What is the equivalent resistance in this circuit?

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

What is the equivalent resistance in this circuit?
. 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 6.2.1). 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.

(Figure 6.2.1)  

What is the equivalent resistance in this circuit?
Figure 6.2.1 (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 are said to be in series whenever the current flows through the resistors sequentially. Consider Figure 6.2.2, which shows three resistors in series with an applied voltage equal to

What is the equivalent resistance in this circuit?
. 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.

(Figure 6.2.2)  

What is the equivalent resistance in this circuit?
Figure 6.2.2 (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 6.2.2, 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

What is the equivalent resistance in this circuit?
 across a resistor when a current flows through it is calculated using the equation
What is the equivalent resistance in this circuit?
, where  is the current in amps (
What is the equivalent resistance in this circuit?
) and
What is the equivalent resistance in this circuit?
 is the resistance in ohms (
What is the equivalent resistance in this circuit?
). 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:

   

What is the equivalent resistance in this circuit?

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 6.2.2, the sum of the potential drop of each resistor and the voltage supplied by the voltage source should equal zero:

   

What is the equivalent resistance in this circuit?

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

What is the equivalent resistance in this circuit?
 resistors are connected in series, the equivalent resistance is

(6.2.1)  

What is the equivalent resistance in this circuit?

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.

A battery with a terminal voltage of

What is the equivalent resistance in this circuit?
is connected to a circuit consisting of four
What is the equivalent resistance in this circuit?
 and one 
What is the equivalent resistance in this circuit?
 resistors all in series (Figure 6.2.3). 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.

(Figure 6.2.3)  

What is the equivalent resistance in this circuit?
Figure 6.2.3 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

What is the equivalent resistance in this circuit?
, 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
What is the equivalent resistance in this circuit?
.

Solution

a.    The equivalent resistance is the algebraic sum of the resistances:

   

What is the equivalent resistance in this circuit?

b.    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:

   

What is the equivalent resistance in this circuit?

c.    The potential drop across each resistor can be found using Ohm’s law:

   

What is the equivalent resistance in this circuit?

   

What is the equivalent resistance in this circuit?

   

What is the equivalent resistance in this circuit?

Note that the sum of the potential drops across each resistor is equal to the voltage supplied by the battery.

d.    The power dissipated by a resistor is equal to

What is the equivalent resistance in this circuit?
, and the power supplied by the battery is equal to
What is the equivalent resistance in this circuit?
:

   

What is the equivalent resistance in this circuit?

   

What is the equivalent resistance in this circuit?

   

What is the equivalent resistance in this circuit?

   

What is the equivalent resistance in this circuit?

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.

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:

       

    What is the equivalent resistance in this circuit?

  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.

Figure 6.2.4shows 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

What is the equivalent resistance in this circuit?
, 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.

(Figure 6.2.4)  

What is the equivalent resistance in this circuit?
Figure 6.2.4 (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 6.2.4 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

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
. As the charges flow from the battery, some go through resistor
What is the equivalent resistance in this circuit?
 and some flow through resistor
What is the equivalent resistance in this circuit?
. The sum of the currents flowing into a junction must be equal to the sum of the currents flowing out of the junction:

   

What is the equivalent resistance in this circuit?

This equation is referred to as Kirchhoff’s junction rule and will be discussed in detail in the next section. In Figure 6.2.4, the junction rule gives

What is the equivalent resistance in this circuit?
. There are two loops in this circuit, which leads to the equations
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 Note the voltage across the resistors in parallel are the same (
What is the equivalent resistance in this circuit?
) and the current is additive:

   

What is the equivalent resistance in this circuit?

Generalizing to any number of

What is the equivalent resistance in this circuit?
 resistors, the equivalent resistance
What is the equivalent resistance in this circuit?
 of a parallel connection is related to the individual resistances by 

(6.2.2)  

What is the equivalent resistance in this circuit?

This relationship results in an equivalent resistance

What is the equivalent resistance in this circuit?
 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.

Three resistors

What is the equivalent resistance in this circuit?
What is the equivalent resistance in this circuit?
, and 
What is the equivalent resistance in this circuit?
 are connected in parallel. The parallel connection is attached to a
What is the equivalent resistance in this circuit?
 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

What is the equivalent resistance in this circuit?
.
(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

What is the equivalent resistance in this circuit?
 for the total resistance
What is the equivalent resistance in this circuit?
.

(c) The individual currents are easily calculated from Ohm’s law

What is the equivalent resistance in this circuit?
, since each resistor gets the full voltage. The total current is the sum of the individual currents:
What is the equivalent resistance in this circuit?
.

(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

What is the equivalent resistance in this circuit?
, since each resistor gets full voltage.

(e) The total power can also be calculated in several ways, use

What is the equivalent resistance in this circuit?
.

Solution

a.    The total resistance for a parallel combination of resistors is found using Equation 6.2.2. Entering known values gives

   

What is the equivalent resistance in this circuit?

