l1 ch22 coulomb law

Department of Physics and Applied PhysicsPHYS.1440 Lecture 1 A.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/


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?


Homework & Attendance & Book

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: Physics2F20Course ID: danylov54686The online HW is typically due midnight on Sunday(You are penalized 25% for each day late)

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

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


Course Grading


Today we are going to discuss:

Chapter 22:

Charges, Insulators, Conductors: Section 22.1-3 Coulomb’s Law: Section 22.4


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.


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


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 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 = (1/9)F


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 Superposition

A) 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 +Q upward, but with a stronger force. Therefore, the net force is up and to the right, but mostly up.

+2Q

+4Q

+Q

1 23

45d

d

+2Q

+4Q


Charged ions

Ifanatomlosesoneormoreelectrons,itbecomespositivelychargedcalledapositiveion

Ifanatomgainsoneormoreelectrons,itbecomesnegativelychargedcalledanegativeion

electrons

Usuallyanatomisneutral


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.


Now to describe processes of charging insulators and conductors we will need a charged object, a charged rod.

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

End of class


Induced charge in conductor

Althoughthemetalasawholeis stillelectricallyneutral,wesaythattheobjecthasbeenpolarized.

MetalChargepolarizationisaslightseparationofthepositiveandnegativechargesinaneutralobject.

Demo: rod/metallized balloon

F


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


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

__

_ ___

++++

+++

+


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.

l1 ch22 coulomb law

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