DC Circuits Lab
Professor Ahmadi， Xin Xu
ECE 1020
George Washington University
Outline
 DC and AC circuit
 Omh’s Law
 Equivalent resistance in series and parallel cuicuits
 Exercises & Homework
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Direct Current (DC) & Alternating
Current (AC)
 DC: uni-directional movement of electric charges
 AC: the direction of the movement of electric charges
reverses periodically
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Ohm’s Law
 Quantifying the relationship between voltage and
current
V IR
 V voltage, volt (V)
I
 I current, amp (A)
 R resistance ohm (Ω)
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V
R
An Example
 Determine the current using Ohm’s Law
V IR
I = ? Amps
3V
3V
R = 1K Ω

=I
x 1000
Ω
Solve for I:
I=3V / 1000 Ω = 3milliAmps
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Resisters in Series
Apply Ohm’s Law on each resister
I
R1
V1  I1  R1
V2  I2  R2
V
V3  I3  R3
R3
Just one path, currents are equal

I1  I2  I3  I
Voltage drops can be added up

R2
V  I1R1  I2 R2  I3 R3  I(R1  R2  R3 )
Equivalent resistance
Req  R1  R2  R3

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Principles for Resisters in Series
 Current remains same in one circle
 Voltage can be added up
 Equivalent Resistance equal to sum of all resisters in
series
I
R1
V
Req  R1  R2  R3
R2
R3
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Examples
Online examples for resistances
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Resisters in Parallel
Apply Ohm’s Law on each resister
I
V1  I1  R1
V2  I2  R2
V3  I3  R3
V
R1
R2
R3
Voltages are same on each path
V1  V2  V3  V



Current is divided to 3 paths
V1 V2 V3
1 1
1
1
 
 V  (   )  V /(
)
1
1
1
R1 R2 R3
R1 R2 R3
 
R1 R2 R3
Equivalent resistance
I  I1  I2  I3 
1
Req 
1 1
1
 
R1 R2 R3
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Principles for Resisters in Parallel
 Voltages remain same in parallel paths
 Current can be added up
I
V
R1
R2
R3
1
Req 
1 1
1
 
R1 R2 R3
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Examples
Online examples for resistances
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Combined
I
Find equivalent resistance for parallel
resistances
R2 R3
V
Req
1
1
 1/(

)
R2 R3
R1
Solve the serial circuit
I
V  I  (R  Req )
Req
V
R1
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Examples
Online examples for resistances
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Exercises
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Exercises – Ohm’s Law
 Given a circuit with a power source V = 9V,find the loop
current if the resistance is 100 Ohm
V  9V,R1  100,I  ?
 Find the loop current by hand, and then build the
circuit using the online interactive simulator

 What is the current if the resistance is doubled
 What is the current if the resistances is reduced by 50%
 What is the current if the voltage is doubled?
 What is the current if the voltage is reduced by 50%
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Exercises - series
 Given a circuit with a power source V = 12V,find the
loop current if resistances are in series
R1  300,R2  150,R3  600
 Find the loop current by hand, and then build the
circuit using the online interactive simulator

 What is the loop current if all resistances are doubled
 What is the loop current if all resistances are reduced
by 50%
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Exercises - parallel
 Given a circuit with a power source V = 12V,find the
loop current I if 2 resistances are connected in parallel
R1  100,R2  100
 Find the current I by hand, and
then build the circuit using the
online interactive simulator
I=?

V=12V R1
R2
 What is the current I if all
resistances are doubled?
 What is the current I if all
resistances are reduced by 50%
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Exercises - parallel
 Given a circuit with a power source V = 12V,find the
loop current if 3 resistances are connected in parallel
R1  100,R2  100,R3  100
 Find the current I by hand, and
then build the circuit using the
online interactive simulator
I=?

V
R1
R2
R3
 What is the current I if all
resistances are doubled
 What is the current I if all
resistances are reduced by 50%
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Exercises – series/parallel
 Given a circuit with a power source V = 5V,find the loop
current I if R1, R2 are connected in parallel, and then
connected to R3 in series
R1  100,R2  100,R3  50
I=?
 Find the current I by hand

R1 R2
V
 Build the circuit using the
online interactive simulator to
verify your results
R3
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Exercises – series/parallel
 Given a circuit with a power source V = 5V,find the loop
current if R1, R2, R3 are connected in parallel, and then
connected to R4 in series
R1  300,R2  300,R3  300,R4  100
I=?
 Find the loop current I
by hand

V
R1
R4
R2
R3
 Build the circuit using
the online interactive
simulator to verify your
results
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Exercises – series/parallel
 Find the loop current I by hand, and then build the
circuit using the online interactive simulator
V  9V
R1  200,R2  50
I=?
R4
V
R3
R3  600,R4  300
R1

R2
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Homework
 Find the loop current I, and voltage and current for
each resistor V1,I1;V2,I2 ;V3,I3 ;V4 ,I4
 Build the circuit using the online interactive simulator
to verify
your results

I=?
R4
V
V  9V
R3
R1  100,R2  100
R3  200,R4  400
R1
R2

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