Lab: Compound Circuits Physics Name: Period: Northeast Community College Diversified Manufacturing Technology Complex (Compound) Circuits PURPOSE: This lab activity provides experience in constructing and measuring complex circuits. DISCUSSION: In a series circuit, there is only one path for current to flow. There is no limit to the number of components that make up the circuit, but, based upon Ohm's law, the more components connected in series the lower the current flowing through the circuit. A simple parallel circuit is composed of two or more branches. Each branch will have a current determined by the amount of resistance in the branch. The sum of the branch current will be equal to the source current. The source voltage will be the same through each branch. As you know, electrical circuits do not come in neat little packages but rather a combination of series and parallel circuits. To solve a compound circuit you have to break the circuit down into manageable units, series and parallel. The problem lies in determining which part is series and which part is parallel. Once determined, the rules for series circuits apply to the series portion of the circuit and the rules for parallel circuits apply to the parallel portion. One rule to bear in mind is to always solve for parallel circuits first. R1 TB1 PB4 L1 PB2 PB3 PB5 PB6 L2 Figure 8-1 Source: http://www.supershareware.com/preview/basic-electrical-troubleshooting.html If you look at the pictorial view of a circuit (Figure 8-1) you likely recognize the many components. But can you determine which components are in series and which are in parallel? It is much easier to make this determination looking at a schematic of the system. With practice you will become more at ease at identifying and solving complex circuits. PROCEDURE: 1. Study the circuit in Figure 8-2 and predict the answers to the following questions based upon your experiences with series and parallel circuits. 6V L3 L4 Figure 8-2 a. Are lamps L1 and L2 in series or parallel? b. Are lamps L3 and L4 in series or parallel? c. Are lamps L2 and L3/L4 in series or parallel? d. Which lamp(s) will be the brightest? e. Which lamp(s) will be the dimmest? 2. Now construct the circuit in Figure 8-2 using the PHET Circuit Construction Kit to check your answers to questions 1(d) and 1(e) and answer the following questions. Again think in terms of your experience with series and parallel circuits. a. Why is L1 brighter than L2? b. Why is L2 brighter than L3 or L4? c. Why are the brightness of L3 and L4 equal? 3. Again study Figure 8-2 and predict what will happen to the brightness of the lamps when lamp 1 (L1) burns out or is removed from the socket. a. Did L2 get brighter or dimmer? b. Did L3 and L4 get brighter or dimmer? 4. You can design your PHET circuit to test your predictions. You can simulate this by setting the lamp resistance to zero. Do not forget to reset the lamp to the original resistance when finished with the question. a. Did your predictions match the simulation? 5. Again study Figure 8-2 and predict what will happen to the brightness of the lamps when lamp 2 (L2) burns out or is removed from the socket. a. Did L1 get brighter or dimmer? b. Did L3 and L4 get brighter or dimmer? c. Explain why L1 and L3 changed as they did when L2 was removed from service. 6. Design your PHET circuit as shown in Figure 8-5 to test your predictions to questions 5(a) and 5(b). 7. Using Figure 8-2 as your circuit, predict the answers to the following questions regarding how the various components will react when Lamp 3 (L3) or Lamp 4 (L4) is burned out or removed from their sockets. a. If L3 burns out, will L2 get brighter or dimmer? b. If L4 burns out, will L2 get brighter or dimmer? c. Why did L1 get brighter when either L3 or L4 are burned out? d. Why did L3 get brighter when L4 is burned out? 8. Use your PHET circuit to test your predictions. 9. Look at the complex circuit identified in Figure 8-6. Because it is a complex circuit you will need to break it down into pieces in which you can manage. Begin by visualizing how Figure 8-6 could be reduced to an equivalent circuit containing two parallel 3 20V R1 100 www 53 R2 10 ww R3 1002 Figure 8-6 Junction 2 wwwww R4 Junction 1 2002 resistors. a. Using the image, calculate the total resistance of the two parallel resistors R3 and R4. b. Will the total resistance (RT) be greater than or less than 1002? 10. Using the total resistance calculated in 9(a), visualize resistors R3 and R4 as a single resistor (call it R5). Your circuit should now resemble Figure 8- R2 10 ww 20V R1 100 www Junction 2 Figure 8-7 www R5 Calculated 7. 11. Look at the complex circuit identified in Figure 8-7. Because it is still a complex circuit you will need to break it down into pieces in which you can manage. Begin by visualizing how Figure 8-7 could be reduced to an equivalent circuit containing two parallel resistors. a. Using the image, calculate the total resistance of resistors R2 and R5. 12. Using the total resistance calculated in 11(a), visualize resistors R1 and R5 as a single resistor (call it R6). Your circuit should now resemble Figure 8-8. Notice you now have a simple series circuit. You could reduce Figure 8-8 one additional time to simplify the circuit even further as shown in Figure 8-9 by summing the two resistors. 20V 20V R1 100 Junction 2 www Figure 8-8 www www R6 Calculated R7 Calculated Figure 8-9 13. Now that you have simplified the complex circuit, determine the voltage through R1, R2, R4 and the total voltage (VAT) of the circuit. Also determine the current through R1, R2, R3, R4, and the total circuit current (IRT) by applying Ohm's law and your knowledge of series and parallel circuits. Location Voltage Current Resistance RT R1 R R3 R4 Rs R6 Rz 14. Upon completing your calculations for 15, construct Figure 8-6. 15. Using the voltmeter and the non-contact ammeter compare the results you obtained from the simulation to those you calculated in question 15. If they are different you need to check your math. 16. Using the circuit in which you constructed in Step 15 answer the following questions. a. What is the voltage between junction 1 and junction 2? Check your answer by connecting the voltmeter between the two junctions. b. Does the current entering junction 1 equal the current leaving junction 1? c. Would removing R1 from the circuit change the value of the current passing through R3? d. Temporarily remove R1 and verify your answer by measuring the current through R3. Reset R1 when you are finished with this question. 6 e. Would removing R4 from the circuit change the value of the current passing through R1? Temporarily remove R4 and verify your answer by measuring the current through R1. f. Does removing R4 change the current through R2? Reset R4 when you are finished with this question. g. If a 1000 resistor were added in parallel with R4, what would happen to the voltage across R2? 7