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

Exam 1 Tue. Sep. 29, 5:30-7 pm, 145 Birge Covers Chap. 21.5-7, 22, 23.1-4, 23.7, 24.1-5, 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

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

+ 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.

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.

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 10-19 C Electron: q = -e Proton: q = +e

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

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.

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

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

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

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.

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

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

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

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 +

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 = 8.854 x 10-12 C2 / N.m2 Gravitational force: FG=GM1M2/ r2 G=6.7x10-11 Nm2/kg2

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 - + -

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

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 ?

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.

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).