Electric Charges and Forces...Department of Physics and Applied Physics PHYS.1440 Lecture 1...
Transcript of Electric Charges and Forces...Department of Physics and Applied Physics PHYS.1440 Lecture 1...
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Lecture 1
Chapter 22
Electric Charges and Forces
PHYSICS II
I am ready.Let’s start.
Course website:https://sites.uml.edu/andriy-danylov/teaching/physics-ii/
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Everything you need to know about the course can be found on the course website or Blackboard
https://sites.uml.edu/andriy‐danylov/teaching/physics‐ii/ Blackboard
https://www.uml.edu/IT/Services/Academic‐Technology/Learning‐Management‐System.aspx
TherearetwoBlackboards(BB).Howtofindtherightone?
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
PhysicsIILectures:8:00‐9:50
M‐FrFalmouth205
The textbook: “Physics for Scientists and Engineers, a Strategic Approach,” Fourth Edition by Randall Knight (Pearson, 2016)
Quizzeswillbeannounced
Noquizzeswillbedroppedattheendof
thesemester.
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Homework & Attendance
Online HW (Mastering Physics)Registration MUST be done through BB(just click on any link inside the folder and you will be guided. You need to have an access number, which comes with a book or you can buy it with a credit card during your registration.)Course name: PHYS1440SU19Course ID: danylov43455The online HW is typically due midnight on Wednesday and Sunday(You are penalized 25% for each day late)
http://www.pearsonmylabandmastering.com/northamerica/
Lecture attendance is mandatory and it will be checked using iClickers. They are provided.Please, return them at the end of the semester.
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Course Grading
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Today we are going to discuss:
Chapter 22:
Charges,Insulators,Conductors:Section 22.1-3 Coulomb’sLaw:Section 22.4
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Electricity
• Electricity and magnetism is all around us.microphones, calculators, televisions, radio, computers.
• The colors of the rainbow in the blue sky are there because of electricity
• Your nerve system is driven by electricity.• You could not see without electricity.
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Electrostatics
Electrical forces arise from particles in atoms Electrons – negatively chargedProtons – positively charged (by convention)
Like charges repel Opposite charges attract
Observations: charges interact
Coulomb found an expression to describe these interactions
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Coulomb’s law
In SI units K = 8.99 109 N m2/C2.
When two charged particles are a distance, r, apart, they each experience a force.
Enormous!!!
F1on2
r
q1
q2F2on1
Rewriting Coulomb’s law in terms of 0 gives us:
Let’s define a new constant, called the permittivity constant 0:
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ConcepTest Coulomb Force
A) 9F
B) 3F
C)F
D)F/3
E)F/9
The force between two charges separatedby a distance d is F. If the charges arepulled apart to a distance 3d, what is theforce on each charge?
QF
QF
dQ
?Q
?
3dOriginally we had:
Fbefore = k(Q)(Q)/d2 = F
Now we have:
Fafter = k(Q)(Q)/(3d)2 = 1/9F
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Principle of superposition
F1 F2F3
F4
q1 q2
q3
q4
+ + +
q
If multiple charges are present, the net electric force on a charge q due to all other charges is
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ConcepTest SuperpositionA) 1
B) 2
C)3
D)4
E)5
Whichofthearrowsbestrepresentsthedirectionofthenetforceoncharge+Qduetotheothertwocharges?
Thecharge+2Q repels+Q towardtheright.Thecharge+4Q repels+Q upward,butwithastrongerforce.Therefore,thenetforceisupandtotheright,butmostlyup.
+2Q
+4Q
+Q
1 23
45
d
d
+2Q
+4Q
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Charged ions
Ifanatomlosesoneormoreelectrons,itbecomespositivelychargedcalledapositiveion
Ifanatomgainsoneormoreelectrons,itbecomesnegativelychargedcalledanegativeion
electrons
Usuallyanatomisneutral
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Insulators and conductors
The electrons in an insulator are all tightlybound to the positive nuclei and not free tomove around.
In metals, the outer atomic electrons are onlyweakly bound to the nuclei.These outer electrons become detached fromtheir parent nuclei and are free to wanderabout through the entire solid.
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Now to describe processes of charging insulators and conductors we will need a charged object, a charged rod.
Whenaplasticrodisrubbedwithatowel,theplasticacquiresanegativechargeandthetowelacquiresanequalamountofpositivecharge.(Thechargesareseparatedbutthesumiszero)
It leads to CONSERVATION OF CHARGEThe net amount of electric charge produced in any process is zero
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Induced charge in conductor
Althoughthemetalasawholeisstillelectricallyneutral,wesaythattheobjecthasbeenpolarized.
MetalChargepolarizationisaslightseparationofthepositiveandnegativechargesinaneutralobject.
Demo: rod/metallized balloon
F
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Charge induction in an insulator
A neutral atom is polarized by an external charge, forming an electric dipole.
Center of negative charge
negatively charged surface
positively charged surface
electric dipole
F
Demo:rod/paper
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Demonstrations
A Van de Graff generator (named after its inventor) is a high voltage generator. It basically loads a spherical hollow conductor with static charges which brings the conductor to a very high (or very low) potential.
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DepartmentofPhysicsandAppliedPhysicsPHYS.1440Lecture1A.Danylov
Thank you
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ConcepTest Polarization/DipoleX andY aretwounchargedmetalspheresoninsulatingstands,andareincontactwitheachother.ApositivelychargedrodR isbroughtclosetoX asshowninFigure(a).
A) Both X and Y are neutral
B) X is positive and Y is neutral
C) X is neutral and Y is positive
D) X is negative and Y is positive
E) Both X and Y are negative
So we created a dipole.
SphereY isnowmovedawayfromX,asinFigure(b)
WhatarethefinalchargestatesofX andY?
+ + + + + + + X Y + + + + + + + X Y___
__ ++++
One piece of metalTwo pieces of metal
__
_ ___
++++
+++
+