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University Physics Volume 2

10.2 Resistors in Series and Parallel

University Physics Volume 210.2 Resistors in Series and Parallel
  1. Preface
  2. Unit 1. Thermodynamics
    1. 1 Temperature and Heat
      1. Introduction
      2. 1.1 Temperature and Thermal Equilibrium
      3. 1.2 Thermometers and Temperature Scales
      4. 1.3 Thermal Expansion
      5. 1.4 Heat Transfer, Specific Heat, and Calorimetry
      6. 1.5 Phase Changes
      7. 1.6 Mechanisms of Heat Transfer
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    2. 2 The Kinetic Theory of Gases
      1. Introduction
      2. 2.1 Molecular Model of an Ideal Gas
      3. 2.2 Pressure, Temperature, and RMS Speed
      4. 2.3 Heat Capacity and Equipartition of Energy
      5. 2.4 Distribution of Molecular Speeds
      6. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    3. 3 The First Law of Thermodynamics
      1. Introduction
      2. 3.1 Thermodynamic Systems
      3. 3.2 Work, Heat, and Internal Energy
      4. 3.3 First Law of Thermodynamics
      5. 3.4 Thermodynamic Processes
      6. 3.5 Heat Capacities of an Ideal Gas
      7. 3.6 Adiabatic Processes for an Ideal Gas
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 4 The Second Law of Thermodynamics
      1. Introduction
      2. 4.1 Reversible and Irreversible Processes
      3. 4.2 Heat Engines
      4. 4.3 Refrigerators and Heat Pumps
      5. 4.4 Statements of the Second Law of Thermodynamics
      6. 4.5 The Carnot Cycle
      7. 4.6 Entropy
      8. 4.7 Entropy on a Microscopic Scale
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  3. Unit 2. Electricity and Magnetism
    1. 5 Electric Charges and Fields
      1. Introduction
      2. 5.1 Electric Charge
      3. 5.2 Conductors, Insulators, and Charging by Induction
      4. 5.3 Coulomb's Law
      5. 5.4 Electric Field
      6. 5.5 Calculating Electric Fields of Charge Distributions
      7. 5.6 Electric Field Lines
      8. 5.7 Electric Dipoles
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    2. 6 Gauss's Law
      1. Introduction
      2. 6.1 Electric Flux
      3. 6.2 Explaining Gauss’s Law
      4. 6.3 Applying Gauss’s Law
      5. 6.4 Conductors in Electrostatic Equilibrium
      6. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    3. 7 Electric Potential
      1. Introduction
      2. 7.1 Electric Potential Energy
      3. 7.2 Electric Potential and Potential Difference
      4. 7.3 Calculations of Electric Potential
      5. 7.4 Determining Field from Potential
      6. 7.5 Equipotential Surfaces and Conductors
      7. 7.6 Applications of Electrostatics
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 8 Capacitance
      1. Introduction
      2. 8.1 Capacitors and Capacitance
      3. 8.2 Capacitors in Series and in Parallel
      4. 8.3 Energy Stored in a Capacitor
      5. 8.4 Capacitor with a Dielectric
      6. 8.5 Molecular Model of a Dielectric
      7. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    5. 9 Current and Resistance
      1. Introduction
      2. 9.1 Electrical Current
      3. 9.2 Model of Conduction in Metals
      4. 9.3 Resistivity and Resistance
      5. 9.4 Ohm's Law
      6. 9.5 Electrical Energy and Power
      7. 9.6 Superconductors
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    6. 10 Direct-Current Circuits
      1. Introduction
      2. 10.1 Electromotive Force
      3. 10.2 Resistors in Series and Parallel
      4. 10.3 Kirchhoff's Rules
      5. 10.4 Electrical Measuring Instruments
      6. 10.5 RC Circuits
      7. 10.6 Household Wiring and Electrical Safety
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    7. 11 Magnetic Forces and Fields
      1. Introduction
      2. 11.1 Magnetism and Its Historical Discoveries
      3. 11.2 Magnetic Fields and Lines
      4. 11.3 Motion of a Charged Particle in a Magnetic Field
      5. 11.4 Magnetic Force on a Current-Carrying Conductor
      6. 11.5 Force and Torque on a Current Loop
      7. 11.6 The Hall Effect
      8. 11.7 Applications of Magnetic Forces and Fields
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    8. 12 Sources of Magnetic Fields
      1. Introduction
      2. 12.1 The Biot-Savart Law
      3. 12.2 Magnetic Field Due to a Thin Straight Wire
      4. 12.3 Magnetic Force between Two Parallel Currents
      5. 12.4 Magnetic Field of a Current Loop
      6. 12.5 Ampère’s Law
      7. 12.6 Solenoids and Toroids
      8. 12.7 Magnetism in Matter
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    9. 13 Electromagnetic Induction
      1. Introduction
      2. 13.1 Faraday’s Law
      3. 13.2 Lenz's Law
      4. 13.3 Motional Emf
      5. 13.4 Induced Electric Fields
      6. 13.5 Eddy Currents
      7. 13.6 Electric Generators and Back Emf
      8. 13.7 Applications of Electromagnetic Induction
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    10. 14 Inductance
      1. Introduction
      2. 14.1 Mutual Inductance
      3. 14.2 Self-Inductance and Inductors
      4. 14.3 Energy in a Magnetic Field
      5. 14.4 RL Circuits
      6. 14.5 Oscillations in an LC Circuit
      7. 14.6 RLC Series Circuits
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    11. 15 Alternating-Current Circuits
      1. Introduction
      2. 15.1 AC Sources
      3. 15.2 Simple AC Circuits
      4. 15.3 RLC Series Circuits with AC
      5. 15.4 Power in an AC Circuit
      6. 15.5 Resonance in an AC Circuit
      7. 15.6 Transformers
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    12. 16 Electromagnetic Waves
      1. Introduction
      2. 16.1 Maxwell’s Equations and Electromagnetic Waves
      3. 16.2 Plane Electromagnetic Waves
      4. 16.3 Energy Carried by Electromagnetic Waves
      5. 16.4 Momentum and Radiation Pressure
      6. 16.5 The Electromagnetic Spectrum
      7. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  4. A | Units
  5. B | Conversion Factors
  6. C | Fundamental Constants
  7. D | Astronomical Data
  8. E | Mathematical Formulas
  9. F | Chemistry
  10. G | The Greek Alphabet
  11. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
    13. Chapter 13
    14. Chapter 14
    15. Chapter 15
    16. Chapter 16
  12. Index

