Physics 2020 Lab: DC Circuits II page 1 of 7Lab: DC Circuits IIINTRODUCTION:This week we will continue with DC circuits, but now with an emphasis on currentrather than voltage. Of course, in order to fully understand any circuit, you need totake into account both voltage and current. Additionally, we will be using a computersimulation called the Circuit Construction Kit (CCK) instead of real wires, bulbs, andresistors. The goals of this lab are to complete our understanding of how voltage,current, and resistance relate to each other in circuits, and to learn how to use anammeter to measure current directly.PRECAUTIONS & NOTES:To measure how much current is flowing through acircuit, the current needs to flow through the ammeter.Recall that when we measured voltage differences, weattached the voltmeter in parallel with whatever we weremeasuring. To measure current, the ammeter needs to beplaced in series with the element (resistor, bulb, battery)that we are measuring (see figure at right).PART I: MEASURING CURRENT DIRECTLY WITH THE AMMETERDrag and drop one 2 resistor into the work area (ifyou right click on a resistor, you can change itsresistance) and two light bulbs into your work area.Construct the circuit shown on the right, consisting oftwo light bulbs (with the same resistance) in serieswith a battery. (The resistor R will be added later).Increase the voltage across the battery to 20 V (rightclick on the battery to change its voltage). University of Colorado at Boulder, Department of PhysicsAmmeter configured to measurecurrent.Physics 2020 Lab: DC Circuits II page 2 of 7Predict what will happen to the current flowing from the battery when you place a (R= 2 ) resistor in parallel with bulb #2 as shown in the schematic. Explain yourreasoning.Measure the voltage difference across the battery, then put the ammeter in the circuitso that you can measure the current flowing from the battery. Measure and recordthe battery voltage difference and current here. Add in the R = 2 resistor, and describe what happens and why. Were yourpredictions correct?PART II: COMPUTING THE RESISTANCE OF A LIGHT BULBBuild a circuit consisting of a battery in series with a 15 resistor and one light bulb(see diagram below).Set the voltage difference across the battery to 30 V. Using the voltmeter with needleprobes, measure the voltage difference VBATT across the battery, the voltagedifference VR across the 15 resistor, and the voltage difference VBULB across thelight bulb. Using the known resistance of the resistor, compute the current IR flowingthough the resistor.Now set up the ammeter to directly measure the current IR flowing though theresistor. Does this value match the calculated IR from the previous step?University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: DC Circuits II page 3 of 7How is the current flowing though the light bulb (IB) related to the current flowingthrough the resistor (IR)? What is the relationship between the three measuredvoltage differences VR, VBULB, and VBATT?From these measurements, compute RBULB. Using the measurements and calculations from this section, how much power isdissipated in the 15 resistor? How much power is dissipated in the light bulb?PART III: 2 BULBS IN SERIESConstruct the circuit shown at right, containing a single lightbulb. Set the battery voltage to 20 V. Using the voltmeter andthe needle probes, measure the voltage difference across thelight bulb. Then use the ammeter to measure the current flowingout of the battery. Record your results.Predict what will happen if a second bulb is added in series withthe first bulb, as shown at right. Will the bulb brightnesschange? Will the current flowing through the first bulb change?Will the current coming out of the battery change? Will thevoltage difference across the first bulb change? How will thepower change (for each bulb and in total) if at all? Clearlyexplain your reasoning.University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: DC Circuits II page 4 of 7Add a second bulb to the circuit in series with the first bulb (as shown in the previousdiagram). How does the brightness compare to the single-bulb case?Measure the voltage difference across each bulb and record the result. Using theammeter, measure the current coming from the battery and record the result. Dothese results match your predictions? If not, explain the measurement.PART IV: 2 BULBS IN PARALLELConstruct the circuit shown at right, containing a single lightbulb. Set the battery voltage to 10 V. Using the voltmeter andthe needle probes, measure the voltage difference across thelight bulb. Then use the ammeter to measure the current flowingout of the battery. Record your results.Predict what will happen if a second bulb is added inparallel with the first bulb, as shown at right. Will thebulb brightness change? Will the current flowingthrough the first bulb change? Will the current comingout of the battery change? Will the voltage differenceacross the first bulb change? How will the powerchange (for each bulb and in total) if at all? Clearlyexplain your reasoning.University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: DC Circuits II page 5 of 7Add a second bulb to the circuit in parallel with the first bulb (as shown in theprevious diagram). How does the brightness compare to the single-bulb case?Measure the current flowing out of the battery, and measure the current flowingthrough each of the bulbs individually. Record your results here. How are thesethree measurements related?PART V: FOUR BULB CIRCUITDo not build the following circuit. First, rank the brightness of bulbs A, B, C, and D inorder from brightest to dimmest BEFORE the break in the circuit is made (circuit a).What happens to the brightness of each bulb (A, B, C, and D) AFTER the break ismade? Does each bulb get brighter, dimmer, or stay the same? Explain yourreasoning.University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: DC Circuits II page 6
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