06_DC_circuits

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Oregon State University, Corvallis *

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213

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Electrical Engineering

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Dec 6, 2023

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PH213 Lab Activity: Electric Circuits In this activity you use wires and electrical elements to test principles of electric circuits. You are supplied with a battery and battery holder as a source Equipment provided • Batteries & holder w/ leads • Wires & clips • Spring board for connecting elements • Bulbs • Multimeter w/ leads • Resistors You will be asked to sketch configurations that you construct. In some cases you’ll be asked to make a physical sketch of the situation, using rough sketches of what the objects in your construction look like. In other cases you’ll be asked to make a schematic circuit diagram , an electrical representation that focuses on how the elements are electrically connected without regard to the physical location or orientation. In circuit diagrams, the elements are represented by standard symbols. Examples are shown below. The symbols are laid out on a grid (usually rectangular). Wire connections are shown as straight lines that make turns at right angles. Lines that meet indicate a connection between wires, or a junction. Figure 1: Commonly used symbols in circuit diagrams All items below must be addressed in your lab write-up, and a random subset will be graded. Using elements In this part of the activity you must determine a way to make a bulb light up, using only one piece of wire, a bulb, and a battery (not in the holder). Draw a clearly labeled and precise physical sketch (not a circuit diagram) of what you end up with. Make sure to represent the electrically important aspects of your result. Now create a second way to make the bulb light up, in which the current flows 1

PH213 Lab Activity: Electric Circuits in the opposite direction. Draw a new physical sketch that clearly shows what you changed from your first method. 1) Add both sketches to your report. 2) Based on your results, construct an explanation for what must occur in order for the bulb to light up and the effects of reversing the current. The bulb lights up in both of the above sketched positions. An explanation is that current must flow across the terminals of the lightbulb, but the polarity (or current flow direction) does not matter. Measuring voltage and current, Ohm’s law Next you’ll use a meter to (separately) measure voltage and current, and then use your results to test Ohm’s law in a circuit. • Ohm’s law states that the voltage across an element ΔV is the product of the current through the element I and the element’s resistance R : ΔV = IR.

Choose a resistor to use in a circuit. Measure its resistance using the meter, and then measure the voltage across your source (batteries). ü You’ll need to use different settings on the meter for each measurement. 3) Report your values. Measured resistance: 100.6 ohms Battery voltage (in series): 2.57V Now create a circuit consisting of your source and resistor. For this activity, you may put the batteries in the holder and use the spring board for connecting wires and elements. You may also use multiple wires. Draw a clearly labeled and precise circuit diagram of your configuration. 4) Add the circuit diagram to your report, labeling it “ No meter .” 5) Predict the voltage across your resistor and the current through it. Include the direction of voltage change and the direction of current flow. Explain how you made your predictions. Since Vloop=0, and the resistance of the wires is very small, the voltage drop across the resistor must be equal to the voltage of the batteries, that is, 2.57V. The direction of voltage change will occur in the same direction as current flow, from positive to negative, across the resistor. Since the resistance is 100ohms, the current would be about 2.57/100=25.7mA.

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Determine how to add the meter to your circuit so that it will measure the voltage across the resistor. (Check that the leads and meter are configured to measure DC voltage.) Draw a circuit diagram including the meter to measure voltage. Be sure to correctly indicate which lead is which. 6) Add the circuit diagram to your report, labeling it “ Voltage meter .” 7) Measure the voltage across the resistor in your circuit. Compare the value to your prediction. Be sure to address direction. What happens if you switch the meter leads? Measured voltage: 2.48 volts This value is close to the prediction, and remaining voltage drop is likely due to the wires in the circuit and the resistance of the multimeter. When the meter leads are switch, the meter simply reads the ‘negative’ voltage, that is, the leads are measuring the voltage that is the inverse of their designated color (indicating polarity) Determine how to add the meter to your circuit so that it will measure the current through your resistor. (Check that the leads and meter are configured to measure DC current.) Draw a circuit diagram, including the meter to measure current. Be 2

PH213 Lab Activity: Electric Circuits sure to correctly indicate which lead is which. 8) Add the circuit diagram to your report, labeling it “ Current meter .” 9) Measure the current through the resistor in your circuit. Compare the value to your prediction. Be sure to address direction. What happens if you switch the meter leads? Measured current: 19.10mA This is the same order of magnitude, but slightly smaller than expected. This is likely due to internal resistance of the multimeter. When the meter leads are switched, the negative value is shown (the direction of current flow as measured is in the opposite direction as the leads) Testing Kirchhoff’s laws Create a circuit to separately test Kirchhoff’s loop law and junction law. • The loop law states that as you go around any loop in a circuit, the voltage changes across each element will add to zero. • The junction law states that the current into any junction in the circuit will equal the current out of that junction.

The circuit you use for this test must include at least 3 devices in addition to the voltage source. When confirming each law, you should do at least two tests. That is, measure the voltage around at least two loops in your circuit, and then measure the current into and out of at least two junctions. Be sure to create a circuit that allows you to do this. Draw a circuit diagram of your circuit. Don’t include your meters, but be sure to uniquely label each device. 10) Add the diagram to your report, labeling it “ Kirchhoff’s laws test .” 11) Describe how you will use the circuit to test each law. This should include how you will use the meter to make your measurements, including where you will connect it in your circuit for each measurement. (You don’t need to create separate diagrams unless you want to.) By testing the voltage across each resistor, I will sum each loop in order to test the loop law. By testing the current before any resistors, between R2 and R3, immediately after R1, and then after all resistors, I will test the junction law. 12) What results do you expect based on the laws (your predictions)? This should be specific to your circuit, i.e., it should reference the devices you plan to measure and how those specific measurements should be related.

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I predict that the voltage drop across R1 will be about 2.57 volts, and the voltage drop across R2 and R3 will be 2.57/2=1.29V. I predict the current before all resistors will be 2.57/(200/3)=38.55mA, and the current between R2 and R3 will be 12.85mA, the current after R1 will be 25.7mA the and after all resistors will be 38.55. 13) Perform the measurements you described and record your results. Across R1: 2.45V Across R2 and R3: both 1.21V Before all resistors: 33mA Between R2 and R3: 11.4mA After R1: 20.5 After all resistors: 31mA 14) State whether the measurements support your prediction, that is, do they support Kirchhoff’s laws. Provide evidence for your conclusion. These values are all “not quite right” but close to the predicted values, and within the range of real-world measurements. The voltage around each loop is close to zero (0.12V and 0.15V respectively), and the total current added between two loops is roughly equal the current in the system (within 0.9mA). Measurement inaccuracies, a power source that is not constant, and added resistance due to wires and the multimeter all cause potential errors in the measurements. In all, the measurements do support Kirchoff’s loop and junction laws. 3