Physics 160 Lecture 1 DC Circuits - SCIPP

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Physics160
Physics

160
Lecture 1
DC Circuits
R. Johnson
April 2, 2012
Room Change!
•Starting with the 2
nd
lecture, on Wednesday, we will
meet in Room 231 of the ISB building (close to the
PhiDttffi)
Ph
ys
i
cs
D
epar
t
men
t
o
ffi
ce
)
.
April 2, 2012Physics 1602
Course Organization
•45% of the grade is on the lab work
–20% on homework and 30% on the final exam

5% on classroom
p
artici
p
ation and readin
g

q
uizzes
ppgq
•Be sure to make use of the course web page,
including eCommons. Included are
Clt
t
dtllb

C
omp
l
e
t
e up-
t
o-
d
a
t
e sy
ll
a
b
us
–Course calendar
–Class email
Cltidt

Cl
ass no
ti
ces an
d
announcemen
t
s
–Course handouts (I generally will not hand out hardcopy)
•PSpicetutorial
AClifliiit

AC
ana
l
ys
i
s o
f

li
near c
i
rcu
it
s
–Homework assignments and solutions
–Lecture notes and analysis examples; PSpicedownloads
Iftiltit
April 2, 2012Physics 1603

I
n
f
orma
ti
on on e
l
ec
t
ron
i
cs par
t
s
–Posted grades
Course Organization
•The homework will include problems that require you
to simulate circuits using PSpice.
•You will need to download and install the program
and model libraries into your own Windows PC, or
elseusethePC

sthatwillbesetupinthelab.
else

use

the

PCs

that

will

be

set

up

in

the

lab.
•See my posted tutorial on how to get started. I will
also demonstrate this in lecture.
April 2, 2012Physics 1604
Lab Organization
•One official section:
–M,W 2:00 to 6:00 pm; TA: Peyton Rose
•One additional section:
–F 1:00 to 4:00 pm; staffed by myself

Thiswillalsoserveasmyofficialofficehours,butIamalso
This

will

also

serve

as

my

official

office

hours,

but

I

am

also

generally available in my office in Nat Sci2, Room 323, in
case you have urgent questions.

YouarewelcometousethetimeonFridaystofinishupyour
You

are

welcome

to

use

the

time

on

Fridays

to

finish

up

your

lab work or complete your homework assignment, including
PSpicesimulations.
April 2, 2012Physics 1605
Labs
•This week you must do lab #1, DC circuits. The lab
will be open today through Friday.
Youneedtwoboundlabnotebookswithquadrule

You

need

two

bound

lab

notebooks

with

quad

rule

paper (the same as used in Physics 5 labs).
–Each will be used on alternate weeks, to allow time for
grading
grading
.
•You need a kit of parts from me (provided by Prof.
Kuttner in Room 176).
•The lab writeup, done in the same notebook in which
you take data, will be due by 2 pm the following
Monda
y
.
y
–The writeupmust be neat enough to be easily read and must
demonstrate that you did all measurements, clearly show the
results, and answer all of the questions in the manual.
April 2, 2012Physics 1606

Graphs must be neat, by hand or by computer.
The kit
parts list:
April 2, 2012Physics 1607
Lecture 1 Notes
Vltfttil
diff

V
o
lt
age: measure o
f
po
t
en
ti
a
l

diff
erence
–Voltage “at” a node in a circuit generally means the potential
difference between that point and “ground.”
–Connect a voltmeter between two points to measure the
potential difference
•Ideally the voltmeter should draw negligible current through the
parallel path created.
•Current (Amps): measure of the charge per unit time
flowing
through
adeviceorwire.
flowing

through
a

device

or

wire.
–You have to break the circuit to insert the ammeter, so that
the current flows through the ammeter.
Ideallytheammeterwillhaveanegligiblevoltagedrop

Ideally

the

ammeter

will

have

a

negligible

voltage

drop
.
–You can also measure current by connecting a voltmeter
across a resistor in a circuit and making use of Ohm’s law:
P
PIV
April 2, 2012Physics 1608•
P
ower:
P
=
IV
R
V
I


Measuring Current & Voltage
Must have very
lowresistance, so
itdoesn

tdisturb
it

doesnt

disturb

the circuit.
Must have very
highresistance, so
itd’tditb
April 2, 2012Physics 1609
it

d
oesn
’t

di
s
t
ur
b

the circuit.
Kirchhoff’s loop rule
R1
R
2
I
V
•The sum of potential changes around a closed loop is zero.
•The is simply a special case of the rule for electrostaticfields:
0




dE
Around any closed loop in space.
0
IR
IR
V
V
I
April 2, 2012Physics 16010
0
21

IR
IR
V
21
RR
I


Kirchhoff’s Junction Rule
I3
R1
R2
I2
I
1
2
1
3
I
I
I


2
1
3
This is simply charge conservation for steady currents:

Charge cannot be created or disappear, and
April 2, 2012Physics 16011
•Charge cannot pile up at the junction or any other point in the wire.
Series and Parallel Combinations
•Series: The equivalent resistance is greater than that
of the largest of the resistors.

321eq
RRRR



•Parallel: the equivalent resistance is less than that of
the smallest of the resistors.

