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Ampere’s Law Physics 102 Professor Lee Carkner Lecture 18

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Currents and Magnetism It is also true that moving charged particles produce magnetic fields Serious magnetic fields are produced by currents What is the magnitude and direction of these fields?

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Magnetic Field from a Current in a Wire Needle deflected tangentially to the wire cross section How can we find the direction and magnitude of the B field for any situation?

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Right Hand Rule Revisited Grasp the wire with your thumb in the direction of the current and your curled fingers indicate the direction of the field

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Ampere’s Law To find the magnitude of the B field we use Ampere’s law The sum of the product of L and B around the entire path is equal to 0 I Where 0 = 4 X 10 -7 T m /A and is called the permeability of free space B L = 0 I

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B Field for a Wire B L = 0 I or B L = 0 I Since B is the same everywhere around the circle B 2 r = 0 I B = 0 I/2 r Magnetic field a distance r from a long straight wire with current I

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Force on Two Parallel Wires The B field then will exert a force on the other wire B = 0 I/2 d For two wires of equal length but different currents: F = 0 I 1 I 2 L/2 d

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Magnetic Field: Loop Can apply the right hand rule all the way around Loop acts as a bar magnet

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Magnetic Field: Solenoid What happens if you stack several loops up? You produce a solenoid Field inside the solenoid is strong and uniform (far from the ends)

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Electromagnet We can write an expression for the solenoid magnetic field: B = 0 nI If you put a piece of iron in the center you get an electromagnet

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Next Time Read: 21.1-21.2 Homework: CH 21 P: 1, 2 CH 20 P: 38, 50

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