That Natural Attraction

Inductance and Magnetic Storage

• A circular aperture of diameter 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)

A Learning Summary

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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 S N

Bits representing 1

S N N S

Domains

N S

S N

S N S NN S N S N S N S N S N SS N S N S N S N

Bits representing 0

N S N S

S N S N

A string of 0s

A 1 followed by a string of 0s

Examples of magnetic data

Bits representing 0

N S N S

S N S N

A string of 0s

S N S NN S N S N S N SS N S N

N S S N

Bits representing 1

S N N S

N S S NN S S NN S S NN S S N

S N N SS N N S S N N SS N N S

A string of 1s

Examples of magnetic data

N S N S

N S S N

N S N S

N S S N N S S N

S N S N S NS N N S

N S

0 0 01

1 1 1

Examples of magnetic data

0 0 01 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:

• B is the magnetic flux, given by

• A is the area of the loop with B through it• second equality holds only in simple cases

dtdiR B

cos BAdABB

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