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Transcript
Physics of Circuits Created for BW Physics Middle School Science Teachers By Dick Heckathorn 23 April 2K + 8 1 Equipment 1. 2. 3. 4. Meter Resistors Battery Connecting Wire 2 Table of Contents 4 A. Finding Total or Equivalent Resistance of Resistors in Series 12 B. Finding Total or Equivalent Resistance of Resistors in Parallel 19 C. Circuit Analysis One Resistor 29 D. Circuit Analysis Two Resistors Connected in Series 38 E. Circuit Analysis Two Resistors Connected in Parallel 49 Final Summary with Some Problems 64 Connecting a 3-Way Switch 3 A. Finding Total or Equivalent Resistance of resistors connected in Series 4 1. Measure and record the resistance of three resistors. R1 _______ R2 _______ R3 _______ 5 2. Connect two of the resistors end to end (in series) and measure the resistance across each pair. R1 + R 2 R1 + R 3 _______ _______ R2 + R _______ 6 3. A new resistor equal to the sum of the two resistors can replace the two resistors and has the same effect in a circuit as the two resistors. We call the sum, an equivalent resistance or a total resistance. 7 4. Develop a rule from which you can predict the equivalent resistance of two resistors in series. R1 + R 2 R1 + R 3 R2 + R 3 8 5. Connect all three resistors end to end and measure the resistance across all three. R 1 + R2 + R 3 _____________ 9 6. Does your rule for three resistors connected in series the same as for two resistors connected in series? Yes ___ No ___ 10 7. Your instructor hopes that you found the following to be true. RT R1 R2 R3 ...... Remember this is for ONLY resistors in series! 11 B. Finding Total or Equivalent Resistance of Resistors in Parallel 12 1. Record the resistance of the three resistors that you measured before. R1 _________ R2 R3 _________ _________ 13 2. Connect two of the resistors as shown (in parallel) and measure the resistance across each end. R1 : R2 R1 : R3 R2 : R3 _________ _________ _________ 14 3. Develop a rule from which can predict the equivalent resistance of two resistors in parallel? 15 4. Connect all three resistors as shown, then measure the resistance across all three. R1 : R2 : R3 ____________ 16 5. Is the rule for finding the equivalent resistance of three resistors connected in parallel the same for finding the equivalent resistance of two resistors connected in parallel? Yes ___ No ___ 17 6. The rule is: 1 1 1 1 ...... RT R1 R2 R3 Remember this is ONLY for resistors in parallel! 18 C. Circuit Analysis One Resistor 19 1. Measure the value of a resistor. R = ___ Set the ammeter to its greatest value. Then construct the following circuit. A R 20 Measure and record: A I VA VB R VR Current I Voltage VR Voltage VA Voltage VB 21 Conclusion(s) A I VA VB R VR I? Compare VB with VA and VR and VB, I, and R? 22 6. Divide the voltage (VB) by the resistance (R). VB __________ R Compares to ____________________ 7. How does VB/R compare to the measured current? Be sure to keep track of units. 23 8. Remove Ammeter VB R VR 24 9. Connect and measure the voltage across the battery and then the resistor. Record the values. VB VB = _______ R VR VR = _______ 25 Conclusion(s)? VB R VR How do VB and VR compare? 26 10. Calculate: VR1 __________ R1 VR 2 __________ R2 How do they compare? They compares to ____________________ 27 11. What does your instructor say about this? Your instructor says that VB and VR should be the same as the energy per charge given to the electrons by the battery VB and then removed by the resistor VR. 28 D. Circuit Analysis Two Resistors Connected in Series 29 1. Measure the value of two resistors. R1 = _________ R2 = _________ Then construct the following circuit. 30 2. Measure and record the current. I = ____________ A R1 R2 31 3. Measure and record the voltage across the battery and both of the resistances. A I R1 V1 R2 V2 VB VB = _______ VR! = _______ VR2 = _______ 32 4. How does VB compare to VR1 and VR2? 