1. From Last Time… Lenses and image formation Optical Instruments p q
Object
Image q p
Optical Instruments

2. Exam 1
Tue. Sep. 29, 5:30-7 pm, 145 Birge
Covers Chap , 22, , 23.7, , 26 + lecture, lab, discussion, HW
8 1/2 x 11 handwritten note sheet (both sides) allowed
Students with scheduled class conflicts: stay after lecture today to arrange time
Study ideas:
Exam 1 Practice problems at Mastering Physics
Sample exams on website (blank & solutions)
Group/Quiz (blank & solutions).
Review lab question sheets. 2

3. Chapter 26: Electric Charges and forces
Two different kinds of electric charges
Benjamin Franklin called these positive, negative
Negative charges are electrons
Positive charges are protons
Often bound in atoms:
Positive protons in central nucleus r~10-15 m
Negative electrons orbit around the nucleus r~10-10 m
Example: Lithium 3 protons in nucleus, 3 electrons orbiting

4. + and - charges can be separated
Triboelectric
Charge is transferred as a result of mechanical (frictional) action
Conduction
charge transfer by contact (spark)
Need a cute little graphic of someone pulling an electron from a proton.

5. Separating charge
Rubber / fur: electrons transferred to rod
Rubber has negative charge
Glass / silk: electrons taken from plastic
Plastic has positive charge
Charge is conserved. Can be moved around, but not created or destroyed.

6. Electric Charges units and quantization
The SI unit of charge is Coulomb (C )
The electric charge, q, is said to be quantized
quantized = it is some integer multiple of a fundamental amount of charge e
q = Ne
N is an integer
e is the magnitude of charge of electron = +1.6 x C
Electron: q = -e
Proton: q = +e

7. Charge by conduction (touching)
Neutral metal + -
Positively charged rod (too few electrons) + -
electron flow
Electroscope demo - wipe charge onto electroscope. +
Less positively charged rod
Positively charged metal

8. Positive charged rod results in positive leaves.
Measuring charge
Touch charged rod to electroscope.
Charge transferred from rod to electroscope.
Everything positively charged.
Like charges on leaves repel.
Positive charged rod results in positive leaves. + + + +
Show that this can measure positive and negative charge. Can put positive charge on leaves. Then can put opposite charge on leaves.

9. Charge motion and materials
Insulators (e.g. plastic, wood, paper)
electrons bound to atoms, do not move around
Even extra charge is stuck
Extra charge cannot move around on insulator
Metals (e.g. copper, aluminum)
Some electrons free, positive ions stuck in place
Additional charge free to move, distributes over surface
Ionic solutions (e.g. saltwater)
Like conductor, but both positive, negative ions free to move

10. Forces between charges
Like charges repel
Unlike charges attract
All of this without touching — a ‘noncontact’ force
Comment that this is pretty amazing.
Show that interaction decreases with distance.
What is the interaction between these electric charges? Mention modern field-theory view.
Attraction, repulsion decreases with distance

11. Induced charge
Charging by induction requires no contact with the object inducing the charge
charged rubber rod
Bring negative charge close. Electrons on sphere move away from rod.
neutral metallic sphere Electrons (-) free to move

12. Quick quiz What is the sense of the force between these two objects?
Attractive
Repulsive
Zero
Ask what would happen with a positively-charged rubber rod.

13. Lightning doorbell + - Ben Franklin’s ‘door bell’.
Announced presence of lightning so knew to go out and do his experiments!

14. Positive charged rod results in positive leaves.
Quick Quiz
Positive charged rod results in positive leaves.
A charged rod is brought close to an initially uncharged electroscope without touching
The leaves
A. move apart
B. only one moves away
C. move closer together
D. depends on sign of rod
E. do nothing
Electroscope demo
This is an induced dipole

15. Vector Nature of Electric Force
a)The force is repulsive if charges are of like sign
b)The force is attractive if charges are of opposite sign
Multiple forces add as vectors.
Electrical forces obey Newton’s Third Law:
F21 = -F12

16. Quick Quiz
Two charges are arranged as shown. What is the direction of the force on the the positively charged ‘test’ particle? C B D + E A +

17. Magnitude of force: Coulomb’s Law
Electrical force between two stationary charged particles
The SI unit of charge is the coulomb (C ), µC = 10-6 C
1 C corresponds to 6.24 x 1018 electrons or protons
ke = Coulomb constant ≈ 9 x 109 N.m2/C2 = 1/(4πeo)
eo  permittivity of free space = x C2 / N.m2
Gravitational force: FG=GM1M2/ r2 G=6.7x10-11 Nm2/kg2

18. Quick Quiz - - Left Right Up Down Zero
Equal but opposite charges are connected by a rigid insulating rod. They are placed near a negative charge as shown. What is the net force on the two connected charges?
Left
Right Up
Down
Zero - + -

19. The electric dipole Can all be approximated by electric dipole.
Two opposite charges magnitude q separated by distance s
Dipole moment
Vector
Points from - charge to + charge
Has magnitude qs

20. Force on an electric dipole
What is the direction of the force on the electric dipole from the positive point charge? Up
Down
Left
Right
Force is zero +
How does the magnitude of the force depend on ?

21. Induced dipoles (charge redistribution)
charged rubber rod
Talk about approximating these as two different charges, separated by a small distance. An electric dipole. Say why charges should be equal and opposite in magnitude.
Bring negative charge close. Electrons on sphere move away from rod.

22. Induced dipole in insulators
A process similar to induction can take place in insulators
The charges within the molecules of the material are rearranged
Talk about hanging a balloon from the wall with static electricity. (can do this as a demo in class). Maybe first try rubbing, then try with rod and transfer to balloon).