Ohm s Law DC Fundamentals Exercise 3: Ohm s Law Circuit Voltage EXERCISE OBJECTIVE When you have completed this exercise, you will be able to determine voltage by using Ohm s law. You will verify your results with a multimeter. DISCUSSION When the circuit current and circuit resistance are known, determine the applied voltage by using Ohm s law. E = I x R VOLTAGE (IN VOLTS) = CURRENT x RESISTANCE (IN AMPS) (IN OHMS) NOTE: Voltage is not directly related to both current and resistance because a change in resistance also affects circuit current. When the circuit resistance is held constant, circuit voltage varies in direct proportion to circuit current. In this circuit, circuit resistance is doubled and circuit current decreases by half. What is the new applied voltage? a. 20 Vdc b. 10 Vdc c. 5 Vdc In this circuit, current is 5 ma and the total resistance is 1600. According to Ohm s law (E = I x R), the applied voltage is 8 Vdc. The voltage appearing across any resistor in a circuit (voltage drop) can be determined when circuit current and the resistor value are known. Use Ohm s law formula E = I x R. 5 ma x 1000 = 5 Vdc 5 ma x 600 = 3 Vdc 110 FACET by Lab-Volt
DC Fundamentals Ohm s Law The sum of each voltage drop equals the value of the applied circuit voltage. 5 ma x 1000 = 5 Vdc 5 ma x 600 = 3 Vdc PROCEDURE Locate the OHM S LAW circuit block, and connect the circuit shown. Set the multimeter to measure current. Connect the multimeter between the voltage source and R1. Adjust the positive supply for a current reading of approximately 3.68 ma. Enter your reading. I T = ma (Recall Value 1) FACET by Lab-Volt 111
Ohm s Law DC Fundamentals Based on your circuit current of ma (Step 3, Recall Value 1) and a nominal circuit resistance of 1510, calculate and enter the expected applied voltage. NOTE: E = I x R V A = Vdc (Recall Value 2) Measure the applied voltage of your circuit. Compare your reading to the calculated value of V (Step 4, Recall Value 2). Do your results indicate that Ohm s law can be used to determine circuit voltage when current and resistance are known? a. No, applied voltage must be measured. b. Yes. c. Based on a circuit current of ma (Step 3, Recall Value 1), use Ohm s law (I x R) to calculate the voltage drop of R1 of your circuit. V R1 = Vdc (Recall Value 3) 112 FACET by Lab-Volt
DC Fundamentals Ohm s Law Based on a circuit current of ma (Step 3, Recall Value 1), use Ohm s law (I x R) to calculate the voltage drop of R2. V R2 = Vdc (Recall Value 4) Energize your circuit with a two-post connector. Measure the voltage drop of R1. V R1 = Vdc (Recall Value 5) NOTE: Your circuit current should be about 3.7 ma. Measure the voltage drop of R2. V R2 = Vdc (Recall Value 6) Based on your data recalled below, which statement applies with respect to Ohm s law? a. Ohm s law cannot be used to determine individual resistive voltage drops. b. Ohm s law can be used only to determine the circuit applied voltage. c. Ohm s law can be used to determine both applied voltage and individual resistive voltage drops. CALCULATED DROPS MEASURED DROPS V R1 Vdc (Step 6, Recall Value 3) Vdc (Step 8, Recall Value 5) V R2 Vdc (Step 7, Recall Value 4) Vdc (Step 9, Recall Value 6) FACET by Lab-Volt 113
Ohm s Law DC Fundamentals Adjust the positive supply for 10 Vdc (use both COARSE and FINE controls for a more precise adjustment). Place CM switch 19 in the ON position. Measure and enter the circuit current in milliamps. I T = ma (Recall Value 7) What new circuit applied voltage is required to reduce the current from 12, Recall Value 7) to 1.35 ma? a. Applied voltage cannot be changed unless circuit resistance is changed. b. 15 Vdc c. 5 Vdc ma (Step Adjust the positive supply to 5 Vdc. Measure the circuit current (CM 19 is activated) and compare your reading to ma (Step 12, Recall Value 7). Does a 50% reduction in applied voltage reduce the current by a like amount? a. Yes. If the resistance is held constant, a 50% applied voltage reduction decreases the circuit current by a like amount. b. Yes, provided that the circuit resistance is decreased by a like amount. c. Yes, provided that the circuit resistance is increased by a like amount. Based on your observations, which statement about voltage, current, and resistance is correct? a. If voltage increases, current decreases when resistance is held constant. b. If voltage increases, current increases when resistance is held constant. c. If voltage increases, both current and resistance must decrease. Make sure all CMs are cleared (turned off) before proceeding to the next section. 114 FACET by Lab-Volt
DC Fundamentals Ohm s Law CONCLUSION Ohm s law (E = I x R) can be used to determine the value of a circuit s applied voltage. Ohm s law can be used to determine the voltage drops of individual circuit resistors. If resistance is held constant, an increase in applied voltage increases the circuit current. If resistance is held constant, a decrease in applied voltage decreases the circuit current. REVIEW QUESTIONS 1. Voltage and resistance are a. measured in the same unit. b. inversely proportional. c. directly proportional. d. smaller than circuit current. 2. An electric circuit with 500 a. 5 Vdc. b. 10 Vdc. c. 15 Vdc. d. 50 Vdc. 3. If the voltage and resistance in a circuit are both doubled, the current will a. increase. b. decrease. c. double. d. remain the same. 4. On the OHM s circuit block, adjust the postive supply for a circuit current of 6 ma. Place CM switch 20 in the ON position. Based on the change of circuit current, voltage drops, and Ohm s law, which component in the circuit block changed? a. R1 increased in value. b. R2 increased in value. c. R1 decreased in value. d. R2 decreased in value. 5. This circuit normally has a current of 4 ma. Suppose you measured a circuit current of 5 ma, the reason for the increased current could be that a. R1 changed to 2500. b. R2 changed to 1000. c. R1 changed to 1500. d. R2 changed to 400. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. FACET by Lab-Volt 115