Learning Objectives

By the end of the section, you will be able to:
  • Define the term equivalent resistance
  • Calculate the equivalent resistance of resistors connected in series
  • Calculate the equivalent resistance of resistors connected in parallel

In Current and Resistance, we described the term ‘resistance’ and explained the basic design of a resistor. Basically, a resistor limits the flow of charge in a circuit and is an ohmic device where V=IR.V=IR. Most circuits have more than one resistor. If several resistors are connected together and connected to a battery, the current supplied by the battery depends on the equivalent resistance of the circuit.

The equivalent resistance of a combination of resistors depends on both their individual values and how they are connected. The simplest combinations of resistors are series and parallel connections (Figure 10.11). In a series circuit, the output current of the first resistor flows into the input of the second resistor; therefore, the current is the same in each resistor. In a parallel circuit, all of the resistor leads on one side of the resistors are connected together and all the leads on the other side are connected together. In the case of a parallel configuration, each resistor has the same potential drop across it, and the currents through each resistor may be different, depending on the resistor. The sum of the individual currents equals the current that flows into the parallel connections.

Par a shows four resistors connected in series and part b shows four resistors connected in parallel.
Figure 10.11 (a) For a series connection of resistors, the current is the same in each resistor. (b) For a parallel connection of resistors, the voltage is the same across each resistor.

Resistors in Series

Resistors are said to be in series whenever the current flows through the resistors sequentially. Consider Figure 10.12, which shows three resistors in series with an applied voltage equal to Vab.Vab. Since there is only one path for the charges to flow through, the current is the same through each resistor. The equivalent resistance of a set of resistors in a series connection is equal to the algebraic sum of the individual resistances.

Part a shows original circuit with three resistors connected in series to a voltage source and part b shows the equivalent circuit with one equivalent resistor connected to the voltage source.
Figure 10.12 (a) Three resistors connected in series to a voltage source. (b) The original circuit is reduced to an equivalent resistance and a voltage source.

In Figure 10.12, the current coming from the voltage source flows through each resistor, so the current through each resistor is the same. The current through the circuit depends on the voltage supplied by the voltage source and the resistance of the resistors. For each resistor, a potential drop occurs that is equal to the loss of electric potential energy as a current travels through each resistor. According to Ohm’s law, the potential drop V across a resistor when a current flows through it is calculated using the equation V=IR,V=IR, where I is the current in amps (A) and R is the resistance in ohms (Ω).(Ω). Since energy is conserved, and the voltage is equal to the potential energy per charge, the sum of the voltage applied to the circuit by the source and the potential drops across the individual resistors around a loop should be equal to zero:

i=1NVi=0.i=1NVi=0.

