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# July 2 2013 16 now finally comes the superposition

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Unformatted text preview: - C. Palmer July 2, 2013 18   A field “creates a ‘condition’ in space” such that when you put a charged object in it, the charged object “feels a force”. -Richard Feynman   It is a vector proportional to force on a test charge. Elec 0 F There Exists Charged Stuﬀ [C] - + - - + Charged Stuﬀ Creates a Field [N/C] =q E Other Charged Stuﬀ Feels the Field [N] - - + - + Physics 1B - Summer Session 1 - C. Palmer - July 2, 2013 19   Below are the fields of point particles or spherically symmetric objects (m1, q1) for gravitational and electric forces. (specific example)   The fields (g, E) have the ability to act on a “test charge” (m2, q2) that could exist anywhere in space. (general case) q1q2 m1m2 ˆ ˆ FElec,1on 2 = ke 2 r Fgrav,1on 2 = −G r 2 r r ke q1 −Gm1 ˆ = q2 2 r ˆ = m2 r 2 r r = m2 gPoint1 = q2 EPoint1 Gm1 ke q1 ˆ ˆ gPoint1 = − 2 r EPoint1 = 2 r r r Physics 1B - Summer Session 1 - C. Palmer July 2, 2013 20   Field lines should be   More lines in a given area proportional to charge in means a stronger field the same diagrams   Directional convention   The number drawn is complete arbitrary   Away from (+)   Toward (-) ++ Physics 1B - Summer Session 1 - C. Palmer - July 2, 2013 21   http://www.cco.caltech.edu/~phys1/java/phys1/ EField/EField.html   They don’t use arrows, but you have to! Physics 1B - Summer Session 1 - C. Palmer July 2, 2013 22   There are two charged objects with charge -1.0µC and 4.0 µC located at -2.0cm and +2.0cm respectively. Where is the electric field equal to 0 on the x-axis? x=0 q1=- 1.0μC x=- 2cm q2=+4.0μC x=+2cm +x Physics 1B - Summer Session 1 - C. Palmer July 2, 2013 23 What is the sign of the field in Region III along the x-axis? A)  It is always positive. B)  It is positive until the fields from the two charges balance and then it becomes negative. C)  It is always negative. D)  It is negative until the fields from the two charges balance and then it becomes positive. Region I Region II q1=- 1.0μC x=- 2cm x=0 Region III q2=+4.0μC x=+2cm +x Physics 1B - Summer Session 1 - C. Palmer July 2, 2013 24 What is the sign of the field in Region III along the x-axis? Region III is always positive because |q2|>|q1| and q2 will always be the closest charge. Region I Region II q1=- 1.0μC x=- 2cm x=0 Region III q2=+4.0μC x=+2cm +x Physics 1B - Summer Session 1 - C. Palmer July 2, 2013 25 In Region II, the field from q2 is in the –x direction (away from positive charge) and the field from q1 is also in the –x direction (toward negative charge). Region I Region II q1=- 1.0μC x=- 2cm x=0 Region III q2=+4.0μC x=+2cm +x Ph...
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## This note was uploaded on 09/10/2013 for the course PHYS 2B 2b taught by Professor Hirsch during the Summer '10 term at UCSD.

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