Electric Charges and Forces
Transcript of 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
__
_ ___
++++
+++
+