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Combination Circuits

Simplifying Resistors in Combination Circuits

3Ω 11Ω

4 Ω 18 Ω

4 Ω18 Ω

12

62

3

4

12V

4

2

3

4

12V

6

3

4

12V

ComboCircuitsQuiz

Now we can analyze other aspects of a circuit…in order to do this we must first simplify.

R1 = 1 ΩR2 = 8 Ω

R3 = 8 Ω

1 = 1 + 1 R2,3 R2 R3

1 = 1 + 1 = 2 = 4 ΩR2,3 8 Ω 8 Ω 8 Ω

R1 = 1 Ω R2,3 = 4 Ω

100 V 0 V

100 V 0 V

Next simplified the circuit down to one resistor.

R1 = 1 Ω R2,3 = 4 Ω100 V 0 V

Series = RT = R1 + R2,3

RT = 1 Ω + 4 Ω

RT = 5 Ω

RT = 5 Ω100 V 0 V

We went from

R1 = 1 ΩR2 = 8 Ω

R3 = 8 Ω

100 V 0 V

toR1 = 1 Ω R2,3 = 4 Ω

100 V 0 V

toRT = 5 Ω

100 V 0 V

Next find the total current flowing through the simplified circuit.

RT = 5 Ω100 V 0 V

IT = VT RT

IT = 100 V 5 Ω

IT = 20 A

Now we can go back and un-simplify the circuit and find the current and voltage through specific resistors…

Find the current and voltage through R1. Hint: Always find current first, then voltage.

R1 = 1 Ω R2,3 = 4 Ω100 V 0 V

RT = 5 Ω100 V 0 V

We already found the current through RT and since this is a series circuit IT = I1 = I2,3 so I1 = 20 A.

Next find voltage…

R1 = 1 Ω R2,3 = 4 Ω100 V 0 V

I1 = 20 A I2,3 = 20 A

V1 = I1R1

In a series circuit VT = V1 + V2,3 so the voltages across both of these resistors will add up to 100 V.

V1 = (20A)(1 Ω)

V1 = 20 V

V2,3 = I2,3R2,3

V2,3 = (20A)(4 Ω)

V2,3 = 80 V

Let’s check to see if the voltages make sense: VT = V1 + V2,3 100 V = 20 V + 80 V

Now we can find the separate currents and voltages through R2 and R3.

R1 = 1 Ω R2,3 = 4 Ω100 V 0 V

I1 = 20 A I2,3 = 20 A

R1 = 1 ΩR2 = 8 Ω

R3 = 8 Ω

100 V 0 V

I1 = 20 A

I2,3 = 20 A

R1 = 1 ΩR2 = 8 Ω

R3 = 8 Ω

100 V 0 V

I1 = 20 A

I2,3 = 20 A

R2 and R3 are in parallel so we know that V2,3 = V2 = V3 therefore

V2 and V3 are 80 V.Now we can find I2 and I3.

I2 = V2,3

R2

I2 = 80 V 8 Ω

I2 = 10 A

I3 = V2,3

R3

I3 = 80 V 8 Ω

I3 = 10 A

V1 = 20 V

V2,3 = 80 V

Let’s try another one…

R3 = 6 Ω

R1 = 4 Ω R2 = 2 Ω

Resistors R1 and R2 are in series so R1,2 = R1 + R2

R1,2 = R1 + R2

R1,2 = 4 Ω + 2 Ω = 6 Ω

R3 = 6 Ω

R1,2 = 6 Ω

100 V 0 V

100 V 0 V

R3 = 6 Ω

R1,2 = 6 Ω

100 V 0 V

1 = 1 + 1 RT R1,2 R3

1 = 1 + 1 = 2 = 3 ΩRT 6 Ω 6 Ω 6 Ω

RT = 3 Ω100 V 0 V

R3 = 6 Ω

R1 = 4 Ω R2 = 2 Ω

100 V 0 V

R3 = 6 Ω

R1,2 = 6 Ω

100 V 0 V

RT = 3 Ω100 V 0 V

What do we do next?

RT = 3 Ω100 V 0 V

IT = VT

RT

IT = 100 V 3 Ω

IT = 33.33 A

Next we un-simplify the circuit and find the rest…

R3 = 6 Ω

R1,2 = 6 Ω

100 V 0 VIT = 33.33 A

Since R1,2 and R3 are in parallel they have voltage in common: VT = V1,2 = V3 therefore

V1,2 and V3 are both 100 V. We can now find each resistors individual current flow, IT = I1,2 + I3.

