Series and Parallel Resistors Today you will investigate how connecting resistors in series and in parallel affects the properties of a circuit. You will assemble several circuits and measure the voltage and current across each element. Equipment: You ll need the following equipment for this lab: A battery eliminator, a digital multimeter (to measure current and voltage), a student breadboard, some resistors, and wire connectors. 1. Before you begin analyzing either of the two circuits A and B you will construct, you need to know the emf of your battery eliminator. Assuming that the internal resistance of the battery eliminator is negligible, you can measure its emf E by measuring the voltage between the terminals. Use the digital multimeter set to V DC to measure the voltage between the terminals. This is the battery eliminator s emf. It should be close to 3V. Record this value above each data table on the line provided. 2. You also need to know the resistance for each resistor you will be using. Using the digital multimeter set to Ω to measure the resistance of each of the resistors you will be using. You may want to label your resistors #1, #2, and #3 or something similar to keep track of them throughout the experiment. Record the measured values on the data sheets for circuit A and circuit B For each of the two circuits, do the following: - Calculate the effective resistance of the circuit and the net current. Neatly show all your steps and calculations in the space provided. Record these values in the data table in the Predicted column - Calculate what the current and potential difference across each resistor should be. You ll need to use Kirchhoff s rules and Ohm s law. Record these values in the table in the Predicted column - Assemble the circuit and measure the current and voltage across each resistor. Remember that an ammeter is connected in series and a voltmeter is connected in parallel. Record your measured values in the table - Measure the net current out of the battery and effective resistance of the circuit. Record your measured values in the table - Calculate the percent difference between your measured and predicted values. 3. Finally, answer the questions on the last sheet using the space provided and turn in with your results on the two circuits. 1
Important notes on setting up your circuit and measuring voltage and current: 1. On the breadboards, each column of 5 holes is connected. Consecutive holes along the same row are not connected. Therefore, to set up two resistors in parallel, they should both start on the same column and end on the same column. For example, if you had a resistor with one end in hole E1 and the other end in hole E5, you could connect another resistor between holes D1 and D5, and the two would be in parallel. An example of two resistors in series would be one connected to holes E1 and E5, and the other connected to holes D5 and D9. 2. Remember, in order to measure voltage, you connect the multimeter in parallel with the circuit element you are measuring. To measure current, you must break the circuit and connect the multimeter in series with the circuit element you are measuring. Also, note that you have to plug the probes into different input jacks for measuring current than you do when measuring voltage or resistance. Circuit A Circuit B 2
Series and Parallel Resistors Name: Date: TA: Group Members: Circuit A R 1 = Ω R 2 = Ω R 3 = Ω Show your calculations for the net current and effective resistance in the space below. E = I net R eff ΔV 1 ΔV 2 ΔV 3 I 1 I 2 I 3 Predicted Measured % diff. 3
Circuit B R 1 = Ω R 2 = Ω R 3 = Ω Show your calculations for the net current and effective resistance in the space below. E = I net R eff ΔV 1 ΔV 2 ΔV 3 I 1 I 2 I 3 Predicted Measured % diff. 4
Questions (use the back of this sheet if necessary) 1. Do your calculated values for current and potential difference in each circuit agree with your measurements? Explain why there is not perfect agreement. 2. Is the resistance of an ammeter high or low? Explain. 3. Is the resistance of a voltmeter high or low? Explain. 4. A series string of Christmas lights, each with a resistance of 20 Ω, is connected to a 120 V source. If the current through the circuit is 0.6 A, how many lights are there? What is the potential difference across each light? 5. (a) Two resistors are connected in series across a battery. Is the power delivered to each resistor (i) the same, or (ii) not necessarily the same? Explain. (b) Two resistors are connected in parallel across a battery. Is the power delivered to each resistor (i) the same or (ii) not necessarily the same? Explain. 6. Connecting batteries in series increases the emf applied to a circuit. What advantage might there be to connecting them in parallel? 5