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