I1,2 = V1,2

R1,2

I1,2 = 100 V 6 Ω

I1,2 = 16.67 A

I3 = V3

R3

I3 = 100 V 6 Ω

I3 = 16.67 A

Now let’s separate R1 and R2…

R3 = 6 Ω

R1,2 = 6 Ω

100 V 0 V

R3 = 6 Ω

R1 = 4 Ω R2 = 2 Ω

100 V 0 V

I3 = 16.67 A

I1,2 = 16.67 A

R3 = 6 Ω

R1 = 4 Ω R2 = 2 Ω

100 V 0 V

I3 = 16.67 A

I1,2 = 16.67 A

R1 and R2 are in series with each other so they have the same amount of current flowing through them: I1,2 = I1 = I2

therefore the current flowing through both of them is 16.67 A which will help us find the voltage drop across each resistor.

V1 = I1,2R1

V1 = (16.67A)(4 Ω)

V1 = 66.68 V

V2 = I1,2R2

V2 = (16.67A)(2 Ω)

V2 = 33.34 V

Let’s try another…

R1 = 2 ΩR2 = 4 Ω

R3 = 4 Ω

21 V 0 V

R4 = 3 Ω

Remember the steps:1. Simplify the circuit so that there is only 1 resistor.2. Find the total current of the simplified resistor.3. Work backward, un-simplifying the circuit, finding the current and voltage of each resistor along the way.4. Be sure to do this using the correct equations that go with that section of the circuit.

1. Simplify the circuit so that there is only 1 resistor.

R1 = 2 ΩR2 = 4 Ω

R3 = 4 Ω

21 V 0 V

R4 = 3 Ω

RT = 7 Ω21 V 0 V

R1 = 2 Ω21 V 0 V

R2,3 = 2 Ω R4 = 3 Ω

2. Find the total current of the simplified resistor.

RT = 7 Ω21 V 0 V

IT = VT RT

IT = 21 V 7 Ω

IT = 3 A

3. Work backward, un-simplifying the circuit, finding the current and voltage of each resistor along the way.

RT = 7 Ω21 V 0 V

R1 = 2 Ω21 V 0 V

R2,3 = 2 Ω R4 = 3 Ω

I1 = 3 A I2,3 = 3 A I4 = 3 A

V1 = I1R1

V1 = (3 A)(2 Ω)

V1 = 6 V

V2,3 = I2,3R2,3

V2,3 = (3 A)(2 Ω)

V2,3 = 6 V

V4 = I4R4

V4 = (3 A)(3 Ω)

V4 = 9 V

R1 = 2 Ω21 V 0 V

R2,3 = 2 Ω R4 = 3 Ω

I1 = 3 A I2,3 = 3 A I4 = 3 AV1 = 6 V V2,3 = 6 V V4 = 9 V

R1 = 2 ΩR2 = 4 Ω

R3 = 4 Ω

21 V 0 V

R4 = 3 Ω

V2 = 6 V

V3 = 6 V

I2 = V2 R2

I2 = 6 V 4 Ω

I2 = 1.5 A

I3 = V3 R3

I3 = 6 V 4 Ω

I3 = 1.5 A

I2,3 = 3 A

Let’s try another…

R1 = 1 ΩR2 = 3 Ω

R4 = 6 Ω

30 V 0 V

R5 = 1 Ω

Remember the steps:1. Simplify the circuit so that there is only 1 resistor.2. Find the total current of the simplified resistor.3. Work backward, un-simplifying the circuit, finding the current and voltage of each resistor along the way.4. Be sure to do this using the correct equations that go with that section of the circuit.

R3 = 3 Ω

R1 = 1 ΩR2 = 3 Ω

R4 = 6 Ω

30 V 0 V

R5 = 1 ΩR3 = 3 Ω

R1 = 1 ΩR2,3 = 6 Ω

R4 = 6 Ω

30 V 0 V

R5 = 1 Ω

RT = 5 Ω30 V 0 V

R1 = 1 Ω30 V 0 V

R2,3,4 = 3 Ω R5 = 3 Ω

More practice…

a)Which letter shows the graph of voltage vs. current for the smallest resistance?

b) Which letter shows the graph of voltage vs. current for the largest resistance?

a) What is the total resistance of this circuit?

b) What is the total current of this circuit?

c) What is the amount of current running through each resistor?

a) What is the total resistance of this circuit?b) What is the total current of this circuit?

a) Find the equivalent resistance. b) Find the current (IT) going through this circuit. c) Find potential drop across R1 & R2

a) Find combined resistance (RT).b) Find the current in R1.

c) Find I3.

d) Find R2.

e) Find value of the second resistor.

a) If the voltage drop across the 3 ohm resistor is 4 volts, then what would the voltage drop be across the 6 ohm resistor?b) Find the total voltage in this series circuit.c) Find combined resistance in this circuit. d) Find the total current in this circuit.

a) Current in this circuit?b) Potential difference in 20 ohm resistor?c) Equivalent resistance in the circuit?

Find R2.