Lecture 11.2 : Magnetic Force

Transcription

1 Lecture 11.2 : Magnetic Force Lecture Outline: The Hall Effect Forces on Current-Carrying Wires Forces on Current Loops Magnetic Properties of Matter Textbook Reading: Ch March 28,

2 Announcements Homework #9 due on Monday, April 1 in Mastering Physics. Exam #3 is Thursday. Covers Ch Remember your calculator and one-sheet of notes. Review session for Exam #3 will be offered Tue (April 2), from 7-9pm in Stolkin. NOTE: If you are in the Tue. lab section (from 6:30-8:30pm), you can go to another lab section this week, or you can join the review session late. Apologies for this overlap (again). 2

3 Last Lecture... Evaluating integral of B ds around closed loop. Independent of the shape of the curve around the current. Independent of where the current passes through curve. Depends only on the total current passing through the area enclosed by the integration path. Ampere s Law Note that the integral here is around a closed loop, and not over a closed surface like in Gauss s Law. 3

4 Last Lecture... Magnetic Force also depends on how moving charge s velocity is oriented relative to a magnetic field. 4

5 Last Lecture... We discussed magnetic field of solenoids... MRI + Magnetic Material = Bad 5

6 The Hall Effect We know charges moving through a magnetic field experience a force. Consider charges moving through a conductor located in a magnetic field: 6

7 The Hall Effect Sign of Charge carrier matters here. Hall voltage is established across conductor. Hall probes can measure magnetic field strengths. How else might we measure magnetic fields? 7

8 Forces on Current-Carrying Wires F on q = qv B q = I t = I l v F wire = I l B F wire = IlB sin α Direction from right-hand rule. 8

9 Forces on Current-Carrying Wires 9

10 Clicker Question #1 The horizontal wire can be levitated held up against the force of gravity if the current in the wire is A. Right to left. B. Left to right. C. It can t be done with this magnetic field. 10

11 Forces on Current-Carrying Wires Example: What magnetic field strength and direction will levitate the wire? 11

12 Forces on Current-Carrying Wires 12

13 Forces on Current Loops Recall the current loop and its magnetic field. 13

14 Forces on Current Loops Two neighboring current loops will attract/repel each other depending on the orientation of their current. 14

15 Clicker Question #2 If released from rest, the current loop will A. Move upward. B. Move downward. C. Rotate clockwise. D. Rotate counterclockwise. E. Do something not listed here. Net torque but no net force 15

16 Forces on Current Loops A current loop placed in an external magnetic field will rotate. 16

17 Forces on Current Loops This is the basis for a simple electric motor. 17

18 Magnetic Properties of Matter We now understand how moving charges create magnetic fields, but what about materials that seem to have inherent magnetism? Atoms, like hydrogen, can have a magnetic moment due to the orbiting electron current. Electrons have their own intrinsic magnetic moment! Unpaired electrons in an atom contribute to the atoms magnetic moment. 18

19 Magnetic Properties of Matter Even though atoms may have a magnetic moment, the atoms in a material can be randomly arranged, canceling each other out. 19 Ferromagnets are materials where the atomic magnetic moments line up.

20 Magnetic Properties of Matter Magnetic domains in ferromagnets can be altered by external field. Moving domain walls in a grain of silicon steel caused by an increasing external magnetic field in the "downward" direction. White areas are domains with magnetization directed up, dark areas are domains with magnetization directed down. - Source: Wikipedia 20

21 Magnetic Properties of Matter A slice of ferromanganese crust from the Pacific Ocean was analyzed using a SQUID microscope at Vanderbilt University. The top image shows a portion of the image of the slice taken with an electron microprobe. The second image shows the magnetized regions in the slice, with red areas showing one direction of magnetization and blue the opposite direction. 21

22 Magnetic Properties of Matter Probably all of you are carrying an example of a magnetic material where the arrangement of domains matters. 22

23 Reminders Homework #9 is due Monday (April 1). Exam #3 next week (April 4), and a review session on April 2 from 7-9 pm in Stolkin. 23