Inductance, Magnetoresistance, and You

1. Inductance, Magnetoresistance, and You Different approaches to magnetic storage

2. + -q +q V Capacitance • The battery provides the work needed to move the charges around and increase their potential energy • The ratio of charge stored to potential difference maintained is the capacitance of the object V • q is the charge on one plate

3. What determines capacitance? • Capacitance depends solely on the geometry of the capacitor and the medium between the plates • Larger plates can collect more charge at the same potential difference than smaller ones • For the same charge, the potential difference between plates will increase with separation between them • For an empty, parallel plate capacitor C=0 A/d

4. Combining capacitors • Capacitors in parallel • Capacitors in series

5. Electrical Storage • Capacitors are used in RAM: • A charged capacitor is a 1 • An uncharged capacitor is a 0 • Reading a capacitor discharges it, so you must continually re-write when reading • Capacitors lose charge over time (charge will slowly leak into air or other insulator), so capacitors are not good for long-term storage

6. A Learning Summary • 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)

7. Magnetic Fields • Magnetic fields are created two ways: • By moving charges (currents) • Intrinsic property of elementary particles • In most materials, the intrinsic magnetic fields of nuclei and electrons each cancel • In ferromagnetic materials, the intrinsic magnetic fields of electrons can be aligned and added up • Magnetic field lines point from “North” to “South”

8. Storing data magnetically • The electron spins (intrinsic magnetic fields) in a ferromagnetic material are aligned to give a net magnetization • The smallest region with same alignment 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

9. 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

10. N S N S S N N S N S S N S N S N N S S N Domains Bits representing 0 Bits representing 1 N S N S S N S N N S N S S N S N N S N S S N S N Examples of magnetic data A string of 0s A 1 followed by a string of 0s

11. N S N S S N S N N S N S S N S N S N N S N S N S N S S N S N S N Bits representing 0 Bits representing 1 S N N S S N N S S N N S S N N S N S S N N S S N N S S N N S S N Examples of magnetic data A string of 0s A string of 1s

12. S N N N S S N N S S S N S N S N N S N S N N N S S S S S S N N N Examples of magnetic data 0 1 0 0 1 1 1

13. Examples of magnetic data 0 1 0 0 1 1 1

14. 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

15. 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

16. 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

17. Faraday’s Law • The conclusion: • FB is the magnetic flux, given by • A is the area of the loop with B through it • second equality holds only in simple cases

18. 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

19. Magnetic Forces • Charges moving through a magnetic field experience a force (Fact #10) • This force is perpendicular to both the magnetic field and the direction of motion • If the charge is at rest, it experiences no magnetic force • If the charge moves parallel (or antiparallel) to magnetic field, it experiences no magnetic force

20. Magnetic Forces • Mathematically, FB = qv x B |FB| = |qv| |B| sin q ( q is angle between v and B) direction given by right-hand rule

21. Magnetoresistance • Electrons moving through a current-carrying wire are moving charges • If a magnetic field is present in the wire (not in the direction of current flow), the conduction electrons will experience a magnetic force perpendicular to direction of current • This force pushes electrons off track, increasing resistance

22. Conduction electrons Magnetic field pointing into page (screen) Direction of velocity v of electrons Direction of qv of (negative) electrons Current-Carrying Wire

23. Direction of force on conduction electrons Magnetic field pointing into page (screen) Direction of velocity v of electrons Direction of qv of (negative) electrons Current-Carrying Wire

24. So where’s the application? • The presence of a magnetic field increases the resistance of a wire • If a potential difference is applied to the wire, current will flow inversely proportional to resistance (i=V/R) • A change in magnetic field produces a change in current which can be measured • This yields a sensitive indicator of change in magnetic field

25. Comparison between Magnetoresistance and Induction • Magnetoresistance is a much larger effect than induction • Magnetoresistance detects magnetic field, not just the change in magnetic field, so it is less sensitive to changes in tape/disk speed and other variables • Equipment needed to detect magnetoresistance simpler than coils for inductance • Magnetoresistance replaced induction several years ago

26. 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

27. 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

28. What else have we learned? • A changing magnetic field induces a current in a loop of wire: iR = - A(dB/dt) cos q • 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

29. More Stuff to remember • A charge moving through a magnetic field experiences a force perpendicular to the field and the direction of motion of the charge • The magnetic force is proportional to the charge, the magnitude of the field, the velocity of the charge, and the sine of the angle between v and B • The effects of this force on charges in a current-carrying wire lead to effect of magnetoresistance

30. Before the next class, . . . • Start Homework 14 (due March 4) • Do Activity 12 Evaluation by Midnight tonight • Read Preface and Chapter 1 in Turton • Do Reading Quiz