This equation is often referred to as Kirchhoff’s loop law, which we will look at in more detail later in this chapter. For Figure 10.12, the sum of the potential drop of each resistor and the voltage supplied by the voltage source should equal zero:

VV1V2V3=0,V=V1+V2+V3,=IR1+IR2+IR3,I=VR1+R2+R3=VReq.VV1V2V3=0,V=V1+V2+V3,=IR1+IR2+IR3,I=VR1+R2+R3=VReq.

Since the current through each component is the same, the equality can be simplified to an equivalent resistance, which is just the sum of the resistances of the individual resistors.

Any number of resistors can be connected in series. If N resistors are connected in series, the equivalent resistance is

Req=R1+R2+R3++RN1+RN=i=1NRi.Req=R1+R2+R3++RN1+RN=i=1NRi.
(10.2)

One result of components connected in a series circuit is that if something happens to one component, it affects all the other components. For example, if several lamps are connected in series and one bulb burns out, all the other lamps go dark.

Example 10.2

Equivalent Resistance, Current, and Power in a Series Circuit A battery with a terminal voltage of 9 V is connected to a circuit consisting of four 20-Ω20-Ω and one 10-Ω10-Ω resistors all in series (Figure 10.13). Assume the battery has negligible internal resistance. (a) Calculate the equivalent resistance of the circuit. (b) Calculate the current through each resistor. (c) Calculate the potential drop across each resistor. (d) Determine the total power dissipated by the resistors and the power supplied by the battery.

The figure shows four resistors of 20 Ω and one resistor of 10 Ω connected in series to a 9 V voltage source.
Figure 10.13 A simple series circuit with five resistors.

Strategy In a series circuit, the equivalent resistance is the algebraic sum of the resistances. The current through the circuit can be found from Ohm’s law and is equal to the voltage divided by the equivalent resistance. The potential drop across each resistor can be found using Ohm’s law. The power dissipated by each resistor can be found using P=I2RP=I2R, and the total power dissipated by the resistors is equal to the sum of the power dissipated by each resistor. The power supplied by the battery can be found using P=IεP=Iε.

Solution

  1. The equivalent resistance is the algebraic sum of the resistances:
    Req=R1+R2+R3+R4+R5=20Ω+20Ω+20Ω+20Ω+10Ω=90Ω.Req=R1+R2+R3+R4+R5=20Ω+20Ω+20Ω+20Ω+10Ω=90Ω.
  2. The current through the circuit is the same for each resistor in a series circuit and is equal to the applied voltage divided by the equivalent resistance:
    I=VReq=9V90Ω=0.1A.I=VReq=9V90Ω=0.1A.
  3. The potential drop across each resistor can be found using Ohm’s law:
    V1=V2=V3=V4=(0.1A)20Ω=2V,V5=(0.1A)10Ω=1V,V1+V2+V3+V4+V5=9V.V1=V2=V3=V4=(0.1A)20Ω=2V,V5=(0.1A)10Ω=1V,V1+V2+V3+V4+V5=9V.

    Note that the sum of the potential drops across each resistor is equal to the voltage supplied by the battery.
  4. The power dissipated by a resistor is equal to P=I2RP=I2R, and the power supplied by the battery is equal to P=IεP=Iε:
    P1=P2=P3=P4=(0.1A)2(20Ω)=0.2W,P5=(0.1A)2(10Ω)=0.1W,Pdissipated=0.2W+0.2W+0.2W+0.2W+0.1W=0.9W,Psource=Iε=(0.1A)(9V)=0.9W.P1=P2=P3=P4=(0.1A)2(20Ω)=0.2W,P5=(0.1A)2(10Ω)=0.1W,Pdissipated=0.2W+0.2W+0.2W+0.2W+0.1W=0.9W,Psource=Iε=(0.1A)(9V)=0.9W.

Significance There are several reasons why we would use multiple resistors instead of just one resistor with a resistance equal to the equivalent resistance of the circuit. Perhaps a resistor of the required size is not available, or we need to dissipate the heat generated, or we want to minimize the cost of resistors. Each resistor may cost a few cents to a few dollars, but when multiplied by thousands of units, the cost saving may be appreciable.

Check Your Understanding 10.2

Some strings of miniature holiday lights are made to short out when a bulb burns out. The device that causes the short is called a shunt, which allows current to flow around the open circuit. A “short” is like putting a piece of wire across the component. The bulbs are usually grouped in series of nine bulbs. If too many bulbs burn out, the shunts eventually open. What causes this?