The total resistance with the correct number of significant digits is

What is the equivalent resistance in this circuit?
. As predicted,
What is the equivalent resistance in this circuit?
 is less than the smallest individual resistance.

b.    The total current can be found from Ohm’s law, substituting

What is the equivalent resistance in this circuit?
 for the total resistance. This gives

   

What is the equivalent resistance in this circuit?

Current 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.

c.    The individual currents are easily calculated from Ohm’s law, since each resistor gets the full voltage. Thus,

   

What is the equivalent resistance in this circuit?

Similarly,

   

What is the equivalent resistance in this circuit?

and

   

What is the equivalent resistance in this circuit?

The total current is the sum of the individual currents:

   

What is the equivalent resistance in this circuit?

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

What is the equivalent resistance in this circuit?
, since each resistor gets full voltage. Thus,

   

What is the equivalent resistance in this circuit?

Similarly,

   

What is the equivalent resistance in this circuit?

and

   

What is the equivalent resistance in this circuit?

e.    The total power can also be calculated in several ways. Choosing

What is the equivalent resistance in this circuit?
 and entering the total current yields

   

What is the equivalent resistance in this circuit?

Significance

Total power dissipated by the resistors is also

What is the equivalent resistance in this circuit?
:

   

What is the equivalent resistance in this circuit?

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

Consider the same potential difference

What is the equivalent resistance in this circuit?
 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?

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

       

    What is the equivalent resistance in this circuit?

    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 6.2.1 summarizes the equations used for the equivalent resistance and equivalent capacitance for series and parallel connections.

(Table 6.2.1)  

Series combination Parallel combination
Equivalent capacitance
What is the equivalent resistance in this circuit?
What is the equivalent resistance in this circuit?
Equivalent resistance
What is the equivalent resistance in this circuit?
What is the equivalent resistance in this circuit?

Table10.1 Summary for Equivalent Resistance and Capacitance in Series and Parallel Combinations

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 inFigure 6.2.5. 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

What is the equivalent resistance in this circuit?
.

(Figure 6.2.5)  

What is the equivalent resistance in this circuit?
Figure 6.2.5 (a) The original circuit of four resistors. (b) Step 1: The resistors
What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
are in series and the equivalent resistance is
What is the equivalent resistance in this circuit?
. (c) Step 2: The reduced circuit shows resistors
What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
are in parallel, with an equivalent resistance of
What is the equivalent resistance in this circuit?
. (d) Step 3: The reduced circuit shows that
What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
are in series with an equivalent resistance of
What is the equivalent resistance in this circuit?
, which is the equivalent resistance
What is the equivalent resistance in this circuit?
. (e) The reduced circuit with a voltage source of
What is the equivalent resistance in this circuit?
with an equivalent resistance of
What is the equivalent resistance in this circuit?
. This results in a current of
What is the equivalent resistance in this circuit?
from the voltage source.

Notice that resistors

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 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
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 is

   

What is the equivalent resistance in this circuit?

The circuit now reduces to three resistors, shown in Figure 6.2.5(c). Redrawing, we now see that resistors

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 constitute a parallel circuit. Those two resistors can be reduced to an equivalent resistance:

   

What is the equivalent resistance in this circuit?

This step of the process reduces the circuit to two resistors, shown in in Figure 6.2.5(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:

   

What is the equivalent resistance in this circuit?

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

What is the equivalent resistance in this circuit?
. This current runs through resistor
What is the equivalent resistance in this circuit?
 and is designated as
What is the equivalent resistance in this circuit?
. The potential drop across
What is the equivalent resistance in this circuit?
 can be found using Ohm’s law:

   

What is the equivalent resistance in this circuit?

Looking at Figure 6.2.5(c), this leaves

What is the equivalent resistance in this circuit?
 to be dropped across the parallel combination of
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
. The current through
What is the equivalent resistance in this circuit?
 can be found using Ohm’s law:

   

What is the equivalent resistance in this circuit?

The resistors

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 are in series so the currents
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 are equal to

   

What is the equivalent resistance in this circuit?

Using Ohm’s law, we can find the potential drop across the last two resistors. The potential drops are

What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
. 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

   

What is the equivalent resistance in this circuit?

The total energy is constant in any process. Therefore, the power supplied by the voltage source is

What is the equivalent resistance in this circuit?
. 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.

Figure 6.2.6 shows resistors wired in a combination of series and parallel. We can consider

What is the equivalent resistance in this circuit?
 to be the resistance of wires leading to
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 (a) Find the equivalent resistance of the circuit. (b) What is the potential drop
What is the equivalent resistance in this circuit?
 across resistor
What is the equivalent resistance in this circuit?
? (c) Find the current
What is the equivalent resistance in this circuit?
 through resistor
What is the equivalent resistance in this circuit?
. (d) What power is dissipated by
What is the equivalent resistance in this circuit?
?

(Figure 6.2.6)  

What is the equivalent resistance in this circuit?
Figure 6.2.6 These three resistors are connected to a voltage source so that
What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
are in parallel with one another and that combination is in series with
What is the equivalent resistance in this circuit?
.

