1. Electricity and Magnetism plus Optics and Modern Physics
Spring 2018
Electricity and Magnetism
plus Optics and Modern Physics
Instructor: Robin Côté

2. Course Info Course has several components:
Lecture: (me talking, demos and Active learning).
Homework Sets: problems from the book.
Clicker questions (AA): short concept questions in class
Team Problems: small questions solved on blackboard
Tests: two midterms and a final.
Questions on tests will look like those we do in the rest of the class; in homework and during lectures.
No surprises
Office hours: to answer additional questions
Labs: (group exploration of physical phenomena). 2

3. How to do well in the course ?
FINAL GRADE WILL BE MADE OF:
2 Midterms 30%
Final Exam 20%
Homeworks 15%
Labs 25%
Clicker questions 7%
Team problems 3%
Remember:
if you miss 1 HW (out of ~10 given during the semester), you miss 2% of the final score !
if you miss more than two LABs => fail

4. Announcements Most of the info about the class will be posted on:
lecture notes (.ppt and .pdf formats)
Exam samples and solutions
Syllabus
Follow the link to 1202
Homework on Masteringphysics
Labs start during the week of January 23.

5. Announcements Register for MasteringPhysics Course ID: cote90751
Homeworks will be posted on Mastering Physics
Register for MasteringPhysics
Course ID: cote90751
Clicker frequency: AA
HW will be due usually Fri. afternoons (5:00 pm)
No Late HW accepted
HELP:
Become familiar with the Physics Resource Center for help with problem sets.
Also, the Q-Center.
Note that the ACT icon is actually a hyperlink to ACT 1. Select it to view the ACT (once the lecture has been given). ACTs can also be viewed from the syllabus and the ACTs link on the homepage.

6. Format of Lectures
Roughly 1/2 to 2/3 of the time in class devoted to presentation of material by instructor
InterACTive periods during lectures where students work together on problems
an “ACT”
Note that the ACT icon is actually a hyperlink to ACT 1. Select it to view the ACT (once the lecture has been given). ACTs can also be viewed from the syllabus and the ACTs link on the homepage.
Clicker questions to check understanding of concepts
A few team problems presented (~5 minutes each)
Occasional demos to illustrate key concepts

7. The World According to Physics 1201
• Things
Specified by geometry and mass
• Forces
Gravity:
Others: Tension, Normal, Friction
• Space and Time
Euclidean with Galilean Invariance
“ordinary” 3D space; “slow” velocities

8. The World According to Physics 1202
• Things -- Bodies and Fields (E,B)
Specified by geometry and mass and charge
Forces
Gravity:
Electromagnetic:
Space and Time
Euclidean with Lorentz Invariance
“ordinary space” but can be really really fast...

9. Chapter 19: Electric charge
The effects of electric charge were first observed as static electricity:
After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.

11. Where Does Our Study Start?
Benjamin Franklin
The Phenomena (known for a long time)
Silk on glass Þ glass ® positive
Fur on rubber Þ rubber ® negative
The Concept
Unlike charges attract
Like charges repel

12. More facts about charges
2 types of charges
Electric charge is always conserved
Charge is not created, only exchanged
Origin of charges
Electrons and protons (later)
Charges quantized ( e )
electron = - e
proton = + e
e = 1.6 x C
SI unit: C = Coulomb

13. How it works ... Polarization & attraction
Some materials can become polarized
their atoms “rotate” in response to an external charge.
This is how a charged object can attract a neutral one.

