Lecture 14: DC Circuits, Kirchhoff's laws

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Dr. W. Pezzaglia Physics 2702, Winter 2012 Page 1
CSUEB Physics Lec #14: DC Circuits, Kirchhoff’s Laws 2012Feb29


Lecture 14: DC Circuits, Kirchhoff’s laws
Schedules:
 Feb 20 (Mon) Lecture #11
o Topic: XVI. Voltage, Part B & C (sections 21.1-21.5)
o Recommended: View Mechanical Universe #31 Voltage, Energy, Force
 Feb 22 (Wed): Lecture #12
o Topic: XVII Capacitance (sections 21.6-21.8)
o For RC Circuits read section 23.7
o Recommended: View Mechanical Universe #30 Potential & Capacitance
 Feb 27 (Mon) Lecture #13 (last time)
o Topic: XVIII Electric Current, Ohms Law (Chapter 22)
o Part A Current (22.1, 22.2), Part B Resistance (22.4, 22.5) Part C Power (22.3, 22.6)
o Recommended: View Mechanical Universe #32 The Electric Battery
 Feb 29 (Wed) Lecture #14 (today)
o Topic: XVIII. C.3 Finish Joule Heating (slides 23-24)
o Topic: XIX. DC Circuits, Kirchhoff’s Laws (Chapter 23)
o Recommended: View Mechanical Universe #33 The Electric Circuit
 Mar 5 (Mon) Lecture #10 (next time)
o Quiz on chapters 21, 22, 23. See sample formula sheet at end.

Note:
 Office Hours: Wed after class, but you can also find me in lab Tuesday 12-2:30, Wed
9:20-11:50 and Friday 2:40-5:10 (except in the first half hour). I am usually around
earlier in day before our lectures, but right before class is a bad time.
==================================================================
Exam 1 plot (solutions on blackboard). More or less, the border between A/B is around
45%, between B/C around 33%, below 20% is probably D.


Dr. W. Pezzaglia Physics 2702, Winter 2012 Page 2
CSUEB Physics Lec #14: DC Circuits, Kirchhoff’s Laws 2012Feb29


Sample Quiz Questions on Voltage, E fields, etc

12. Equipotentials are perpendicular to electric field lines,
a. only when the field is constant b. always c. never d. everywhere except on
conductors e. none of these
10. Midway between a proton and electron, we can say
a. voltage is zero b. electric field is zero c. energy density is zero d. all of these e. none of these
11. An electron leaves a cathode (at -50 V), passes through a grid (at -55 V) and ends up at an anode
(at -10 V). The kinetic energy of the electron at the anode is ________ electron volts
a. -40 b. -10 c. -5 d. 40 e. 45
Sample Quiz Questions on Capacitors
1. Two capacitors in series have the same _____;two in parallel have the same _____
a. current, voltage b. charge, voltage c. voltage, current
d. resistivity, conductance e. none of these
7. An isolated capacitor is holding 24 Joules of energy. The plates are pulled apart to twice the
distance. The new energy would be:
(a) 12 (b) 24 (c) 48 (d) need more info (e) none of these
8. An air-spaced capacitor is rated to a maximum of 40 volts. A dielectric is inserted which has K=5,
and dielectric breakdown which is 3 times that of air. The maximum voltage the capacitor can handle is
now,
(a) 600 (b) 200 (c) 120 (d) 24 (e) none of these
9. The energy stored in a capacitor is stored in
a. plates b. charge distribution c. electric field d. dielectric
Sample Quiz Questions on Circuits and Current
1. When we say a particular appliance “uses up” electricity, we really mean that
(a) electric charges are dissipated (b) the voltage is lowered
(c) the energy associated with the electric charges is dissipated
(d) current is dissipated, usually in the form of heat (e) none of these
2. A “Khr=Kilowatt-Hour” is a unit of
(a) Power (b) Electric Field (c) Energy (d) Current (e) none of these
3. The name of a minus pole of a battery is
(a) cathode (b) dynode (c) electrode (d) anode (e) none of these
4. The resistivity of an ideal conductor would be
(a) infinity (b) zero (c) 1 (d) 377 (e) none of these
5. For the measurement of current and voltage of an element in a circuit (e.g. a resistor), you would put
an ammeter in ____ and a voltmeter in _____ with the element.
a. series, series b. parallel, parallel c. parallel, series
d. series, parallel e. none of these
6. A voltmeter of resistance 10,000 ohms reads 72 volts when put across a battery. When a current of
3 amps is being pulled out of the battery the voltmeter reads 60 V. The internal resistance of the
battery is, a. 24 b. 20 c. 4 d. 0.0072 e. none of these
7. A circuit of a battery plus light bulb has an old style ammeter (like you have in lab) inserted. The
meter reads 120 mA. If the meter were removed, the “true” current in the circuit would be ______
120 mA. (e) none of these
(a) slightly more than (b) slightly less than (c) equal to (d) need more info
8. A 4 volt battery (2 ohm resistance) is connected backwards to a 6 volt batter (3 ohm resistance).
The net voltage would be____ and net internal resistance_____.
(a) 10, 5 (b) 2, 5 (c) 10, 1 (d) 2, 1 (e) none of these
9. A 3 volt battery is connected in series to a 4 ohm resistor and 5 uF capacitor. The time constant in
µs (micro-seconds) is?
(a) 12,000,000 (b) 60 (c) 20 (d) 15 (e) none of these
Dr. W. Pezzaglia Physics 2702, Winter 2012 Page 3
CSUEB Physics Lec #14: DC Circuits, Kirchhoff’s Laws 2012Feb29


SAMPLE Quiz FORMULA SHEET (revised Feb 29, 2012)
Sample Formula for Exam:
Quiz covers chapters 21, 22, 23



Constants, all in SI Units

2.54 cm = 1 inch
23
0
1002.6 N
R=N
0
k k=1.3810
-23
J/K
c=3x10
8
m/s G=6.67x10
-11
m
e
=9x10
-31
kg

0
=8.85x10
-12
k=1/(4π
0
)=9x10
9
e=1.6x10
-19
Coul

FORMULAS
kTKE
2
3

4
TA
t
Q




EqF




2
0
4 r
qQ
F


U=QV
r
Q
V
0
4


r
LQ
rE
0
2
/
)(



0
A
Q
E 

2
0
4 r
Q
E



x
V
E
x






cos

 EAEAd



0


Q


CVQ 

21
CCC



21
21
CC
CC
C



d
A
C



VQCVU
2
1
2
2
1


0


e
K

t
V
CI




IRV


t
Q
I




VIP 

JAI


JE



d
nevJ 

Ev
d



QLp 

A
L
R



21
RRR



21
111
RRR


 sin
pEEp 




cos
pEEpU 





x
Ep
F







=RC
2
0
2
1
Eu






  


0000
1exp)(
TTRTTRTR 