5. Calculate VR1/R1 and VR2/R2 ______ ______ What do these values compare to? 33 5. Calculate both VR1/R1 and VR2/R2. VR1 VR 2 __________ __________ R1 R2 How do they compare? What do these values compare to? 34 6. Write a conclusion about the current and voltage in a circuit when two resistors are connected in series with the battery. IT I R1 I R 2 VB VR1 VR 2 35 7. What does your instructor say about this? Resistance in Series 1. Current (I) in the circuit is everywhere the same. 2. Potential difference (VB) supplied by the battery equals the sum of the potential difference (V1+V2) lost in the components connected in series. 36 E. Circuit Analysis Two Resistors Connected in Parallel 38 1. Measure the value of both resistor. R1 = _______ R2 = _______ Then construct the following circuit. 39 2. Record the value of the current A I R1 R2 IT = ________ 40 3. If you have access to two meters, create the circuit below and measure the current through both resistors. Otherwise, hook up A1 measure the current I1 and then repeat for A2. A1 I1 A2 I2 R1 R2 41 Measure and record both currents. A1 I1 A2 I2 R1 R2 IR1 = ____________ IR2 = ____________ 42 4. How is the does the current through each meter, IR1 and IR2 compare to the total current IT? IT = _______ IR1 = _______ IR2 = _______ A1 IR1 AR2IR2 R1 R2 43 5. Remove all ammeters which will give you the following circuit. R1 R2 44 6. Measure and record the voltage across the battery VB, and the voltage across the two resistors VR1 and VR2 VB VR1 VB _______ VR1 _______ R1 R2 VR2 VR2 _______ 45 7. How is the voltage of the battery VB related to the voltage through each resistor VR1 and VR2? VB _______ VR1 _______ VR2 _______ Calculate the following: VB VR1 VR 2 ______ ______ ______ RT RR1 RR 2 What do the values represent ? How are they related ? 46 11. Write a conclusion about the current and voltage in a circuit where 2 resistors are connected in parallel with the battery. IT I R1 I R 2 VB VR1 VR 2 47 Summary 1. Current (I) from the battery equals the sum of the currents (I1 + I2) through the separate resistances. 2. Potential difference (VB) supplied by the battery equals the potential difference (V1 = V2) lost in the resistances connected in parallel. 48 Final Summary with some problems 49 Summary What governs the amount of electric potential energy an electron will lose in each load? The conservation of energy. The amount gained is equal to the sum of the total energy lost. 50 Summary What governs the number of electrons that will take each path? The conservation of charge. There is no gain or loss of electrons nor any accumulation at any point. 51 Series Circuit VT R1 R2 R3 V1 V2 V3 Energy VT VConservati V3 1 V2 on V IR IT RT I1 R1 I 2 R2 I 3 R3 Charg I T eI1Conservati I 2 Ion 3 RT R1 R2 R3 52 Resistance in Series RT R1 R2 R3 12 Ω, 25 Ω, and 42 Ω in series The equivalent resistance is 79 Ω 53 Resistance in Series Three 30-Ω light bulbs and two 20-Ω heating elements connected in series The equivalent resistance is …. 130 ohms 54 Resistance in Series two strings of Christmas tree lights connected in series, if the 1st string has eight 4-Ω bulbs & the 2nd has twelve 3-Ω bulbs The equivalent resistance is …. 68 ohms 55 Resistance in Series Find the value of the unknown resistance in the following: a 20-Ω, a 18-Ω, and an unknown resistor are connected in series to give an equivalent resistance of 64-Ω 26 ohms 56 Parallel Circuit R1 I1 IT VT R2 I2 R3 I3 V1 V2 V3 V I R I T eIConservati Charg 1 I 2 I 3 on VT VConservati Energy on 1 V2 V 3 VT V1 V3 V2 R2 RT R1 R3 1 1 1 1 RT R1 R2 R3 57 Resistance in Parallel 1 1 1 1 RT R1 R2 R3 Find the equivalent resistance when a 4-Ω and an 8-Ω bulb are connected in parallel. 2.7 ohm 58 Resistance in Parallel Find the equivalent resistance when a 16-Ω and an 8-Ω bulb are connected in parallel. 5.3 ohm 59 Resistance in Parallel Find the equivalent resistance when a 20-Ω, a 10- Ω and a 5-Ω bulb are connected in parallel. 2.9 ohm 60 Resistance in Parallel What resistance would have to be added in parallel with a 40- Ω hair dryer to reduce the equivalent resistance to 8- Ω? 10 ohm 61 Resistance in Parallel Find the equivalent resistance of two, three, four, and five 60-Ω bulb are connected in parallel. 30 Ω 20 Ω 15 Ω 12 Ω Is there a simple relationship for the equivalent resistance of ‘n’ resistors in parallel? R/n 62 That’s all folks! 63 64 65