Let’s briefly summarize the major features of resistors in series:

  1. Series resistances add together to get the equivalent resistance:
    Req=R1+R2+R3++RN1+RN=i=1NRi.Req=R1+R2+R3++RN1+RN=i=1NRi.
  2. The same current flows through each resistor in series.
  3. Individual resistors in series do not get the total source voltage, but divide it. The total potential drop across a series configuration of resistors is equal to the sum of the potential drops across each resistor.

Resistors in Parallel

Figure 10.14 shows resistors in parallel, wired to a voltage source. Resistors are in parallel when one end of all the resistors are connected by a continuous wire of negligible resistance and the other end of all the resistors are also connected to one another through a continuous wire of negligible resistance. The potential drop across each resistor is the same. Current through each resistor can be found using Ohm’s law I=V/R,I=V/R, where the voltage is constant across each resistor. For example, an automobile’s headlights, radio, and other systems are wired in parallel, so that each subsystem utilizes the full voltage of the source and can operate completely independently. The same is true of the wiring in your house or any building.

Part a shows original circuit with two resistors connected in parallel to a voltage source and part b shows the equivalent circuit with one equivalent resistor connected to the voltage source.
Figure 10.14 (a) Two resistors connected in parallel to a voltage source. (b) The original circuit is reduced to an equivalent resistance and a voltage source.

The current flowing from the voltage source in Figure 10.14 depends on the voltage supplied by the voltage source and the equivalent resistance of the circuit. In this case, the current flows from the voltage source and enters a junction, or node, where the circuit splits flowing through resistors R1R1 and R2R2. As the charges flow from the battery, some go through resistor R1R1 and some flow through resistor R2.R2. The sum of the currents flowing into a junction must be equal to the sum of the currents flowing out of the junction:

Iin= Iout. Iin= Iout.

This equation is referred to as Kirchhoff’s junction rule and will be discussed in detail in the next section. In Figure 10.14, the junction rule gives I=I1+I2I=I1+I2. There are two loops in this circuit, which leads to the equations V=I1R1V=I1R1 and I1R1=I2R2I1R1=I2R2. Note the voltage across the resistors in parallel are the same (V=V1=V2)(V=V1=V2) and the current is additive:

I=I1+I2=V1R1+V2R2=VR1+VR2=V(1R1+1R2)=VReqReq=(1R1+1R2)−1.I=I1+I2=V1R1+V2R2=VR1+VR2=V(1R1+1R2)=VReqReq=(1R1+1R2)−1.

Generalizing to any number of N resistors, the equivalent resistance ReqReq of a parallel connection is related to the individual resistances by

Req=(1R1+1R2+1R3++1RN1+1RN)−1=(i=1N1Ri)−1.Req=(1R1+1R2+1R3++1RN1+1RN)−1=(i=1N1Ri)−1.
(10.3)

This relationship results in an equivalent resistance ReqReq that is less than the smallest of the individual resistances. When resistors are connected in parallel, more current flows from the source than would flow for any of them individually, so the total resistance is lower.

Example 10.3

Analysis of a Parallel Circuit Three resistors R1=1.00Ω,R2=2.00Ω,R1=1.00Ω,R2=2.00Ω, and R3=2.00Ω,R3=2.00Ω, are connected in parallel. The parallel connection is attached to a V=3.00VV=3.00V voltage source. (a) What is the equivalent resistance? (b) Find the current supplied by the source to the parallel circuit. (c) Calculate the currents in each resistor and show that these add together to equal the current output of the source. (d) Calculate the power dissipated by each resistor. (e) Find the power output of the source and show that it equals the total power dissipated by the resistors.

Strategy (a) The total resistance for a parallel combination of resistors is found using Req=(i1Ri)−1Req=(i1Ri)−1.

(Note that in these calculations, each intermediate answer is shown with an extra digit.)

(b) The current supplied by the source can be found from Ohm’s law, substituting ReqReq for the total resistance I=VReq.I=VReq.

(c) The individual currents are easily calculated from Ohm’s law (Ii=ViRi)(Ii=ViRi), since each resistor gets the full voltage. The total current is the sum of the individual currents: I=iIi.I=iIi.

(d) The power dissipated by each resistor can be found using any of the equations relating power to current, voltage, and resistance, since all three are known. Let us use Pi=V2/Ri,Pi=V2/Ri, since each resistor gets full voltage.

(e) The total power can also be calculated in several ways, use P=IVP=IV.

