SCS 139 Applied Physics II

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1 SCS 139 Applied Physics II Dr. Prapun Suksompong 1. Magnetic Forces and Fields 2. Electromagnetic Induction 3. Alternating Current 4. Electromagnetic Wave 1 Office Hours: Library (Rangsit) Mon 16:20-16:50 BKD Wed 9:20-11:20

2 About Me Ph.D. from Cornell University, USA In Electrical and Computer Engineering Minor: Mathematics (Probability Theory) Ph.D. Research: Neuro-Information Theory Modeling and analyzing neurons in human brain from communication engineering perspective. Current Research: Wireless Communication Mobile Communications, WiFi (802.11) 2009 SIIT Best Teaching Award 2011 SIIT Research Award i 2 prapun.com

3 i Course Web Site Please check the course website regularly. Announcements References Handouts (Posted before corresponding lectures) Annotated Notes/Slides (Posted after corresponding lectures) Assignments and Solutions 3 www2.siit.tu.ac.th/prapun/scs139/

4 Course Website: Notes & Slides Some PDF notes/slides will be posted before the corresponding lectures. Hard copies can be purchased from the copy center. In lectures PDF notes/slides will be highlighted and updated with examples / comments. The annotated pdf files will be posted after the corresponding lectures. Put all of your energy into understanding the material. Remind me the day after the lecture if the notes/slides from the day before are still not posted on the web. i 4

SCS 139 II.1 Magnetic Forces and Fields (I) Dr. Prapun Suksompong prapun@siit.tu.ac.

5 SCS 139 II.1 Magnetic Forces and Fields (I) Dr. Prapun Suksompong F qv B B F il B 5 Office Hours: Library (Rangsit) Mon 16:20-16:50 BKD Wed 9:20-11:20

Physics? 28-2 What Produces a Magnetic Field?

6 6 Reference Principles of Physics Ninth Edition, International Student Version David Halliday, Robert Resnick, and JearlWalker Chapter 28 Magnetic Fields 28-1 What Is Physics? 28-2 What Produces a Magnetic Field? 28-3 The Definition of B 28-6 A Circulating Charged Particle 28-8 Magnetic Force on a Current- Carrying Wire 28-9 Torque on a Current Loop

7 7 Magnet

8 What Produces a Magnetic Field? 8 Use moving electrically charged particles, such as a current in a wire, to make an electromagnet. Magnetic field is a basic characteristic of elementary particles (such as electrons) just as mass and electric charge (or lack of charge) are basic characteristics. These particles have an intrinsic magnetic field around them. Permanent magnet: The magnetic fields of the electrons in certain materials add together to give a net magnetic field around the material. In other materials, the magnetic fields of the electrons cancel out, giving no net magnetic field surrounding the material.

9 Applications You are surrounded by magnets. Old: Magnetic recording of music and images on audiotape and videotape. Magnets control CD and DVD players and computer hard drives. Magnets drive the speaker cones in headphones, TVs, computers, and telephones. A modern car comes equipped with dozens of magnets because they are required in the motors for engine ignition, automatic window control, sunroof control, and windshield wiper control. Most security alarm systems, doorbells, and automatic door latches employ magnets. 9

10 Application: Neodymium magnet toys NeoCube, BuckyBalls, CyberCube, EuroCube, MagCube 10

11 Magnetic stripe (magstripe) Credit, debit, and ATM cards: All of these cards have a magnetic strip on one side. The magstripe is made up of tiny iron-based magnetic particles (about 20 millionths of an inch long) in a plastic-like film. 11

12 Hard disk drives Hard disk drives record data on a thin magnetic coating 12

13 13 Magnetic resonance imaging

14 Collect and transport scrap metal Using an electromagnet to collect and transport scrap metal at a steel mill. 14

15 Magnetic Field Lines the field around an ordinary bar magnet 15

16 Viewing the field N N N S N N N N 16

17 Magnetic Field Viewers Magnetic field viewing film 17

18 ipad 2 - Smart Cover + Magnets Turn off the ipad 2's screen. 18 The rest are used to either clamp to the ipad on the right side (the far-right column of magnets), or to form the triangular shape used to create a stand for the ipad 2.

