Last time…

1. I2 I1 I3 I1=I2+I3 R1 I1 e R2 R3 I2 I3 Last time… Kirchoff’s junction law Equivalent resistance (parallel, series) Kirchoff’s loop law Physics 208 Lecture 13

2. Exam 2 is Tuesday Oct. 27 5:30-7 pm, 145 Birge Students w / scheduled academic conflict please stay after class Tues. Oct. 20 to arrange alternate time. Covers: all material since exam 2. Bring: Calculator One (double-sided) 8 1/2 x 11 note sheet Exam review: Thursday, Oct. 22, in class Physics 208 Lecture 13

3. Time t = 0: t increases: Time t = : Charging a capacitor • Again Kirchoff’s loop law: Looks like resistor & battery: uncharged cap acts like short circuit VC increases, so VR decreases Fully charged capacitor acts like open circuit Physics 208 Lecture 13

4. Question The circuit contains three identical light bulbs and a fully-charged capacitor. Which is brightest? A B C A & B All equally bright Physics 208 Lecture 13

5. Question The circuit contains three identical light bulbs and an uncharged capacitor. Which is brightest? A B C A & B All equally bright Physics 208 Lecture 13

6. C B A D Discharging the capacitor • Kirchoff’s loop law Charges in the current I come from capacitor: Physics 208 Lecture 13

7. RC discharge • RC time constant Physics 208 Lecture 13

8. Charging a capacitor Physics 208 Lecture 13

9. Extracellular fluid Plasma membrane Cytoplasm Human capacitors • Cell membrane: • ‘Empty space’ separating charged fluids (conductors) • ~ 7 - 8 nm thick • In combination w/fluids, acts as parallel-plate capacitor 100 µm Physics 208 Lecture 13

10. Cell Membrane RC circuit Nerve signal is an action potential that propagates down RC cell-membrane network Physics 208 Lecture 13

11. Magnetism Physics 208 Lecture 13

12. N S Magnets • Clearly magnets interact with each other • Sometimes attracting, sometimes repelling • But the magnetic particles are sort of a ‘composite’ positive and negative ‘magnetic charge’. • Visualized as a bar with positive pole (North) at one end and negative pole (South) at other. • These ‘magnetic charges’ cannot be broken apart — always appear in N-S pairs. Physics 208 Lecture 13

13. Let’s Break A Magnet! • North Pole and South Pole • Are inseparable Physics 208 Lecture 13

14. Magnetic field N • Similar in spirit to electric field • Exerts torque on a mangetic dipole • Magnetic field exerts a torque on compass needle • Aligns it with magnetic field lines. • Magnetic field lines indicate direction of local magnetic field • Field lines • leave magnet at N pole • enter magnet at S pole S Physics 208 Lecture 13

15. N N N S S S S N Magnetism: Permanent magnets Opposites attract Likes repel • North Pole and South Pole • This is the elementarymagnetic particle • Called magnetic dipole(North poleand South pole) • Poles interact with each other similar to charges. Physics 208 Lecture 13

16. N N N N S S S S Alignment force Which way will these magnets tend to align? • But really one of the magnets is trying to align the other one. N S S N N S N S A B C D Physics 208 Lecture 13

17. ( Compare electric dipole: ) Magnetic dipoles • Magnetic charges (monopoles) have never been observed. • Magnetic dipole characterized by dipole moment • Torque on magnetic dipole Torque tends to aligns magnetic dipole with magnetic field Physics 208 Lecture 13

18. - + Magnetic Field Lines • Magnetic Field Lines • Arrows give direction • Density gives strength • Looks like dipole Physics 208 Lecture 13

19. Electric vs Magnetic Field Lines • Similarities • Density gives strength • Arrow gives direction • Leave +, North • Enter -, South • Differences • Start/Stop on electric charge • No Magnetic Charge, lines are continuous! • Convention for 3-D situations: • x x x x x x x INTO Page • ••••••••••••• OUT of Page Physics 208 Lecture 13

20. Magnets and magnetic fields Physics 208 Lecture 13

21. Definition of Magnetic Field Defined electric field based on electric force on a charged particle. • Observation: • Magnetic field has no effect on stationary charged particle Is there a magnetic force on charged particle? Physics 208 Lecture 15

22. S q v F mag Magnetic force on moving charged particle • Empirical facts: a) magnitude: µ to velocity of q b) direction: ^ to direction of q Physics 208 Lecture 15

23. Magnetic force on electric charges • Effect of uniform B-field on charged particle • Charged particle not moving • no effect • Charged particle is moving: • force exerted perpendicularto both field and velocity Physics 208 Lecture 15

24. Magnetic force on charged particle • Magnitude of force is proportional to • Charge of particle, q • Speed of particle, v • Strength of magnet field, B • sin(), =angle between and Direction of force is perpendicular to both and vector ‘cross product’ Physics 208 Lecture 15

25. FB on a Charge Moving in a Magnetic Field, Formula FB = q v x B • FB is the magnetic force • q is the charge • v is the velocity of the moving charge • B is the magnetic field • SI unit of magnetic field: tesla (T) • CGS unit: gauss (G): 1 T = 104 G (Earth surface 0.5 G) Physics 208 Lecture 15

26. B 3 2 1 Quick Quiz The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. Which particle experiences the greatest magnetic force? 1 2 3 All same F = q v B sin(q) Physics 208 Lecture 15

27. B 3 Magnitude F = q v B sin(q) 2 All forces are into page 1 Quick Quiz The three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. The force on all the particles is in the same direction. True False Physics 208 Lecture 15

28. intermediate Force on moving charged particle Physics 208 Lecture 15