Solution

  1. The total resistance for a parallel combination of resistors is found using Equation 10.3. Entering known values gives
    Req=(1R1+1R2+1R3)−1=(11.00Ω+12.00Ω+12.00Ω)−1=0.50Ω.Req=(1R1+1R2+1R3)−1=(11.00Ω+12.00Ω+12.00Ω)−1=0.50Ω.

    The total resistance with the correct number of significant digits is Req=0.50Ω.Req=0.50Ω. As predicted, ReqReq is less than the smallest individual resistance.
  2. The total current can be found from Ohm’s law, substituting ReqReq for the total resistance. This gives
    I=VReq=3.00V0.50Ω=6.00A.I=VReq=3.00V0.50Ω=6.00A.

    Current I for each device is much larger than for the same devices connected in series (see the previous example). A circuit with parallel connections has a smaller total resistance than the resistors connected in series.
  3. The individual currents are easily calculated from Ohm’s law, since each resistor gets the full voltage. Thus,
    I1=VR1=3.00V1.00Ω=3.00A.I1=VR1=3.00V1.00Ω=3.00A.

    Similarly,
    I2=VR2=3.00V2.00Ω=1.50AI2=VR2=3.00V2.00Ω=1.50A

    and
    I3=VR3=6.00V2.00Ω=1.50A.I3=VR3=6.00V2.00Ω=1.50A.

    The total current is the sum of the individual currents:
    I1+I2+I3=6.00A.I1+I2+I3=6.00A.
  4. The power dissipated by each resistor can be found using any of the equations relating power to current, voltage, and resistance, since all three are known. Let us use P=V2/R,P=V2/R, since each resistor gets full voltage. Thus,
    P1=V2R1=(3.00V)21.00Ω=9.00W.P1=V2R1=(3.00V)21.00Ω=9.00W.

    Similarly,
    P2=V2R2=(3.00V)22.00Ω=4.50WP2=V2R2=(3.00V)22.00Ω=4.50W

    and
    P3=V2R3=(3.00V)22.00Ω=4.50W.P3=V2R3=(3.00V)22.00Ω=4.50W.
  5. The total power can also be calculated in several ways. Choosing P=IVP=IV and entering the total current yields
    P=IV=(6.00A)(3.00V)=18.00W.P=IV=(6.00A)(3.00V)=18.00W.

Significance Total power dissipated by the resistors is also 18.00 W:

P1+P2+P3=9.00W+4.50W+4.50W=18.00W.P1+P2+P3=9.00W+4.50W+4.50W=18.00W.

Notice that the total power dissipated by the resistors equals the power supplied by the source.

Check Your Understanding 10.3

Consider the same potential difference (V=3.00V)(V=3.00V) applied to the same three resistors connected in series. Would the equivalent resistance of the series circuit be higher, lower, or equal to the three resistor in parallel? Would the current through the series circuit be higher, lower, or equal to the current provided by the same voltage applied to the parallel circuit? How would the power dissipated by the resistor in series compare to the power dissipated by the resistors in parallel?

Check Your Understanding 10.4

How would you use a river and two waterfalls to model a parallel configuration of two resistors? How does this analogy break down?

Let us summarize the major features of resistors in parallel:

  1. Equivalent resistance is found from
    Req=(1R1+1R2+1R3++1RN1+1RN)−1=(i=1N1Ri)−1,Req=(1R1+1R2+1R3++1RN1+1RN)−1=(i=1N1Ri)−1,

    and is smaller than any individual resistance in the combination.
  2. The potential drop across each resistor in parallel is the same.
  3. Parallel resistors do not each get the total current; they divide it. The current entering a parallel combination of resistors is equal to the sum of the current through each resistor in parallel.

In this chapter, we introduced the equivalent resistance of resistors connect in series and resistors connected in parallel. You may recall that in Capacitance, we introduced the equivalent capacitance of capacitors connected in series and parallel. Circuits often contain both capacitors and resistors. Table 10.1 summarizes the equations used for the equivalent resistance and equivalent capacitance for series and parallel connections.

  Series combination Parallel combination
Equivalent capacitance 1Ceq=1C1+1C2+1C3+1Ceq=1C1+1C2+1C3+ Ceq=C1+C2+C3+Ceq=C1+C2+C3+
Equivalent resistance Req=R1+R2+R3+=i=1NRiReq=R1+R2+R3+=i=1NRi 1Req=1R1+1R2+1R3+1Req=1R1+1R2+1R3+
Table 10.1 Summary for Equivalent Resistance and Capacitance in Series and Parallel Combinations

Combinations of Series and Parallel

More complex connections of resistors are often just combinations of series and parallel connections. Such combinations are common, especially when wire resistance is considered. In that case, wire resistance is in series with other resistances that are in parallel.

