Workshop material for week 2
•Material covered in lectures in week 1
•Homework assigned
•Introduction
•Important concepts
•21.1 Electric charge
•21.2 Conductors insulators, and induced charge
•21.3 Coulomb‟s law
•Reflection and formula sheets
•Homework
•Quiz
Homework assigned
Section 21.2: Questions 21.4 and 21.9 (11th Ed)
or 21.5 and 21.10 (12th Ed)
Section 21.3: Probls. 21.17 and 21.21 (11th Ed or 12th Ed)
Workshop Introduction
• Welcome
• Introduction: name e-address, phone & office numbers
• Goals:
• Facilitate learning
• Form a study group
• Review material (Hwk, work selected problems,
prepare cheat sheet, weekly quizzes, review for exams)
• This week‟s news:
• Buy clickers, buy lab manual,
• List sections covered in week 1
• List homework problems assigned in week 1
21.1 Electric charge
• Start with the demo, add your own tricks, let them play with tape, soda
cans, rods, etc.
• Explain the atomic origin of electricity
• Let students „discover‟ main concepts:
• There are two kinds of charges: + and –
• There is charge conservation
• Equal charges repel, unequal attract
• e and p are the source of electricity
• Charge of e = - charge of p
• Charge of e is a unit of charge
• Electrons can be removed from atoms
• “Ions” are atoms with excess charge
•C for parallel plates, spherical capacitor, cylindrical capacitor
•Solve problem 24.5, involve the students (10 min)
•Pose problem 24.10, let students work in groups (10 min)
21.2 Conductors insulators and induced charge
• Some atoms have electrons that are weakly bound that can move from
atom to atom, those are knows as conductors
• Some atoms have all their electrons tightly bound and cannot conduct
electricity, those are insulators
• Some elements are not conductors but can conduct if another elements
is added, those are semi-conductors
• Protons do not move during conduction, only electrons
• Some atoms have 2 or more conducting electrons
Copper atom, 29 electrons
Conduction electron
Charging by induction
Insulators can be charged by friction
Conductors must be charged in isolation
Attraction of neutral insulators by polarization
Post the following question (21.5, 12th Ed) for discussion:
A metal sphere hangs from a nylon thread. When a positively charged rod is
brought close to the sphere, the sphere is drawn towards the rod
(A) is the sphere charged? [Not necessarily, it could contain charge of opposite
sign, or it could be neutral and being attracted by polarization]
(B) What will happen if the sphere touches the rod? [It will move away as it will
get charged with the same type of charge as the rod]
21.3 Coulomb‟s Law
• What would happen to the attractive or repulsive forces between two charges if
• The distance between charges is doubled?
• The magnitude of the charges is doubled?
q1q2
F
r2
• Summarize these findings with the expression and explain that the second
power was discovered by Coulomb using a torsion balance
• Go back in time to the 1700s and ask what are the units of charge?
• Explain that it is a new property of matter and cannot be measure
in the usual units (m or s); Coulomb measured it in „strokes‟ which later
became “Coulombs”, equal to the charge of 6x1018 electrons
• Pose the problem of having Newtons on the left and C2/m2 on the right, how to
balance the units?
• Explain that a constant of proportionality was experimentally determined
• Solve problem 21.17 involving the class
• Pose problem 21.21, let students work in groups (15 min)
Reflection: Important concepts seen in week 1:
Ask students to reflect about what are the most important
concepts of the following sections and write on board (5 min)
•21.1 Electric charge
•21.2 Conductors insulators, and induced charge
•21.3 Coulomb‟s law
Formula/Concept sheet for exam
Ask students to come up with important concepts to
include in the formula sheet for exam 1 (5 min)
Quiz
• Assign a quiz based on problem 21.22, give them a maximum of 15 min
Dismissal