Exercise 2: Circuit Current

Transcription

1 Exercise 2: Circuit Current EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe and measure current by using a simple circuit. You will verify your results with a multimeter. DISCUSSION through a conductor from one point to another. With a negative charge at terminal 1 and a positive charge at terminal 2, electrons are repelled from terminal 1 and attracted to terminal 2. Electrons enter the conductor from terminal 1 of the battery. Electrons leave the conductor and enter the battery at terminal 2. The total amount of current in an electrical circuit is determined by the voltage applied to the circuit and the total resistance (R) of the circuit. If the circuit resistance remains the same while voltage is varied, current is altered. If the circuit voltage remains the same while resistance is varied, current is altered. FACET by Lab-Volt 43

2 DC Fundamentals In this circuit, voltage and / or resistance change. Circuit current a. remains constant. b. is not affected. c. must change. To measure circuit current, place an ammeter in series with the circuit. Place the COM meter terminal at the most negative side of the circuit entry point. The unit of measure for current is the ampere (A). One ampere of current can be expressed as 1000 milliamperes (ma or milliamps). One milliampere of current can be expressed as 1000 microamperes ( A or microamps). Examples of ampere/milliampere/microampere comparisons are: 1000 ma = 1A 100 ma = 0.1A 10 ma = 0.01A 1 ma = 0.001A = 1000 A PROCEDURE Locate the OHM S LAW circuit block. Connect a voltmeter across the variable voltage source. 44 FACET by Lab-Volt

3 Adjust the positive variable supply controls (on the base unit) for a reading of about 0V. The positive variable supply has FINE and COARSE adjustments. The controls allow you to set exact voltage values between 0V and +10V. Remove the voltmeter from your circuit. Select dc milliamps and a low value ma full scale range on your multimeter. Connect the ammeter as shown in the circuit. Observe proper polarity for your meter connection. Connect R1 and R2 with a two-post connector. Your ammeter indicates a near zero current because a. b. circuit source voltage is near 0. c. Adjust the positive supply controls until the ammeter reads 1.0. Does this reading indicate that circuit current is 1A or 1 ma? a. 1A because current can be expressed only in amperes. b. 1 ma because an ammeter set to read milliamps can only indicate 1 ma. c. 1 ma because the ammeter is set to indicate milliamperes and the circuit current is 1 ma. Monitor the current in your circuit as you adjust the positive supply voltage controls from minimum to maximum. FACET by Lab-Volt 45

4 DC Fundamentals What is the relationship between the current in your circuit and the applied voltage? a. Current is directly related to voltage. b. Current is indirectly related to voltage. c. Current and voltage are not related. a. the negative battery terminal, through the load and meter, and into the positive battery terminal. b. both battery terminals and are averaged within the meter. c. the positive battery terminal, through the load and meter, and into the negative battery terminal. CONCLUSION load, and back to the positive terminal of the power source. The ampere, measured by an ammeter, is the unit of measure for current. In a circuit, current increases when voltage increases; current decreases when voltage decreases. Therefore, current and voltage are directly related. REVIEW QUESTIONS 1. An ampere is the unit of measurement for a. resistance. b. c. voltage. d. potential difference. 2. a. negative, negative b. negative, positive c. positive, positive d. positive, negative 3. a. 10 x 10 3 A. b. 1 ma. c. 1.0 x 10 2 A. d. 10 x 10A. 4. A coulomb is the unit of measurement of a. an electron. b. a proton. c. circuit current. d. an electrical charge. 46 FACET by Lab-Volt

5 5. A reading of 8.55 ma is expressed in amperes as a A. b A. c A. d A. FACET by Lab-Volt 47