19 19 ipad 2 - Smart Cover + Magnets

20 20 Ferrofluid

21 Magnetic Field Lines Run from the north pole to the south pole. bar magnet horseshoe magnet C-shaped magnet 21

22 Magnetic Field (B) and Field Lines A vector quantity. We can represent magnetic fields with field lines. Rule 1: The direction of the tangent to a magnetic field line at any point gives the direction of Bat that point Rule 2: The spacing of the lines represents the magnitude of B The magnetic field is stronger where the lines are closer together, and conversely. 22

Nikola Tesla Earlier non-si unit: gauss 1

1 G = 100 T In magnetically shielded room

23 Unit for B B = magnitude of B SI Unit: tesla (T) Defined in 1960 in honour of Nikola Tesla Earlier non-si unit: gauss 1 T (tesla) = 10 4 G (gauss) Some approximate magnetic fields: Earth s magnetic field near the planet s surface: 1 G = 100 T In magnetically shielded room (MSR): T = 10 ft N N s T Cm/s Cm N C/s m Wb 2 m N A m 23

The earth s geographical north pole is actually its magnetic south pole.

24 Earth's Magnetic Field On Earth s surface, we can detect magnetic field with a compass, which is essentially a slender bar magnet. The earth s geographical north pole is actually its magnetic south pole. Magnetoception: Migratory birds and sea turtles can sense the earth s magnetic field, using it for navigation. 24

Work by detecting the tiny (femtotesla) magnetic

25 MEG Magnetoencephalography A powerful and noninvasive method for studying human brain activity. Work by detecting the tiny (femtotesla) magnetic fluctuations at the surface of the head that arise from the brain s electrical activity. 25

26 Uniform B Array of dots (which represent the tips of arrows) represents a field directed out of the plane. Array of Xs represents a field directed into that plane. 26

A magnetic field B is defined in terms of the force F B acting on a

27 Magnetic Forces and Fields Charged particle moving through a magnetic field experience magnetic force. A magnetic field B is defined in terms of the force F B acting on a test particle with charge q moving through the field with velocity v : The magnitude of F B : F qv B B F q vbsin B 27

28 Example A uniform magnetic field with magnitude 1.2 mt, is directed vertically upward throughout the volume of a laboratory chamber. A proton enters the chamber, moving horizontally from south to north with speed m/s. What magnetic deflecting force acts on the proton as it enters the chamber? (Neglect Earth s magnetic field.) 28

29 Direction of F B Right-hand rule: The thumb of the right hand points in the direction of v Bwhen the fingers sweep v into B. Perpendicular to the direction of v Perpendicular to the direction of B F qv B B 29

30 30 Exercise

31 A Charged Particle Circulating in B A charged particle with mass m and charge magnitude q moving with velocity v perpendicular to a uniform magnetic field B will travel in a circle. The magnetic force plays the role of centripetal force. 31

32 32 Uniform B

33 A Charged Particle Circulating in B The radius r of the circle is r mv q B The period T is given by T 2 r 2 m v q B 33

34 Example: Mass spectrometer An ion of mass m (to be measured) and charge q is produced in source S. The initially stationary ion is accelerated by the electric field due to a potential difference V. 34 The ion leaves S and enters a separator chamber in which a uniform magnetic field is perpendicular to the path of the ion. A wide detector lines the bottom wall of the chamber, and the B causes the ion to move in a semicircle and thus strike the detector.

35 Example: Mass spectrometer Suppose that B = mt, V = V, and ions of charge q = C strike the detector at a point that lies at x = m. What is the mass m of the individual ions? 35

36 Magnetic Force on a Current-Carrying Wire A straight wire carrying a current i in a uniform magnetic field experiences a sideways force F il B L is a length vector that has magnitude L and is directed along the wire segment in the direction of the (conventional) current. F B F ilbsin 36

SCS 139 Applied Physics II

SCS 139 Applied Physics II Dr. Prapun Suksompong prapun@siit.tu.ac.th www.prapun.com Office Hours: BKD 3601-7 Monday 9:20-10:20 Wednesday 9:20-10:20 1 i Course Web Site Please check the course website