Combinations of series and parallel can be reduced to a single equivalent resistance using the technique illustrated in Figure 10.15. Various parts can be identified as either series or parallel connections, reduced to their equivalent resistances, and then further reduced until a single equivalent resistance is left. The process is more time consuming than difficult. Here, we note the equivalent resistance as Req.Req.

Part a shows a circuit with four resistors and a voltage source. The positive terminal of voltage source of 24 V is connected to resistor R subscript 1 of 7 Ω which is connected to two parallel branches. The first branch has resistor R subscript 2 of 10 Ω and the other branch has resistor R subscript 3 of 6 Ω in series with resistors R subscript 4 of 4 Ω. Parts b to e of the figure show the steps to simplify the circuit to an equivalent circuit with one equivalent resistor and voltage source.
Figure 10.15 (a) The original circuit of four resistors. (b) Step 1: The resistors R3R3 and R4R4 are in series and the equivalent resistance is R34=10Ω.R34=10Ω. (c) Step 2: The reduced circuit shows resistors R2R2 and R34R34 are in parallel, with an equivalent resistance of R234=5Ω.R234=5Ω. (d) Step 3: The reduced circuit shows that R1R1 and R234R234 are in series with an equivalent resistance of R1234=12Ω,R1234=12Ω, which is the equivalent resistance Req.Req. (e) The reduced circuit with a voltage source of V=24VV=24V with an equivalent resistance of Req=12Ω.Req=12Ω. This results in a current of I=2AI=2A from the voltage source.

Notice that resistors R3R3 and R4R4 are in series. They can be combined into a single equivalent resistance. One method of keeping track of the process is to include the resistors as subscripts. Here the equivalent resistance of R3R3 and R4R4 is

R34=R3+R4=6Ω+4Ω=10Ω.R34=R3+R4=6Ω+4Ω=10Ω.

The circuit now reduces to three resistors, shown in Figure 10.15(c). Redrawing, we now see that resistors R2R2 and R34R34 constitute a parallel circuit. Those two resistors can be reduced to an equivalent resistance:

R234=(1R2+1R34)−1=(110Ω+110Ω)−1=5Ω.R234=(1R2+1R34)−1=(110Ω+110Ω)−1=5Ω.

This step of the process reduces the circuit to two resistors, shown in in Figure 10.15(d). Here, the circuit reduces to two resistors, which in this case are in series. These two resistors can be reduced to an equivalent resistance, which is the equivalent resistance of the circuit:

Req=R1234=R1+R234=7Ω+5Ω=12Ω.Req=R1234=R1+R234=7Ω+5Ω=12Ω.

The main goal of this circuit analysis is reached, and the circuit is now reduced to a single resistor and single voltage source.

Now we can analyze the circuit. The current provided by the voltage source is I=VReq=24V12Ω=2A.I=VReq=24V12Ω=2A. This current runs through resistor R1R1 and is designated as I1.I1. The potential drop across R1R1 can be found using Ohm’s law:

V1=I1R1=(2A)(7Ω)=14V.V1=I1R1=(2A)(7Ω)=14V.

Looking at Figure 10.15(c), this leaves 24V14V=10V24V14V=10V to be dropped across the parallel combination of R2R2 and R34.R34. The current through R2R2 can be found using Ohm’s law:

I2=V2R2=10V10Ω=1A.I2=V2R2=10V10Ω=1A.

The resistors R3R3 and R4R4 are in series so the currents I3I3 and I4I4 are equal to

I3=I4=II2=2A1A=1A.I3=I4=II2=2A1A=1A.

Using Ohm’s law, we can find the potential drop across the last two resistors. The potential drops are V3=I3R3=6VV3=I3R3=6V and V4=I4R4=4V.V4=I4R4=4V. The final analysis is to look at the power supplied by the voltage source and the power dissipated by the resistors. The power dissipated by the resistors is

P1=I12R1=(2A)2(7Ω)=28W,P2=I22R2=(1A)2(10Ω)=10W,P3=I32R3=(1A)2(6Ω)=6W,P4=I42R4=(1A)2(4Ω)=4W,Pdissipated=P1+P2+P3+P4=48W.P1=I12R1=(2A)2(7Ω)=28W,P2=I22R2=(1A)2(10Ω)=10W,P3=I32R3=(1A)2(6Ω)=6W,P4=I42R4=(1A)2(4Ω)=4W,Pdissipated=P1+P2+P3+P4=48W.

