsic
magnetic fields of electrons can be aligned
and add
Storing data magnetically
• The electron spins (intrinsic magnetic fields) in a
ferromagnetic material are aligned to give a net
magnetization
• The smallest region with magnetization is called a
“domain”
• If data is digital and binary, two domains are used to
store a bit of storage
• If data is analog, magnetic field varies continuously
in proportion to the data
Representing data magnetically
• Two domains are used to store a bit of
storage
• Magnetizations in the two domains with the
same direction represents a 0
• Magnetizations in the two domains with
opposite directions represents a 1
• The direction of magnetization changes at
the start of a new bit
Examples of magnetic data
N S N S
S N N S
S N
S N S N
N S S N
Domains
Bits representing 0
Bits representing 1
N S
N S N S S N S N N S N S S N S N N S N S S N S N
A string of 0s
A 1 followed by a string of 0s
Examples of magnetic data
N S N S
N S N S S N S N N S N S S N S N
A string of 0s
S N S N
Bits representing 0
S N N S S N N S S N N S S N N S
S N N S
A string of 1s
N S S N
Bits representing 1
N S S N N S S N N S S N N S S N
Examples of magnetic data
S N N S N SS N N S S N S N N S N S
0
1
0
0
N S S N N S S N N S S N N S
1
1
1
Examples of magnetic data
0
1
0
0
1
1
1
Writing magnetic data
• Ferromagnetic material becomes magnetized in
the presence of a magnetic field
• Currents create magnetic fields
• A loop of current creates a magnetic field passing
through the axis of the loop in a direction given by
the “right-hand rule”
• Outside the loop, the field has the opposite
direction, since it is circling back
Writing magnetic data
• Changing the direction of current in the loop
changes the direction of the magnetic field and so
magnetizes the ferromagnetic material in a
different direction
Reading magnetic data - induction
• Faraday (and Henry) discovered that changing
magnetic fields produce electric fields
• This electric field provides the emf needed to move
charges around a loop of wire (current!)
• They also found that changing the area of the loop
in the electric field induces a current
• Using a larger loop, or a coil of multiple loops,
resulted in a larger current than a smaller loop
Faraday’s Law
• The conclusion:
dF B
  iR  
dt
• FB is the magnetic flux, given by
F B   B  dA  BA cos
• A is the area of the loop with B through it
• second equality holds only in simple cases
Faraday’s Law and you
• A changing magnetic field induces emf in a coil of
wires proportional to
–
–
–
–
The number of turns in the coil
The area of the coil
The angle between the coil’s axis and the field
The rate of change of the field
• Moving a magnetized ferromagnetic material past a
coil of wire will induce a current if the
magnetization changes
• Measuring this current provides info on field
Activity
• Work through today’s activity
What have we learned?
• A piece of ferromagnetic material in a magnetic
field retains the magnetization of the field even
after leaving the field
• Currents create magnetic fields proportional to
current
• Changing the direction of current changes
direction of magnetic field
• Magnetic data is written this way
What else have we learned?
• Magnetic storage uses two domains for each
bit of data: parallel domains represent 0,
antiparallel (opposite) domains represent 1
• The first domain of a new bit will have
magnetization opposite from second domain
of prior bit
• This convention allows errors to be caught
What else have we learned?
• A changing magnetic field induces a current in a
loop of wire:
iR = - A(dB/dt) cos 
• A magnetized material moving past a loop of wire
provides such a changing magnetic field
• If current is induced as bit passes, bit is 1; if no
current induced, bit is 0