1. 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.
11/27/2018

2. 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

3. Solution
11/27/2018

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

5. Charging a Capacitor at at
11/27/2018

6. 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

7. 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

8. Solution
11/27/2018

9. Discharging a Capacitorat at
DEMO
11/27/2018

10. 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

11. 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.

12. 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.

13. 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

14. 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 = V2 = V
C) V1 = V2 = 0
D) V1 = V V2 = 0
11/27/2018

15. 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 > (B) 1 > 3> 2
(C) 1 = 2 = (D) 2 > 3 > 1
(E) 3 > 1 > 2
11/27/2018

16. 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

17. 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

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

19. 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

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

21. 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

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

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

24. 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

25. Earth’s Magnetic Field