31203 Minute Paper

1
3/12/03 Minute Paper

• How does the capacitor behave if it is already
  charged when you turn a switch to connect it to a
  load?
• You take a long time with minute paper sometimes
  of stuff most people know I feel it wastes
  class time.
• Are Capacitor one way? Can you charge them
  backward?
• For surge protecting, wouldnt the RC constant
  destroy the sine wave?
• Check Appendix A for complex number help. It
  helped me a lot.
• Exam time.
• Can I dunk?
• No jokes, sorry.

2
Concept Check RC circuit
Charging Capacitors
Which configuration has the largest final charge
on the capacitor? Which capacitor charges
faster?
Answers (1) same (2) (b)
3
Properties of a Capacitor

• No Sudden Change in voltage, i.e. voltage has to
  change continuously!

Large RC
Rectifying circuit convert ac to dc signal.
4
Example II
28-52P. The flashing lights seen on barricades
at street work and construction sites are driven
by a relaxation oscillator circuit as shown in
the figure, with N being a neon lamp. The lamp is
nonconducting until the voltage across it (And
the capacitor also) rises to a value of 72 V
whereupon it "ignites," dropping its resistance
almost instantly to zero and discharging the
capacitor. Then the lamp again becomes
nonconducting and the process repeats. If the
emf  95 V and C  0.150 microF, what should R be
for the lamp to flash every 0.5 sec?
5
RL Circuit
Switch to position a
t0, i0
tR/L
Switch to position b
Current (iL) must be continuous, i.e. ii-
6
Concept Check
A battery is connected to a solenoid. When the
switch is opened, the light bulb

• Remain off
• Goes off
• Slowly dims out
• Keeps burning as brightly as it did before the
  switch was opened.
• Flares up brightly, then dims and goes out

Answer 5