# Lecture_14_-_no_quiz_answers - Lecture 14 1/25 Midterm Exam...

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Lecture 14 1/25 Midterm Exam Thursday, July 7 th in class Closed book, closed notes Equation sheet will be provided Covers material from Chapter 21 Chapter 26 Covers homework from HW 1 HW 13 12 Questions, Multiple Choice

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Lecture 14 2/25 Coulomb’s Law Charges with the same sign repel each other Charges with opposite signs attract each other 2 , 1 2 2 , 1 2 1 2 , 1 ˆ r r q q k F Force by q 1 on q 2 + + 2 2 9 0 / 10 99 . 8 4 1 C m N k q 1 = charge of particle 1 q 2 = charge of particle 2 r 1,2 = distance between particles 1 and 2 Coulomb constant where ε 0 is the permittivity of free space
Lecture 14 3/25 Electric Field The electric field is independent of the test charge (q) and only depends on position in space r r Q k E ˆ 2 q F E Q P E at P due to a point charge (Q) r Electric field units: N/C

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Lecture 14 4/25 Electric Field and Force Electric field at point P only depends on Q E acting on charge q is produced by the other charges (external field) The charge is not affected by its own electric field What is the direction of the force on q ? If q is positive , same as electric field E (radial outward) If q is negative , opposite of electric field E (radial inward) Q P Q q q E F E E
Lecture 14 5/25 Torque on a Dipole The magnitude of the torque: F F E center of mass θ d -Q Q x d q p sin F r F r sin sin sin sin ) ( sin E p E q d F d F x d F x E p dipole moment torque points into the page

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Lecture 14 6/25 Electric Field from Coulomb’s Law Multiple Point Charges Continuous Charge Distribution http://www.falstad.com/vector3de/ + + + + P i r i q i i i i r r q k E ˆ 2 Summation over discrete charges Integral over continuous charge distribution (cylinder with uniform charge) r r dq k E d E ˆ 2 dq P r dV dA dL dq (line charge) (surface charge) (volume charge)
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## This note was uploaded on 03/14/2012 for the course PHYS 241 taught by Professor Wei during the Summer '08 term at Purdue.

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Lecture_14_-_no_quiz_answers - Lecture 14 1/25 Midterm Exam...

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