14. 19.2: Insulators vs. Conductors
Insulators – wood, rubber, styrofoam, most ceramics, etc.
Conductors – copper, gold, exotic ceramics, etc.
Sometimes just called metals
Insulators – charges cannot move.
Will usually be evenly spread throughout object
Conductors – charges free to move.
on isolated conductors all charges move to surface.
Semiconductors – in between
e.g. : silicon & germanium

15. Charging by conduction
A charged object (the rod) is placed in contact with another object (the sphere)
Some electrons on the rod can move to the sphere
When the rod is removed, the sphere is left with a charge
The object being charged is always left with a charge having the same sign as the object doing the charging

16. Charging by Induction
Grounded: an object connected to a conducting wire or pipe to the
A negatively charged rubber rod is brought near an uncharged sphere
The charges in the sphere are redistributed
electrons in the sphere are repelled from electrons in the rod
A grounded conducting wire is connected
allows electrons to move from the sphere to the ground
The wire to ground is removed, the sphere is left with an excess of induced positive charge
Charging by induction requires no contact with the object inducing the charge

17. 19-3 The Force of Electric Charges
Assume that the electrical force between two charged objects acts along the line joining the centers of the charges (a Central Force).
It increases if the magnitude of one of the charges increases.
It increases if the distance between the charges is decreased, i.e. the charges get closer

18. 19-3 The Force of Electric Charges
Charles Coulomb ( )
The electric force between two charged particles:
is inversely proportional to the square of the distance between particles;
increases if the magnitude of the charges increases;
is attractive if the charges are of opposite sign and repulsive if the charges have the same sign.

19. What We Call Coulomb's Lawq2 r
F12 q1
F21
q1q2 1
F12= r
4pe0 r2
SI Units:
r in meters
q in Coulombs
F in Newtons Þ 1
4pe0
= k = N m2/C2
We call this group of constants “k” as in: F = k q1q2/r2
This force has same spatial dependence as gravitational force, BUT there is NO mention of mass here!!
The strength of the FORCE between two objects is determined by the charge of the two objects.

20. Chapter 19, ACT 1 Q2
A charged ball Q1 is fixed to a horizontal surface as shown. When another charged ball Q2 is brought near, it achieves an equilibrium position at a distance d12 directly above Q1.
d23 Q2 Q3
d12 g Q1
When Q1 is replaced by a different charged ball Q3 , Q2 achieves an equilibrium position at distance d23 (< d12) directly above Q3.
1: A) The charge of Q3 has the same sign as the charge of Q1
B) The charge of Q3 has the opposite sign as the charge of Q1
C) Cannot determine the relative signs of the charges of Q3 & Q1

21. Chapter 19, ACT 2
A charged ball Q1 is fixed to a horizontal surface as shown. When another charged ball Q2 is brought near, it achieves an equilibrium position at a distance d12 directly above Q1. Q2 Q2
d12
d23 g Q1 Q3
When Q1 is replaced by a different charged ball Q3 , Q2 achieves an equilibrium position at distance d23(< d12) directly above Q3.
2: A) The magnitude of charge Q3 < the magnitude of charge Q1
B) The magnitude of charge Q3 > the magnitude of charge Q1
C) Cannot determine relative magnitudes of charges of Q3 & Q1

22. What happens when you consider more than two charges?
If q1 were the only other charge, we would know the force on q due to q1 . F 1 2 -q
+q1
+q2
If q2 were the only other charge, we would know the force on q due to q2 .
What is the force on q when both q1 and q2 are present??
The answer: just as in mechanics, we have the Law of Superposition:
The TOTAL FORCE on the object is just the VECTOR SUM of the individual forces. F = 1 + 2

23. Chapter 19, ACT 3 Q2 Q1 3R +Q
+2Q
Two balls, one with charge Q1 = +Q and the other with charge Q2 = +2Q, are held fixed at a separation d = 3R as shown. Q2 Q1 R +Q
+2Q Q3 2R
Another ball with (non-zero) charge Q3 is introduced in between Q1 and Q2 at a distance = R from Q1 .
Which of the following statements is true?
(a)
The force on Q3 can be zero if Q3 is positive.
(b)
The force on Q3 can be zero if Q3 is negative.
(c)
The force on Q3 can never be zero, no matter what the charge Q3 is.

24. Force Comparison Electrical vs Gravitationalq2 r q1 Þ
For a proton,
q = 1.6 X C
m = 1.67 X kg Þ *
Note: smallest charge seen in nature !
G= N m2/kg2

25. Recap of today’s lecture
Course details
Charges
Two types (+ and -)
Like repel, unlike attract
Type of materials
Conductors / insulators
Coulomb force