The total energy is constant in any process. Therefore, the power supplied by the voltage source is Ps=IV=(2A)(24V)=48W.Ps=IV=(2A)(24V)=48W. Analyzing the power supplied to the circuit and the power dissipated by the resistors is a good check for the validity of the analysis; they should be equal.

Example 10.4

Combining Series and Parallel Circuits Figure 10.16 shows resistors wired in a combination of series and parallel. We can consider R1R1 to be the resistance of wires leading to R2R2 and R3.R3. (a) Find the equivalent resistance of the circuit. (b) What is the potential drop V1V1 across resistor R1R1? (c) Find the current I2I2 through resistor R2R2. (d) What power is dissipated by R2R2?

The figure shows a circuit with three resistors and a voltage source. The positive terminal of voltage source of 12 V is connected to R subscript 1 of 1 Ω with left current I subscript 1 connected to two parallel resistors R subscript 2 of 6 Ω with downward current I subscript 2 and R subscript 3 of 13 Ω
Figure 10.16 These three resistors are connected to a voltage source so that R2R2 and R3R3 are in parallel with one another and that combination is in series with R1.R1.

Strategy (a) To find the equivalent resistance, first find the equivalent resistance of the parallel connection of R2R2 and R3.R3. Then use this result to find the equivalent resistance of the series connection with R1.R1.

(b) The current through R1R1 can be found using Ohm’s law and the voltage applied. The current through R1R1 is equal to the current from the battery. The potential drop V1V1 across the resistor R1R1 (which represents the resistance in the connecting wires) can be found using Ohm’s law.

(c) The current through R2R2 can be found using Ohm’s law I2=V2R2.I2=V2R2. The voltage across R2R2 can be found using V2=VV1.V2=VV1.

(d) Using Ohm’s law (V2=I2R2)(V2=I2R2), the power dissipated by the resistor can also be found usingP2=I22R2=V22R2P2=I22R2=V22R2.

Solution

  1. To find the equivalent resistance of the circuit, notice that the parallel connection of R2R2 and R3R3 is in series with R1R1, so the equivalent resistance is
    Req=R1+(1R2+1R3)−1=1.00Ω+(16.00Ω+113.00Ω)−1=5.10Ω.Req=R1+(1R2+1R3)−1=1.00Ω+(16.00Ω+113.00Ω)−1=5.10Ω.

    The total resistance of this combination is intermediate between the pure series and pure parallel values (20.0Ω20.0Ω and 0.804Ω0.804Ω, respectively).
  2. The current through R1R1 is equal to the current supplied by the battery:
    I1=I=VReq=12.0V5.10Ω=2.35A.I1=I=VReq=12.0V5.10Ω=2.35A.

    The voltage across R1R1 is
    V1=I1R1=(2.35A)(1Ω)=2.35V.V1=I1R1=(2.35A)(1Ω)=2.35V.

    The voltage applied to R2R2 and R3R3 is less than the voltage supplied by the battery by an amount V1.V1. When wire resistance is large, it can significantly affect the operation of the devices represented by R2R2 and R3R3.
  3. To find the current through R2R2, we must first find the voltage applied to it. The voltage across the two resistors in parallel is the same:
    V2=V3=VV1=12.0V2.35V=9.65V.V2=V3=VV1=12.0V2.35V=9.65V.

    Now we can find the current I2I2 through resistance R2R2 using Ohm’s law:
    I2=V2R2=9.65V6.00Ω=1.61A.I2=V2R2=9.65V6.00Ω=1.61A.

    The current is less than the 2.00 A that flowed through R2R2 when it was connected in parallel to the battery in the previous parallel circuit example.
  4. The power dissipated by R2R2 is given by
    P2=I22R2=(1.61A)2(6.00Ω)=15.5W.P2=I22R2=(1.61A)2(6.00Ω)=15.5W.

Significance The analysis of complex circuits can often be simplified by reducing the circuit to a voltage source and an equivalent resistance. Even if the entire circuit cannot be reduced to a single voltage source and a single equivalent resistance, portions of the circuit may be reduced, greatly simplifying the analysis.

Check Your Understanding 10.5

Consider the electrical circuits in your home. Give at least two examples of circuits that must use a combination of series and parallel circuits to operate efficiently.

Practical Implications

One implication of this last example is that resistance in wires reduces the current and power delivered to a resistor. If wire resistance is relatively large, as in a worn (or a very long) extension cord, then this loss can be significant. If a large current is drawn, the IR drop in the wires can also be significant and may become apparent from the heat generated in the cord.

For example, when you are rummaging in the refrigerator and the motor comes on, the refrigerator light dims momentarily. Similarly, you can see the passenger compartment light dim when you start the engine of your car (although this may be due to resistance inside the battery itself).

