Chapter 23: Series and Parallel Circuits - Denton ISD

1 WhichBulbBurnsBrighter?One is a 60-watt bulb and the other a100-watt bulb, and theyare connected in anelectric circuit . Look at the text on page 541 for the desk needs to be well lit. You have a 100-watt lightbulband a 60-watt lightbulb available for your desk of the two lightbulbs would you choose? You knowa watt is a unit of power, so you might expect that a 100-wattlightbulb would always produce more light than a 60-watt would it?You re shopping for a string of holiday lights for a party. Youfind a string of pumpkin lights with a tag that reads If one goesout, the rest stay lit. Why is this important? How are the lightsconnected to each other and the power source to make sure thatthe rest stay lit?Does the type of electric circuit that connects the bulb make adifference?

2 Suppose the lightbulbs are in a circuit unlike the par-allel circuit that normally connects lightbulbs in your home. Forexample, in the photo at the left, the bulbs are connected inseries. Does that make a difference? Can you figure out why? Toanswer these questions, you need to know how a Series circuit dif-fers from a Parallel Chapter 22, you studied Circuits that had one source of elec-tric energy, for example, a battery and one device such as a motoror a lamp that converted the electric energy to another form. Butcircuits can be much more versatile than that. In this Chapter ,you ll explore several ways in which devices may be connected inelectric Circuits . By the end of this Chapter , you ll have no troubledeciding which of the two bulbs at the left is brighter and why therest of the bulbs stay lit.

3 Series and ParallelCircuits WHAT YOU LL LEARN You will distinguish betweenparallel and Series circuitsand Series - Parallel combina-tions and solve problemsdealing with them. You will explain the functionof fuses, circuit breakers,and ground fault interrupters,and describe ammeters and IT S IMPORTANT Electrical Circuits are thebasis of every electricaldevice, from electric lights tomicrowave ovens to comput-ers. Understanding circuitshelps you to use them, andto use them find out more about electrical Circuits , visit the Glencoe ScienceWeb site at you have ever had the chance to explore rivers inthe mountains, such as the river shown below inFigure 23 1,the following description will be familiar. From their sources high in the mountains, rivers make their wayto the plains below. No matter what path they take, the change in eleva-tion, from the mountaintop to the plain, is the same.

4 Some rivers flow in asingle stream, tumbling through a Series of rapids and over rivers split into two or more streams as they flow over a waterfall orthrough the rapids. Part of the river follows one path; other parts find dif-ferent routes. But no matter how many paths the water takes, the totalamount of water flowing down the mountain is the same, and the verticaldrop from mountaintop to plain is the same distance. Mountain rivers can serve as a model for electric Circuits . The distancethe water drops is similar to potential difference. The amount of waterflowing through the river each second is similar to current. Narrow rapidsare like a large resistance. But what is similar to a battery or generator?Just as in an electrical circuit , an energy source is needed to raise waterto the top of the mountain.

5 As you learned in Earth science, the source ofenergy is the sun. Solar energy evaporates water from lakes and seas andforms clouds that release rain or snow to fall on the tops of about the mountain river model as you read about the current inelectrical Describe both a seriesconnection and a parallelconnection and statetheimportant characteristics of each. Calculate current, voltagedrops, and equivalent resis-tance for devices connectedin Series and in Parallel . Describea voltage dividerand solveproblems involving Circuits532 Series and Parallel CircuitsFIGURE 23 1No matter whatpath the water of a river takesdown a mountain, the amount ofwater and the drop in elevationare the CircuitsPat, Chris, and Ali were connecting two identical lamps to a batteryas illustrated inFigure 23 making the final connection to thebattery, their teacher asked them to predict the brightness of the twolamps.

6 They knew that the brightness of a lamp depends on the currentflowing through it. Pat said that only the lamp closer to the + terminalof the battery would light because all the current would be convertedinto light. Chris said that the second lamp would light, but it would bedimmer than the other one because some electrical energy would bechanged into thermal and light energy. Consequently, there would beless electrical energy left for the second lamp. Ali said that both lampswould be equally bright because current is a flow of charge, and becausethe charge leaving the first lamp had nowhere else to go in the circuitexcept through the second lamp, the current would be the same in thetwo lamps. Who do you think is right?If you consider the mountain river model for this circuit , you ll seethat Ali is correct.

7 Charge has only one path to follow. Recall from Chap-ter 20 that charge cannot be created or destroyed, so the same amountof charge must leave a circuit as enters the circuit . This means that thecurrent is the same everywhere in the circuit . If you connect threeammeters into a circuit as shown inFigure 23 3, they all have the samevalue. A circuit such as this, in which all current travels through eachdevice, is called a Series how could you answer Chris? If the current is the same, whatchanges in the lamp to produce the thermal and light energy? Recall thatpower, the rate at which electrical energy is converted, is represented byP IV. Thus, if there is a potential difference or voltage drop across thelamp, then electrical energy is being converted into another form. Theresistance of the lamp is defined as R V/I.

8 Thus, the potential differ-ence, also called the voltage drop, is V is the current in the Series circuit ?From the river model,you know that the sum of the drops in height at each rapid is equal tothe total drop from the top of the mountain to sea level. In the electri-cal circuit , the increase in voltage provided by the generator or otherenergy source, Vsource, is equal to the sum of voltage drops across thelamps A and VA VBBecause the current, I,through the lamps is the same, VA IRAandVB IRB. Therefore, Vsource IRA IRBor Vsource I(RA RB).The current through the circuit is represented by the RVAso urRceB This equation applies to any number of resistances in Series , not justtwo. The same current would exist with a single resistor, R,that has Simple Circuits533+ RARB Vsource+ FIGURE 23 2 What is your prediction about the brightnessof the two lightbulbs?

9 FIGURE 23 3 The ammetersshow that in a Series circuit thecurrent is the same equal to the sum of the resistances of the two lamps. Such aresistance is called the equivalent resistance of the circuit . For resistorsin Series , the equivalent resistanceis the sum of all the individualresistances. Equivalent Resistance for Resistors in SeriesR RA RB ..Notice that the equivalent resistance is larger than any single resis-tance. Therefore, if the battery voltage doesn t change, adding moredevices in Series always decreases the current. To find the current, I,through a Series circuit , first calculate the equivalent resistance,R, andthen use the following equation to calculate VsoRurce 534 Series and Parallel Cavendish used thedirect approach to measurethe strength of an electriccurrent.

10 Lacking the appropriate instruments, he instead shocked himselfwith the current and thenestimated the LabSeries ResistanceHook up a power supply, aresistor, and an ammeter in aseries circuit . Predict what willhappen to the current in thecircuit when a second, identicalresistor is added in Series to thecircuit. Predict the new currentswhen the circuit contains threeand four resistors in your prediction. Try it. Analyze and Conclude Makea data table to show yourresults. Briefly explain yourresults. (Hint:Include the ideaof resistance.) 20- resistors are connected in Series across a 120-V generator. What is the equivalent resistance of the circuit ? What is the current in the circuit ? 10- resistor, a 15- resistor, and a 5- resistor are connectedin Series across a 90-V battery. What is the equivalent resistanceof the circuit ?