Physics 7C Sections A, B, C & D

Professor Lawrence B. Coleman

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Lecture 8 - The Magnetic Field & Electromagnetic Waves

Physics 7C Spring 2014 - Lecture 8

Quote of the Week: “what light through yonder window breaks?”

Wm. Shakespeare, Romeo and Juliet Act 2, scene 2

v  Check Rubric Grades v  Last Call TODAY for Quiz 1

reconsiderations. v  Quiz #4 today 8:20AM & 9AM

v  Memorial Day holiday effects on DL meetings – check calendar and with your TA.

Physics 7C Spring 2014 - Lecture 8 2

Announcements

v  Final Exam is Thursday June 12th 6PM - 8PM v  No early, late or otherwise different exams v  Practice Problems and Questions to Ponder to be

posted v  TA’s Office Hours & Reviews Sessions TBA

Physics 7C Spring 2014 - Lecture 8 3

Announcements-Final Exam

v  Lecture 9 (next week) is last official lecture.

v  Lecture 10, on Thursday June 5th will be a special presentation from 8AM till 10:20AM of the applications of Physics 7 (A, B & C) in biology and the real world. Not officially on the final, but of course it is physics. Come if you like.

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Announcements

Physics 7C Spring 2014 - Lecture 8 5

Putting Magnetic Fields to Use

v  Magnetic Fields are created by iron and other elemental metals and compounds.

v  In addition, magnetic fields are also created by electric currents!

Physics 7C Spring 2014 - Lecture 8 6

The Magnetic Field

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The Magnetic Field

Physics 7C Spring 2014 - Lecture 8 8

The Magnetic Field

v  Strength of field drops off as 1/r

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Magnetic Fields From Current in Wire

B(r) = µoI2πr

µo = 4π ×10−7T •m / A

Physics 7C Spring 2014 - Lecture 8 10

Magnetic Field - Loop

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Magnetic Field- Many Loops a.k.a. Solenoid

Magnitude of field in center ~ I, N & 1/R

v  Lots of loops v  Lots of current v  MRI Magnet

v  Superconducting wire v  8000 loops v  100 amps v  1 Telsa = 2 x 104

earth’s field

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Electromagnet

v  Electric currents create magnetic fields.

v  What does a magnetic field do to an electric current??

v  Let’s start with what a magnetic field does to a charge.

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Magnetism & Electricity

v  If the charge is stationary…

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Magnetism & Electricity

v  If the charge is moving parallel to the B field.

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Magnetism & Electricity

v  If the charge is moving perpendicular to the B field.

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Magnetism & Electricity

Lorentz Force

v  If the charge is moving perpendicular to the B field.

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Magnetism & Electricity

Lorentz Force

FLorentz = q i v i B i sinθ

v  Three ways, pick the one that works the best for YOU.

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Which Way is Force

v  Three ways, pick the one that works the best for YOU.

v  The DL TA method -- Very Bad Finger

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Right Hand Rule for Lorentz Force

v The Flat Palm method

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Right Hand Rule for Lorentz Force

v The curl method: F= qv x B

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Right Hand Rule for Lorentz Force

v What happens if the charge is negative?

v Just flip the force, 180°

Physics 7C Spring 2014 - Lecture 8 22

Right Hand Rule for Lorentz Force

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Magnetic Force on a Moving Charge Special Case when v normal to B

v  What about the charges moving in a wire.

v  They create a magnetic field, but what if the wire is in a magnetic field?

v  Look at two parallel wires that carry current. Look at one creating the B-field and the force that field puts on the moving charges of the other.

Physics 7C Spring 2014 - Lecture 8 24

Magnetic Force on a Moving Charge

Physics 7C Spring 2014 - Lecture 8 25

Magnetic Force on a Moving Charge