METHOD #1
R1 = 100 Ω
R2 = 400 Ω
R3 = 330 Ω
Re = equivalent resistance = to be determined
Equivalent Resistance (resistors in parallel)
1/Re = 1/R1 + 1/R2 + 1/R3 = 1/100 + 1/400 + 1/330 = 0.01 + 0.0025 + 0.00303
1/Re = 0.01553
Re = 1/0.01553 = 64.39 Ω
Ohm's Law
I = V/R
I = V/Re = 40 V / 64.39 Ω = 0.6212 A ANSWER
METHOD #2
V1 = voltage across R1 = 40 V
V2 = voltage across R2 = 40 V
V3 = voltage across R3 = 40 V
Ohm's Law
I = V/R
I1 = V1/R1 = 40 V / 100 Ω = 0.4000 A
I2 = V2/R2 = 40 V / 400 Ω = 0.1000 A
I3 = V3/R3 = 40 V / 330 Ω = 0.1212 A
Circuit Current
Itotal = I1 + I2 + I3 = 0.4000 A + 0.1000 A + 0.1212 A = 0.6121 A ANSWER
2. A circuit contains three resistors connected in series. The value of R1 is 200 ohms, the
value of R2 is 640 ohms, and the value of R3 is 500 ohms. The circuit is supplied with 6 VDC.
What is the total voltage drop across the three resistors?
R1 = 200 Ω
R2 = 640 Ω
R3 = 500 Ω
Re = equivalent resistance = to be determined
Equivalent Resistance (resistors in series)
Re = R1 + R2 + R3 = 200 Ω + 640 Ω + 500 Ω = 1,340 Ω
Ohm's Law
I = V/R
Itotal = V/Re = 6 V / 1,340 Ω
Itotal = 0.0045 A
V1 = Itotal x R1 = 0.0045 A x 200 Ω = 0.90 V Voltage Drop Across R1
V2 = Itotal x R2 = 0.0045 A x 640 Ω = 2.88 V Voltage Drop Across R2
V3 = Itotal x R3 = 0.0045 A x 500 Ω = 2.25 V Voltage Drop Across R3
Total Voltage Drop = 6.03 V ≈ 6V Total Voltage Drop
3. A circuit contains two resistors connected in parallel. The value of R1 is 30 ohms and
the value of R2 is 60 ohms. The supply of voltage is 24 VDC. What is the voltage drop
across R2?
V = V1 = V2 = 24 VDC
Since the above equation is true for voltages of resistors in parallel,
then V2, the voltage drop across R2, is 24 VDC. ANSWER
4. A circuit contains two 2,000-ohm resistors connected in parallel. What is the total
resistance of the circuit?
METHOD #1
1/Re = 1/2,000 + 1/2,000 = 0.0005 + 0.0005
1/Re = 0.001
Re = 1/0.001 = 1,000 Ω ANSWER
NOTE: Re = Rtotal
METHOD #2
Rtotal = (R1 x R2)/(R1 + R2) = (2,000 x 2,000)/(2,000 + 2,000) = 4,000,000/4,000
Rtotal = 1,000 Ω ANSWER
