PHY 184. Spring 2007 Lecture 12. Title: Capacitor calculations. Announcements. Homework Set 3 is due tomorrow morning at 8:00 am. Midterm 1 will take place in class next week on Thursday, February 8. Practice exam will be posted in a few days.
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PHY 184
Spring 2007
Lecture 12
Title: Capacitor calculations
184 Lecture 12
184 Lecture 12
A is the area of each plate
d is the distance between the plates
184 Lecture 12
184 Lecture 12
184 Lecture 12
Note that C depends on geometrical factors.
184 Lecture 12
184 Lecture 12
184 Lecture 12
…makes sense!
184 Lecture 12
184 Lecture 12
…meaning V = q/4pe0R
(we already knew that!)
184 Lecture 12
b=40 mm
a=38 mm
d
A ?
a) Calculate the capacitance.
b) Calculate the area A of a parallel-plate capacitor with the same plate separation and capacitance.
Answers: (a) 84.5 pF; (b) 191 cm2
184 Lecture 12
A) 140 pF
B) 3.5 pF
C) 7 pF
D) 0
184 Lecture 12
The capacitance is the constant of proportionality between change and voltage. It depends on the geometry not on the charge or voltage.
A) C doubles
B) C is cut in half
C) C does not change
184 Lecture 12
184 Lecture 12
.. key point for capacitors in parallel
184 Lecture 12
184 Lecture 12
.. key point for capacitors in series
184 Lecture 12
184 Lecture 12
Ceq = 10 pF
Answer: 90 pC
A) q=90 pC
B) q=1 pC
C) q=3 pC
D) q=180 pC
184 Lecture 12
Ceq = 3 pF
Answer: 30 volts
A) 180 V
B) 10 V
C) 9 V
D) 30 V
184 Lecture 12