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Unformatted text preview: ysics 1B  Summer Session 1  C. Palmer July 2, 2013 26 In Region I, the field from q2 is in the –x direction (away from positive charge) and the field from q1 is in
the +x direction (toward negative charge).
How do they balance? q2>q1 but close to q1 its field is very large, and so close to q1 the total field is 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 27 Far from the charges the distance will be comparable but the q2>q1 and so q2 dominates the field (x
dir) far.
Where is the electric field equal to 0 on the xaxis?
Somewhere in Region I. 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 28 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm Follow rhat ˆ
r1 q1= 1.0μC x= 2cm x=0 q2=+4.0μC x=+2cm +x ˆ
r2
Physics 1B  Summer Session 1  C. Palmer July 2, 2013 29 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm Follow rhat ˆ
r1 ˆ
ˆ
r1 = − x
ˆ
ˆ
r2 = − x q1= 1.0μC x= 2cm x=0 q2=+4.0μC x=+2cm +x ˆ
r2
Physics 1B  Summer Session 1  C. Palmer July 2, 2013 30 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm q1= 1.0μC E1x= 2cm E2 x=0 q2=+4.0μC x=+2cm +x Physics 1B  Summer
Session 1  C. Palmer July 2, 2013 31 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm ETotal = E1 + E2
ke q1
ke q2
ˆ
ˆ
=
r1 +
r2
2
2
( x − x1 )
( x − x2 ) q1= 1.0μC x= 2cm x=0 q2=+4.0μC x=+2cm +x Physics 1B  Summer Session 1  C. Palmer July 2, 2013 32 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm ETotal = E1 + E2 ke q1
ke q2
ˆ
ˆ
=
(− x) +
(− x)
2
2
( x − x1 )
( x − x2 ) q1= 1.0μC x= 2cm x=0 q2=+4.0μC x=+2cm +x Physics 1B  Summer Session 1  C. Palmer July 2, 2013 33 Where is the electric field equal to 0 on the x axis? Somewhere x<2.0cm q1
q2
−
−
=0
2
2
( x − x1 ) ( x − x2 )
q1
q2
=−
2
( x − x1 )
( x − x 2 )2
q
( x − x1 )2 = − 1 ( x − x2 )2
q2 q1= 1.0μC x= 2cm 1
( x + 0.02 m ) = ( x − 0.02 m )2
4
1
x + 0.02 = ( x − 0.02 ) = 0.5 x − 0.01
2
x = −0.06 m = −6 cm
2 x=0 q2=+4.0μC x=+2cm +x Physics 1B  Summer Session 1  C. Palmer July 2, 2013 34 Conductors –
–
–
–
–
– –
– – – –
– Excess negative (or voids resulting in positive) charge on a conductor repel each other.
They move to minimize/balance the force on all
charge. The outcome is that charge is distributed ON THE SURFACE of the conductor.
If the surface is pointy, then that is farther anyway
and the charge accumulates there.
If there is significant symmetry, then on a smooth
conductor the charge will be uniformly distributed.
Physics 2B  Summer Session 2  C. Palmer August 5, 2013 35 Inside of a conductor the electric field is zero. The electric field outside a conductor is normal
(perpendicular) to the
surface. –
– –
E=0 – –
– This is in accordance to the rules for field lines The lines begin/end at charges. Physics 1B  Summer Session 1  C. Palmer July 2, 2013 36 Go to khanacademy.org
You can login with a google account, facebook or your email address
Go to Learn Science & Engineering Physics
Then go to “Electricity and magnetism” on the right This site has some great lectures/walkthrough of problems
“Electrostatics (part 1): Introduction to Charge and Coulomb's Law” ~ 15 minutes
Charge and Coulomb’s Law example Also, start the chapter 21 problems
It’s a good idea to get ahead now.
Physics 2B  Summer Session 2  C. Palmer August 5, 2013 37...
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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.
 Summer '10
 Hirsch

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