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Transcript
From Last Time… Total internal reflection Object Lenses and imaging Image 1 This Friday’s guest lecture Biological Diffraction Prof. Katrina Forest, Bacteriology Friday 8:50 am, 2103 Chamberlin QuickTime™ and a PNG decompressor are needed to see this picture. X-ray diffraction Real-space structure 2 Chapter 25: Electric Charges and forces • Two different kinds of electric charges • • Benjamin Franklin called these positive, negative Negative charges are electrons Positive charges are protons Example: Lithium • Often bound in atoms: 3 protons in nucleus, 3 electrons orbiting Positive protons in central nucleus r~10-15 m Negative electrons orbit around the nucleus r~10-10 m 3 + and - charges can be separated • Triboelectric Charge is transferred as a result of mechanical (frictional) action • Conduction charge transfer by contact (spark) 4 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. 5 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 6 Charge by conduction (touching) + + + + + + + + + + ++ Positively charged rod (too few electrons) + + + + + + + + + + ++ electron flow + + + + + + + Less positively charged rod +-+ - -+ + - + Neutral metal +-+ - -+ + - + + + + + + Positively charged metal 7 Measuring charge • Transfer charge to • • electroscope. Everything equally charge. Like charges on leaves repel. + ++ + Positive charged rod results in positive leaves. 8 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 9 Interactions between charges Why did the electrons flow? attractive force between positive and negative charges. repulsive force between two positive or two negative charges 10 Forces between charges Like charges repel Opposite charges attract All of this without touching — a ‘noncontact’ force Attraction, repulsion decreases with distance 11 Induced charge • Charging by induction requires no contact with the object inducing the charge charged rubber rod neutral metallic sphere Bring negative charge close. Electrons on sphere move away from rod. 12 Quick quiz What is the force between these two objects? A. Attractive B. Repulsive C. Zero 13 Lightning doorbell + + + + - - + + + + • Ben Franklin’s • ‘door bell’. Announced presence of lightning so knew to go out and do his experiments! 14 Electrical machines • Can mechanize the • rubbing process to continually separate charge. This charge can then be transferred to other objects. 15 Quick Quiz A charged rod is brought close to an initially uncharged electroscope without touching The leaves Positive charged rod results in positive leaves. A. move apart B. only one moves away C. move closer together D. depends on sign of rod E. do nothing This is an induced dipole 16 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 The force is a conservative force Electrical forces obey Newton’s Third Law: F21 = -F12 17 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 + A E + 18 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 19 Quick Quiz 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? A) Left B) Right - + - C) Up D) Down E) Zero kq1q2 F 2 r 20 The electric dipole • Can all be approximated • by electric dipole. Two opposite charges magnitude q separated by distance s Dipole moment Vector p Points from - charge to + charge Has magnitude qs 21 Force on an electric dipole • What is the direction of the force on the electric dipole from the positive point charge? A. Up B. Down p + C. Left D. Right E. Force is zero How does the magnitude of the force depend on p ? 22 Induced dipoles (charge redistribution) charged rubber rod Bring negative charge close. Electrons on sphere move away from rod. 23 Induced dipole in insulators • A process similar to • induction can take place in insulators The charges within the molecules of the material are rearranged 24 The idea of electric fields • EM wave made up of oscillating electric and magnetic fields. • But what is an electric field? • Electric field is a way to describe the force on a charged particle due to other charges around it. • Force = charge electric field • The direction of the force is the direction of the electric field. 25 Electric field of a point charge Force on this charge… Q2 + + + Q1Q2 F k 2 rˆ r Q1 …due to this charge Force = Charge Electric field + 26 + Question • Which vector best represents the electric field at the red dot? A B C D E A B E C - D 27