1 / 13

Lecture 23: WED 11 MAR

Physics 2102 Jonathan Dowling. Lecture 23: WED 11 MAR. Ampere’s law. André Marie Ampère (1775 – 1836). Exam 02 and Midterm Grade. Exam 02 Average: 63/100. Q1&P1, Dr. Schafer, Office hours: MW 1:30-2:30 pm, 222B Nicholson Q2&P2, Dr. Gaarde, Office hours: TTh 2:30-3:30 pm, 215B Nicholson

nellis
Download Presentation

Lecture 23: WED 11 MAR

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Physics 2102 Jonathan Dowling Lecture 23: WED 11 MAR Ampere’s law André Marie Ampère (1775 – 1836)

  2. Exam 02 and Midterm Grade Exam 02 Average: 63/100 Q1&P1, Dr. Schafer, Office hours: MW 1:30-2:30 pm, 222B Nicholson Q2&P2, Dr. Gaarde, Office hours: TTh 2:30-3:30 pm, 215B Nicholson Q3&P3, Dr. Lee, Office hours: WF 2:30-3:30 pm, 451 Nicholson Q4&P4, Dr. Buth, Office hours: MF 2:30-3:30 pm, 222A Nicholson You can calculate your midterm letter grade, as posted on paws, via this handy formula used by all sections: [(MT01+MT02)*500/200+HWT/761]*30]/530*100 = Midterm Score Here MT01 and MT02 are your scores on the first and second midterms and HWT is your total homework points for HW01-07 only. Once you have this Midterm Score you then can get your letter grade via: A: 90-100 B: 80-89 C: 60-79 D: 50-59 F: below 50. Details on the grading policy are at: http://www.phys.lsu.edu/classes/spring2009/phys2102/ I have posted both midterm exam grades and your midterm class score on WebAssign.

  3. Magnetic field due to wire 1 where the wire 2 is, I1 L I2 Force on wire 2 due to this field, F a Forces Between Wires Neil’s Rule: Same Currents – Wires Attract! Opposite Currents – Wires Repel!

  4. Summary • Magnetic fields exert forces on moving charges: • The force is perpendicular to the field and the velocity. • A current loop is a magnetic dipole moment. • Uniform magnetic fields exert torques on dipole moments. • Electric currents produce magnetic fields: • To compute magnetic fields produced by currents, use Biot-Savart’s law for each element of current, and then integrate. • Straight currents produce circular magnetic field lines, with amplitudeB=m0i/2pr (use right hand rule for direction). • Circular currents produce a magnetic field at the center (given by another right hand rule) equal toB=m0iF/4pr • Wires currying currents produce forces on each other: Neil’s Rule: parallel currents attract, anti-parallel currents repel.

  5. q q Flux = 0! Remember Gauss Law for E-Fields? Given an arbitrary closed surface, the electric flux through it is proportional to the charge enclosed by the surface.

  6. New Gauss Law for B-Fields! No isolated magnetic poles! The magnetic flux through any closed “Gaussian surface” will be ZERO. This is one of the four “Maxwell’s equations”. There are no SINKS or SOURCES of B-Fields! What Goes IN Must Come OUT!

  7. i4 i3 i2 i1 ds Ampere’s law: Closed Loops The circulation of B (the integral of B scalar ds) along an imaginary closed loop is proportional to the net amount of current traversing the loop. Thumb rule for sign; ignore i4 If you have a lot of symmetry, knowing the circulation of B allows you to know B.

  8. Sample Problem • Two square conducting loops carry currents of 5.0 and 3.0 A as shown in Fig. 30-60. What’s the value of ∫B∙dsthrough each of the paths shown? Path 1: ∫B∙ds = •(–5.0A+3.0A) Path 2: ∫B∙ds = •(–5.0A–5.0A–3.0A)

  9. R Ampere’s Law: Example 1 • Infinitely long straight wire with current i. • Symmetry: magnetic field consists of circular loops centered around wire. • So: choose a circular loop C -- B is tangential to the loop everywhere! • Angle between B and ds = 0. (Go around loop in same direction as B field lines!) Much Easier Way to Get B-Field Around A Wire: No Cow-Culus.

  10. Ampere’s Law: Example 2 i Current out of page, circular field lines • Infinitely long cylindrical wire of finite radius R carries a total current i with uniform current density • Compute the magnetic field at a distance rfrom cylinder axisfor: • r < a (inside the wire) • r > a (outside the wire)

  11. Ampere’s Law: Example 2 (cont) Current out of page, field tangent to the closed amperian loop Need Current Density J! For r>R, ienc=i, so B=m0i/2pR = LONG WIRE! For r < R

  12. B O R r Ampere’s Law: Example 2 (cont)

  13. Solenoids The n = N/L is turns per unit length.

More Related