Minute Paper (2/16/04) Open circuit vs. short circuit Current direction: high voltage to low...
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Transcript of Minute Paper (2/16/04) Open circuit vs. short circuit Current direction: high voltage to low...
Minute Paper (2/16/04)
• Open circuit vs. short circuit• Current direction: high voltage to low
voltage, except inside the source. • Where do KVL & KCL come from?
– Maxwell eqs. and charge conservation.
Good Lab. Practice
• Make circuits neat• Check the circuit before turning on power, and
turn power if you need change circuit elements.
• Do not touch the circuit with bare hand when power is on.
• Make sure the grounding is good• When using meters, start with large range first• Do not short voltage source or open current
source.
Series Resistors and the Voltage Divider Rule
iiii
RRRi
RiRiRi
vvvV
321
321
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3215.1
, i.... ,
:series in resistorsN for general In
5.1
1N1
1
321
N
nn
N
nnEQ
EQ
EQ
vviiRR
RRRR
iRV
• Series Circuit: One branch same current through each resistor.
NRRRR ,......., :resistors seriesa For 21EQ
VR
Rv
VR
Rv
VR
Rv
EQ
EQ
EQ
5.1
5.1
5.1
33
22
11
Voltage Divider:
Example: voltage divider
If you want to use your car battery (12 V) to power your laptop computer which requires 3V power supplier, how would you do it.
3
123
12
21
21
1
11
RR
RR
R
R
Rv
EQ
Practical voltage sources
Ideal voltage source: rs=0
SLS
LLSL
LS
SS
vRr
RRiv
Rr
vi
The output voltage is now depend on the RL
Make rs << RL, so vL vS , independent on RL
Parallel Resistors and the Current Divider Rule
NRRRR ,......., :resistors parallelFor 21EQ
SEQ
SEQ
SEQ i
R
R
R
vii
R
R
R
vii
R
R
R
vi
333
222
111 , , Current Divider:
321
33
22
11
321
321
111
; ;
:KCL
:KVL
RRRvi
R
vi
R
vi
R
vi
iiii
vvvv
S
S
,v.... ,11
:parallel in resistorsN for general In
1111
1N1
1
321
N
nnS
N
n nEQ
EQ
EQS
iivvRR
RRRR
R
vi
•Parallel Circuit: resistors share the same terminal same voltage on each resistor.
Large current though smaller R
Advantage of parallel circuit: a broken branch will not affect other branches
Practical current sources
Ideal current source: rs=∞
SLS
SL
LS
LSEQ
sL
EQL
iRr
ri
Rr
RrR
iR
Ri
The output current is now depend on the RL
Make rs >> RL, so iL iS , independent on RL
Ammeter
• Ammeter: – measure current, – put in series, – The lower the internal resistance is, the better the
meter is
Voltmeter
• Voltmeter: – measure voltage, – put in parallel, – the higher the internal resistance is, the better the meter is.
An example: parallel circuit
There is a circuit break in circuit that supplies power to your kitchen. The circuit break will be triggered if the total current exceeds 25 A. Can you turn on a 1500W toaster, a 1000W dishwasher, a 1000 W microwave oven, and 1000W refrigerator simultaneously?
A
PPPPi refrigmicrowavedishwashertoaster
5.371000100010001500120
1120120120120
.refrigmicrowavedishwashertoaster iiiii
Concept Check
There are three identical batteries rated 1.5V and max. output current is 200 mA.
• If they are connected in series, – what is total output voltage? – What is max. output current?
• If they are connected in parallel, – what is total output voltage? – What is max. output current?