RC Circuits The intermittent wipres are part of an RC circuit whose time constant can be varied by selecting different values of R through a multiposition switch, As it increases with time, the voltage across the capacitor reaches are point at which it triggers the wipers and discharges, ready to begin another charging cycle. The time intervals between sweeps of wipers is determined by the value of the time constant A strobe light is an interesting beast, producing light by a means utterly unlike the hot filament wire in an incandescent light bulb. A strobe light stores up energy in an electrical component called a "capacitor", and then suddenly dumps it all into a lamp bulb filled with xenon gas. The normally insulating gas in the bulb suddenly conducts electricity. This produces a sudden, brief, and intense flash of light. ･A bright light that flashes at regular close intervals, often used to "freeze" motion. An example is an automotive engine timing light.･A bright light that flashes at regular slow intervals, often as a warning. An example is the flashing red light at the top of radio station antennas, to warn airplanes.･A bright light that flashes once, often used for photographic purposes. A protographer's xenon flash lamp is a reusable alternative to "flash bulbs".･Just about anything built with a xenon flash lamp. http://www.catskillhouse.us/blog/files/paulthurst41_self_supporting_communications_tower.jpg http://www.uh.edu/engines/wipers.jpg http://media-2.web.britannica.com/eb-media/55/155-004-1096806E.jpg http://courses.washington.edu/bhrchem/c456/ch11.pdf 11/27/2018

RC Circuits Charging a capacitor: C initially uncharged; connect switch S1 at t=0 Calculate current and charge as function of time. Apply Kirchhoff’s Voltage Law: Short term: q = q0 = 0 Intermediate term: Long term: 11/27/2018

Solution 11/27/2018

Continued Capacitive Time Constant:  The greater the , the greater the charging time. Units of  : 11/27/2018

Charging a Capacitor at at 11/27/2018

Charging a Capacitor DEMO 6C-14 Demo: Circuit Board t=0, capacitor acts as a wire with negligible resistance t=infinity, capacitor behaves as an open circuit (voltage drop over the capacitor) 11/27/2018

RC Circuits Discharging a capacitor: C initially charged with Q=C Connect switch S2 at t=0. Apply Kirchhoff’s Voltage Law: Short term: q = q0 = Ce Intermediate term: Long term: 11/27/2018

Solution 11/27/2018

Discharging a Capacitor at at DEMO 11/27/2018

Behavior of Capacitors Charging Initially, the capacitor behaves like a wire. After a long time, the capacitor behaves like an open switch in terms of current flow. Discharging Initially, the capacitor behaves like a variable battery. After a long time, the capacitor behaves like an open switch 11/27/2018

Question 1 What will happen after the switch is closed? V C S Bulb 1 Bulb 2 R B – both bulbs come on but then bulb 2 fades out The purpose of this Check is to jog the students minds back to when they studied work and potential energy in their intro mechanics class. What will happen after the switch is closed? A) Both bulbs come on and stay on. B) Both bulbs come on but then bulb 2 fades out. C) Both bulbs come on but then bulb 1 fades out. D) Both bulbs come on and then both fade out.

Question 2 Suppose the switch has been closed a long time. V C S Bulb 1 Bulb 2 R Suppose the switch has been closed a long time. Now what will happen after open the switch? Both bulbs come on and stay on. Both bulbs come on but then bulb 2 fades out. Both bulbs come on but then bulb 1 fades out. Both bulbs come on and then both fade out. D – both bulbs come on and then both fade out. The purpose of this Check is to jog the students minds back to when they studied work and potential energy in their intro mechanics class.

Question 3 B. t1 = t2 C. t1 > t2 CIRCUIT 2 CIRCUIT 1  A. t1 < t2 B. t1 = t2 C. t1 > t2  R C Tau= (2R)(C/2)=RC Tau=(2C)*(R/2)=TC 11/27/2018

QUIZ A circuit is wired up as shown. The capacitor is initially uncharged and switches S1 and S2 are initially open. What is the voltage, V1, across the capacitor immediately after the switch S1 is closed and what is the voltage, V2, a long time after S1 is closed. V1=0 and V2=V A) V1 = V V2 = V B) V1 = 0 V2 = V C) V1 = 0 V2 = 0 D) V1 = V V2 = 0 11/27/2018

QUIZ (1) (2) (3) The figure shows three section of circuit that are to be connected in turn to the same battery via a switch. The resistors are identical, as are the capacitors. Rank the sections according to the final charge (t = ) on the capacitor. C is correct 2 = 3 > 1 (B) 1 > 3> 2 (C) 1 = 2 = 3 (D) 2 > 3 > 1 (E) 3 > 1 > 2 11/27/2018

Magnetic Field Large Magnetic fields are used in MRI (Nobel prize for medicine in 2003) Extremely Large magnetic field are found in some stars Earth has a Magnetic Field 11/27/2018

Bar Magnets N S S N Bar magnet ... two poles: N and S Like poles repel; Unlike poles attract. Magnetic Field lines: (defined in same way as electric field lines, direction and density) Attraction Repulsion DEMO From North to South 11/27/2018

DEMO of Magnetic Field Lines Electric Field Lines of an Electric Dipole Magnetic Field Lines of a bar magnet 11/27/2018

S N 3 2 1 Question 4. Which drawing shows the correct field lines for a bar magnet? (1) (2) (3) Magnetic field lines are continuous Arrows go from N to S outside then magnet (S to N inside). Answer is 2. 20 11/27/2018 20

Try cutting a bar magnet in half: Magnetic Monopoles One explanation: there exists magnetic charge, just like electric charge. An entity which carried this magnetic charge would be called a magnetic monopole (having + or - magnetic charge). How can you isolate this magnetic charge? Try cutting a bar magnet in half: S N S N S N In fact no attempt yet has been successful in finding magnetic monopoles in nature but scientists are looking for them. 11/27/2018

Earth’s Magnetic Field By convention, the N end of a bar magnet is what points at the Earth’s North Geographic Pole. Since opposite poles attract (analogous to opposite electric charges), the “North Geomagnetic Pole” is in fact a magnetic SOUTH pole, by convention. Confusing, but it’s just a convention. Just remember that we define N for bar magnets as pointing to geographic North. 11/27/2018

Earth’s Magnetic Field Magnetic North Pole 1999 11/27/2018

Earth’s Magnetic Field Magnetically Quiet Day Disturbed Day 11/27/2018

Earth’s Magnetic Field Since 1904: 750 km, an average of 9.4 km per year. From 1973 to late 1983: 120 km, an average of 11.6 km per year 11/27/2018

Earth’s Magnetic Field