Lecture_18_(Chap.21)

# Lecture_18_(Chap.21) - Exam 2 Wednesday March 30 8-9:30...

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Exam 2 Wednesday, March 30, 8-9:30, PHYS 112. Chap. 18-21 mphasis on conceptual problems Emphasis on conceptual problems. Practice exam on course home page. 1

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Chapter 21 Magnetic Force 2
Magnetic Field of a Moving Charge The Biot-Savart law for a 0 ˆ r v q B moving charge 2 4 r The Biot-Savart law for a ˆ short piece of wire: 2 0 4 r r l I B 3 How magnetic field affects other charges?

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Magnetic Force on a Moving Charge TV and CRT monitors: Direction of the magnetic force depends on: the direction of B the direction of v of the moving charge the sign of the moving charge arge of the particle B v q F magnetic q – charge of the particle v – speed of the particle B – magnetic field 4 m A N m/s C N T
Right Hand Rule for Magnetic Force B v q F magnetic Electron charge = -e: 5 The magnetic force on a moving electron is in opposite direction to the direction of the cross product vB

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Effect of B on the Speed of the Charge B v q F magnetic What is the effect on the magnitude of speed? 0 l d F Kinetic energy does not change Magnetic field cannot change a particle’s energy! agnetic field cannot change a particle’s speed! Magnetic field cannot change a particle s speed! Magnetic force can only change the direction of velocity but 6 not its magnitude
Magnitude of the Magnetic Force B v q F magnetic Move charged tape close to a magnet: can you detect the magnetic force? Single electron in television tube: n vB p d n q v d <<c) sin qvB dt sin vB m dt (v c) e/m e = 1.78 . 10 11 C/kg q/m for charged tape: (10 -8 C)/(10 -4 kg) 10 -4 C/kg 7

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Exercise Net charge on invisible tape is ~ 10 -8 C Magnetic field near your bar magnet is ~0.1 T What is the force on tape if the tape is moved in the direction perpendicular to the field with a speed of 10 m/s ?
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Lecture_18_(Chap.21) - Exam 2 Wednesday March 30 8-9:30...

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