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Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII
Lecture Capture: http://echo360.uml.edu/danylov201415/physics2fall.html
Lecture 1
Chapter 25
Electric Charges and Forces
09.05.2014Physics II
95.144
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Course WebsiteEverything you need to know about the course can be found on the course website: http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII.aspx
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Course Organization
Physics II meets 4 times a week• Lectures : 9:00-9:50 TuFr, Olney 150• Recitation Sections
Various times (M/W) and locations
Lecture slides will be posted on-line
Lecture recording (video and audio) will be posted:
The textbook: “Physics for Scientists and Engineers, a Strategic Approach,” Third Edition by Randall Knight (Pearson, 2012)
http://echo360.uml.edu/danylov201415/physics2fall.html
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Weekly Homework
• Online HW is on www.masteringphysics.com Course title: “95144Fall2014” Course ID: DANYLOVFALL2014
• The online HW is typically due midnight on Sunday(You are penalized 25% for each day late)
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Course Grading
Item PointsHW 100
Quizzes 100Exam 1 100Exam 2 100
Final Exam 200Total 600
Range Grade>80% A
75-80% A-70-75% B+65-70% B60-65% B-55-60% C+50-55% C45-50% C-40-45% D+35-40% D<35% F
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Physics II Resources
Get help early and often. The first person to contact with problems in Physics I is your
Recitation Instructor!! Recitation Instructors will have office hours each week during
which they can be met. Physics Department Tutoring Center: Tutors are available free of charge in the Physics Department
tutoring room (9 am-5 pm), located adjacent to the Department office on the 1st floor of Olney Hall.
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
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.
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Electrostatics
Electrical forces arise from particles in atoms Electrons – negatively chargedProtons – positively charged (by convention)
Charges are at rest
Like charges repel Opposite charges attract
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
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
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Coulomb’s law
We can make many future equations easier to use if we rewrite Coulomb’s law in a somewhat more complicated way.
Let’s define a new constant, called the permittivity constant 0:
Rewriting Coulomb’s law in terms of 0 gives us:
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
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
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Charged ions
If an atom loses one or more electrons, it becomes positively charged called a positive ion
If an atom gains one or more electrons, it becomes negatively charged called a negative ion
electrons
Usually an atom is neutral
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Insulators and conductors
The electrons in an insulator are all tightly bound to the positive nuclei and not free to move around.
In metals, the outer atomic electrons are only weakly bound to the nuclei.
These outer electrons become detached from their parent nuclei and are free to wander about through the entire solid.
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
ChargingWhen a plastic rod is rubbed with a towel, the plastic acquires a negative charge and the towel acquires an equal amount of positive charge.(The charges are separated but the sum is zero)
Rubbed glass positively charged (lack of electrons)Rubbed amber/plastic negatively charged (excess electrons)
It leads to CONSERVATION OF CHARGEThe net amount of electric charge produced in any process is zero
Demo:rod/paper
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Charging metal
The figure shows how a conductor is charged by contact with a charged plastic rod.
Electrons in a conductor are free to move.
Once charge is transferred to the metal, repulsive forces between the electrons cause them to move apart from each other.
Demo:rod/electroscope
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Induced charge in conductor
Although the metal as a whole is still electrically neutral, we say that the object has been polarized.
Metal
Charge polarization is a slight separation of the positive and negative charges in a neutral object.
Demo:rod/electroscope/induction and rod/baloon
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Charge induction in an insulatorThe figure shows how 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
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
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.
Electroscope
Wimshurst generator(Up To 75,000 Volts)
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
The Electric Field
The units of the electric field are N/C. The magnitude E of the electric field is called the electric field strength.
If a probe charge q experiences an electric force at a point in space, we say that there is an electric field at that point causing the force.
Q
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Once E is known, it is easy to get FA charged particle with charge q at a point in space where the electric field is E experiences an electric force:
The force on a negative charge is opposite the direction of E.
E
F
F
If q is positive, the force on the particle is in the direction of E.
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
The Electric Field of a Point Charge
The electric field at a distance r from a point charge q ( - unit vector).
=
14
Let’s find an electric field at a distance r from a point charge q is:
Department of Physics and Applied Physics95.144, Fall 2014, Lecture 1
Thank youSee you on Tuesday