1111
R
R
R
R



321敱
R
R
R
R
=
sumofthe

conductances

April 2, 2012Physics 16012

sum

of

the

conductances
Series or Parallel?
R1
R2
April 2, 2012Physics 16013
Series
Series or Parallel?
R2
R1
April 2, 2012Physics 16014
Parallel
Series or Parallel?
Parallel
April 2, 2012Physics 16015
Parallel
Series vs Parallel
•Are R
1
and R2
in series?
•Are the
y
in
p
arallel?
R
1
yp
R
1
R2
R3
Neither!
April 2, 2012Physics 16016
R2
and R3
are in parallel.
What is the difference between these schematics?
Nothing!
April 2, 2012Physics 16017
Equivalent Resistance?
2

6

2


6


ㄲ1
4

㄰1
3 
This wire is ~zero
resistance in parallel
withthe10

牥獩獴潲r
April 2, 2012Physics 16018
with

the

10


牥獩獴潲r
R

㴷=
Circuit Analysis Example
6 
25
4 A
175 V
8 
25



c
㈴⁖
㜠7
㌠3
㤮㤵9
A
㈰2
199 V
V
16.95 A
 7.11
A 95.16
V 199
eq
R
April 2, 2012Physics 16019
3-Loop Circuit
This circuit cannot be reduced through series/parallel combinations.
1.0 
2.0 
I2
I1-I2
14 V
1.0 
I
1
1
I3-I2
2.0 
1.0 
I3
I1-I3
April 2, 2012Physics 16020
Use the Computer to Solve the Equations
April 2, 2012Physics 16021
A Trivial Job for PSpice




4.1
10
14
I
V
R
eq
April 2, 2012Physics 16022
10
1
I
eq
Thévenin Equivalent Circuit
•Any two-terminal network of
resistors and voltage and
currentsourcesisequivalentto
current

sources

is

equivalent

to

a series combination of a single
voltage source and a single
resistor.
resistor.
•Measure or calculate the output voltage VAB
with no load
(open circuit).
)
iit
(
V
V
•Measure or calculate the output current with A shorted to
B (short circuit).
)
c
i
rcu
it
open
(
Th
V
V

)捩牣畩c潰敮(

V
R

April 2, 2012Physics 16023
)circuitshort (
Th
I
R

Norton Equivalent Circuit
•Measure or calculate the output current IAB
when the
outputs are shorted together. This is INo.

Measureorcalculatetheoutputvoltage
V
whenthe

Measure

or

calculate

the

output

voltage

V
AB
when

the

outputs are open.
N
AB
No
I
V
R
•Note:
N
o
I
NoTh
RR
April 2, 2012Physics 16024
NoNoTh
R
I
V
Lab Precautions
•Do not work alone in the lab.
•Never open any chassis that uses A/C power while it
ispluggedin
is

plugged

in
.
–In fact, don’t open any such chassis, period, without talking
to me or the TA first.
Tthttthdidtti
bf

T
urn
th
e me
t
er
t
o
th
e
d
es
i
re
d
se
tti
ng
b
e
f
ore
connecting it to the circuit.
–Otherwise you are likely to blow a fuse or even damage the
t
me
t
er.
•Measure the power supply voltage, and then turn it
off, beforeconnecting it to your circuit.
•Do not make changes to your circuit wiring while it is
powered.

Poweroffallequipmentwhenfinished
April 2, 2012Physics 16025
Power

off

all

equipment

when

finished
.
More Lab Precautions
•The return side of the oscilloscope input must always be
connected to ground.
–Do not try to use the scope to measure the voltage across a
lidfhid
component un
l
ess one s
id
e o
f
t
h
at component
i
s connecte
d
to
ground.
•Do not connect a diode directly across a voltage source.
Diillbblikldtit

D
o
i
ng so w
ill
pro
b
a
bl
y qu
i
c
kl
y
d
es
t
roy
it
.
–Make sure that there is a resistor of at least 1 kohmin series with
the diode.
Ttiitt
b
tblit

T
es
t
your c
i
rcu
it
s
t
age-
b
y-s
t
age as you assem
bl
e
it
.
–Resist the urge to connect everything together all at once and then
turn the power on to see what happens.
ThisisnotsoftwareAbuginyourcircuitcandestroycomponents

This

is

not

software
.
A

bug

in

your

circuit

can

destroy

components

and/or blow fuses!
•No food or drink in the lab, period. This is a strict EH&S
regulationthatfollowsfromtheoccasionaluseofradioactive
April 2, 2012Physics 16026
regulation

that

follows

from

the

occasional

use

of

radioactive

source in the room.
Lab 1 Pointers
•You may have to use a DVM for the current. The black analog
VOM meters don’t seem to have a suitable range.

OrputtheVOMmeteronthe50

A獣慬s慮a捯湮散ca㔶5



灵p

瑨t

噏V

浥瑥m



瑨t



A

獣慬s

慮a

捯湮散c

a

㔶5

牥獩獴潲⁩渠灡牡汬敬⁦潲r愠〮㕭䄠獣慬攠潲‵afor a 5mA scale, etc.
•(Calculated assuming a 5kresistance for the meter winding.)

Don’t connect the meter across a volta
g
e source with it on the
g
ammeter setting, or else the fuse willblow!
•For the diode section, make measurements with the diode both
forward and backward.
–You should be able to make a graph something like Figure 1.67 in
the textbook, except that the negative scale won’t go far enough to
show the breakdown.
Alkdhfjtthitiidlttith

Al
so, ma
k
e a secon
d
grap
h
o
f

j
us
t

th
e pos
iti
ve s
id
e, p
l
o
tti
ng
th
e
logarithm of the current on the vertical scale.
–Be sure to use the 1N4148 (1N914 equivalent) diode (there are 4 in
the kit
),
not one of the zene
r
diodes!
April 2, 2012Physics 16027
),
•My eyes need a magnifying glass to read the labels.
Resistor Color Code
April 2, 2012Physics 16028