What is happening in these high-current situations is illustrated in Figure 10.17. The device represented by R3R3 has a very low resistance, so when it is switched on, a large current flows. This increased current causes a larger IR drop in the wires represented by R1R1, reducing the voltage across the light bulb (which is R2R2), which then dims noticeably.

The figure shows schematic of a refrigerator.
Figure 10.17 Why do lights dim when a large appliance is switched on? The answer is that the large current the appliance motor draws causes a significant IR drop in the wires and reduces the voltage across the light.

Problem-Solving Strategy: Series and Parallel Resistors

  1. Draw a clear circuit diagram, labeling all resistors and voltage sources. This step includes a list of the known values for the problem, since they are labeled in your circuit diagram.
  2. Identify exactly what needs to be determined in the problem (identify the unknowns). A written list is useful.
  3. Determine whether resistors are in series, parallel, or a combination of both series and parallel. Examine the circuit diagram to make this assessment. Resistors are in series if the same current must pass sequentially through them.
  4. Use the appropriate list of major features for series or parallel connections to solve for the unknowns. There is one list for series and another for parallel.
  5. Check to see whether the answers are reasonable and consistent.

Example 10.5

Combining Series and Parallel Circuits Two resistors connected in series (R1,R2)(R1,R2) are connected to two resistors that are connected in parallel (R3,R4)(R3,R4). The series-parallel combination is connected to a battery. Each resistor has a resistance of 10.00 Ohms. The wires connecting the resistors and battery have negligible resistance. A current of 2.00 Amps runs through resistor R1.R1. What is the voltage supplied by the voltage source?

Strategy Use the steps in the preceding problem-solving strategy to find the solution for this example.

Solution

  1. Draw a clear circuit diagram (Figure 10.18).
    The figure shows a circuit with four resistors and a voltage source. The positive terminal of voltage source is connected to resistor R subscript 1 of 10 Ω with right current I subscript 1 of 2 A connected in series to resistor R subscript 2 of 10 Ω connected in series to two parallel resistors R subscript 3 of 10 Ω and R subscript 4 of 10 Ω.
    Figure 10.18 To find the unknown voltage, we must first find the equivalent resistance of the circuit.
  2. The unknown is the voltage of the battery. In order to find the voltage supplied by the battery, the equivalent resistance must be found.
  3. In this circuit, we already know that the resistors R1R1 and R2R2 are in series and the resistors R3R3 and R4R4 are in parallel. The equivalent resistance of the parallel configuration of the resistors R3R3 and R4R4 is in series with the series configuration of resistors R1R1 and R2R2.
  4. The voltage supplied by the battery can be found by multiplying the current from the battery and the equivalent resistance of the circuit. The current from the battery is equal to the current through R1R1 and is equal to 2.00 A. We need to find the equivalent resistance by reducing the circuit. To reduce the circuit, first consider the two resistors in parallel. The equivalent resistance is R34=(110.00Ω+110.00Ω)−1=5.00Ω.R34=(110.00Ω+110.00Ω)−1=5.00Ω. This parallel combination is in series with the other two resistors, so the equivalent resistance of the circuit is Req=R1+R2+R34=25.00Ω.Req=R1+R2+R34=25.00Ω. The voltage supplied by the battery is therefore V=IReq=2.00A(25.00Ω)=50.00V.V=IReq=2.00A(25.00Ω)=50.00V.
  5. One way to check the consistency of your results is to calculate the power supplied by the battery and the power dissipated by the resistors. The power supplied by the battery is Pbatt=IV=100.00W.Pbatt=IV=100.00W.
    Since they are in series, the current through R2R2 equals the current through R1.R1. Since R3=R4R3=R4, the current through each will be 1.00 Amps. The power dissipated by the resistors is equal to the sum of the power dissipated by each resistor:
    P=I12R1+I22R2+I32R3+I42R4=40.00W+40.00W+10.00W+10.00W=100.00W.P=I12R1+I22R2+I32R3+I42R4=40.00W+40.00W+10.00W+10.00W=100.00W.

    Since the power dissipated by the resistors equals the power supplied by the battery, our solution seems consistent.

Significance If a problem has a combination of series and parallel, as in this example, it can be reduced in steps by using the preceding problem-solving strategy and by considering individual groups of series or parallel connections. When finding ReqReq for a parallel connection, the reciprocal must be taken with care. In addition, units and numerical results must be reasonable. Equivalent series resistance should be greater, whereas equivalent parallel resistance should be smaller, for example. Power should be greater for the same devices in parallel compared with series, and so on.

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