Electric Charges and Forces

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII

Lecture 1

Chapter 22

Electric Charges and Forces

PHYSICS II

I am ready.What are you up to now?

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Today we are going to discuss:

Chapter 22:

Course structure Charges: Section 22.1-3 Charges: Section 22.4

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Course Website

Everything 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 PhysicsPHYS.1440 Lecture 1 Danylov

Lectures/Textbook

Physics II Lectures : 8:00-8:50 Tu/Th Ball 214

Lecture slides will be posted on-line

The textbook: “Physics for Scientists and Engineers, a Strategic Approach,” Fourth Edition by Randall Knight (Pearson, 2016)

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Weekly Homework

• Online HW is on • Course title: Physics II S18 UML • The online HW is typically due

midnight on Sunday(You are penalized 25% for each day late)

http://www.pearsonmylabandmastering.com/northamerica/

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Clicker registration

Channel Number of a clicker:

61

through the Blackboard

Then, go here

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Course Grading

Item PointsHW 100

Quizzes 100Exam 1 100Exam 2 100

Final ExamLecture

Attendance

20050

Total 650

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 PhysicsPHYS.1440 Lecture 1 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.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

ElectrostaticsElectrical 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

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 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:

ConcepTest Coulomb Force

A) 9F

B) 3F

C) F

D) F/3

E) F/9

The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force 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

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 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

ConcepTest SuperpositionA) 1

B) 2

C) 3

D) 4

E) 5

Which of the arrows best represents the direction of the net force on charge +Q due to the other two charges?

The charge +2Q repels +Q toward the right. The charge +4Q repels +Qupward, but with a stronger force. Therefore, the net force is up and to the right, but mostly up.

+2Q

+4Q

+Q

1 23

45

d

d

+2Q

+4Q

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

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 PhysicsPHYS.1440 Lecture 1 Danylov

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 PhysicsPHYS.1440 Lecture 1 Danylov

Charging

When 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)

It leads to CONSERVATION OF CHARGEThe net amount of electric charge produced in any process is zero

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

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 PhysicsPHYS.1440 Lecture 1 Danylov

Charge induction in an insulator

The 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 PhysicsPHYS.1440 Lecture 1 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.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 Danylov

Thank you

ConcepTest Polarization/DipoleX and Y are two uncharged metal spheres on insulating stands, and are in contact with each other. A positively charged rod R is brought close to X as shown in Figure (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.

Sphere Y is now moved away from X, as in Figure (b)What are the final charge states of X and Y?

+ + + + + + + X Y + + + + + + + X Y___

__ ++++

One piece of metalTwo pieces of metal

__

_ ___

++++

+++

+