Strategy

(a) To find the equivalent resistance, first find the equivalent resistance of the parallel connection of

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
. Then use this result to find the equivalent resistance of the series connection with
What is the equivalent resistance in this circuit?
.

(b) The current through

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

(c) The current through

What is the equivalent resistance in this circuit?
 can be found using Ohm’s law
What is the equivalent resistance in this circuit?
. The voltage across
What is the equivalent resistance in this circuit?
 can be found using
What is the equivalent resistance in this circuit?
.

(d) Using Ohm’s law

What is the equivalent resistance in this circuit?
, the power dissipated by the resistor can also be found using
What is the equivalent resistance in this circuit?
.

Solution

a.    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

   

What is the equivalent resistance in this circuit?

The total resistance of this combination is intermediate between the pure series and pure parallel values (

What is the equivalent resistance in this circuit?
and
What is the equivalent resistance in this circuit?
, respectively).

b.    The current through

What is the equivalent resistance in this circuit?
 is equal to the current supplied by the battery:

   

What is the equivalent resistance in this circuit?

The voltage across

What is the equivalent resistance in this circuit?
 is

   

What is the equivalent resistance in this circuit?

The voltage applied to

What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
 is less than the voltage supplied by the battery by an amount
What is the equivalent resistance in this circuit?
.
 When wire resistance is large, it can significantly affect the operation of the devices represented by
What is the equivalent resistance in this circuit?
 and
What is the equivalent resistance in this circuit?
.

c.    To find the current through

What is the equivalent resistance in this circuit?
, we must first find the voltage applied to it. The voltage across the two resistors in parallel is the same:

   

What is the equivalent resistance in this circuit?

Now we can find the current

What is the equivalent resistance in this circuit?
 through resistance
What is the equivalent resistance in this circuit?
 using Ohm’s law:

   

What is the equivalent resistance in this circuit?

The current is less than the

What is the equivalent resistance in this circuit?
that flowed through
What is the equivalent resistance in this circuit?
 when it was connected in parallel to the battery in the previous parallel circuit example.

d.    The power dissipated by

What is the equivalent resistance in this circuit?
 is given by

   

What is the equivalent resistance in this circuit?

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.

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.

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

What is the equivalent resistance in this circuit?
 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 6.2.7. The device represented by

What is the equivalent resistance in this circuit?
 has a very low resistance, so when it is switched on, a large current flows. This increased current causes a larger
What is the equivalent resistance in this circuit?
 drop in the wires represented by
What is the equivalent resistance in this circuit?
, reducing the voltage across the light bulb (which is
What is the equivalent resistance in this circuit?
), which then dims noticeably.

(Figure 6.2.7)  

What is the equivalent resistance in this circuit?
Figure 6.2.7 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
What is the equivalent resistance in this circuit?
drop in the wires and reduces the voltage across the light.

  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.

Two resistors connected in series

What is the equivalent resistance in this circuit?
 are connected to two resistors that are connected in parallel
What is the equivalent resistance in this circuit?
. The series-parallel combination is connected to a battery. Each resistor has a resistance of
What is the equivalent resistance in this circuit?
. The wires connecting the resistors and battery have negligible resistance. A current of
What is the equivalent resistance in this circuit?
runs through resistor
What is the equivalent resistance in this circuit?
. 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 6.2.8).

    (Figure 6.2.8)  

    What is the equivalent resistance in this circuit?
    Figure 6.2.8 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
    What is the equivalent resistance in this circuit?
     and
    What is the equivalent resistance in this circuit?
     are in series and the resistors
    What is the equivalent resistance in this circuit?
     and
    What is the equivalent resistance in this circuit?
     are in parallel. The equivalent resistance of the parallel configuration of the resistors
    What is the equivalent resistance in this circuit?
     and
    What is the equivalent resistance in this circuit?
     is in series with the series configuration of resistors
    What is the equivalent resistance in this circuit?
     and
    What is the equivalent resistance in this circuit?
    .
  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
    What is the equivalent resistance in this circuit?
     and is equal to
    What is the equivalent resistance in this circuit?
    . 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
    What is the equivalent resistance in this circuit?
    . This parallel combination is in series with the other two resistors, so the equivalent resistance of the circuit is
    What is the equivalent resistance in this circuit?
    . The voltage supplied by the battery is therefore
    What is the equivalent resistance in this circuit?
    .
  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

       

    What is the equivalent resistance in this circuit?

    Since they are in series, the current through

    What is the equivalent resistance in this circuit?
     equals the current through
    What is the equivalent resistance in this circuit?
    .
     Since
    What is the equivalent resistance in this circuit?
    ,
     the current through each will be
    What is the equivalent resistance in this circuit?
    . The power dissipated by the resistors is equal to the sum of the power dissipated by each resistor:

       

    What is the equivalent resistance in this circuit?


    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

What is the equivalent resistance in this circuit?
 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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