Lab 4: Resistance 1. Introduction How can we better understand what controls the behavior of everyday electronics when we connect them to our electrical grid? The water pipe for your faucet is smaller than the pipe supplying your home, which is smaller than the pipes for your street. Why? The pipe size limits the flow of water to match what you need, and electronics work in a similar manner. In this lab, you will investigate how various circuit components respond to an applied voltage. Sometimes a large current is desirable (e.g., starting your car or running your air conditioner) and other times only a very small current is safe (e.g., in most circuit boards or your body). How can we use the same voltage source (i.e., a wall outlet) to do both successfully? You will gain a deeper understanding of electrical resistance and how it limits current, a feature that makes some key circuit safety features possible. You will also see examples of how to quantify the power usage of various components, and how resistance relates voltage and current. For a review of basic definitions and concepts relevant to this lab, see Chapter 22.1-22.6 in the Knight textbook. 2. Experiment Activity 1a - Introduction to Resistors and LEDs Your Snap Circuits kit includes five different resistors (100Ω ‘R1’, 1kΩ ‘R2’, 5.1kΩ ‘R3’, 10kΩ ‘R4’, and 100kΩ ‘R5’) with resistance values given in Ohms (Ω) or kiloOhms (kΩ; 1kΩ = 103 Ω). Take these out and examine them, noting the circuit symbol for a resistor printed on them (red squiggly line). Components R1-R5 are examples of fixed resistors, and their resistances can also be identified via the color bands printed on them. If you (carefully) open up an electronic device in your home, you will likely find components that look similar. Your two lamps L1 & L2 require a high current to be bright enough to see them light, so we will be using them later as high-current indicators. However, even your smallest resistor R1 would limit the current too much to see anything happening in L1 & L2, so that suggests we need a low-current indicator as well.

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Light-emitting diodes (or LEDs) satisfy this requirement - and this is one reason why LEDs are so common in electronics - but they are very different from light bulbs in that they are constructed from different materials. Can you think of something in your home that uses LEDs? LEDs are less bright and more sensitive to current than other types of light bulbs, but they can also be easily damaged beyond repair if the current gets too high. This means that we will always want to be sure another resistor is in the circuit to act as a ‘safety valve’ limiting the current. Locate your red and green LEDs (‘D1’ and ‘D2’). The arrow symbol on the printed component indicates the direction the LED allows current to flow; they won’t allow current in the other direction, which can be useful in many types of circuits.

Figure 1: voltage reading of battery Figure 2: basic LED circuit Now let’s see how ordinary light bulbs behave in a circuit. Locate your L1 lamp, a battery pack B1 with AA batteries installed, and your slide ON/OFF switch S1 set to OFF, along with your digital multimeter set to read DC voltage (V), as in Lab 3. Construct a simple circuit consisting of L1, S1, and a single battery pack, similar to what you made for Lab 1. With the switch OFF, does the bulb light? Why or why not? Connect the leads of your multimeter to the battery pack and measure V similar to Figure 1. Now set S1 to ON and record your observations, and measure V again. What does this tell you about how a switch controls the flow of charge (i.e., current) in a circuit? Can you think of an example situation where such control might be useful? Did you notice any changes in the V reading when S1 was ON vs. OFF? You should notice that the V value dropped by ~0.1 V when L1 was drawing current from the batteries, which tells you something about how real batteries (with fixed design voltage) behave under load in a circuit. This load-dependent behavior of battery output is referred to as the terminal voltage and is the result of inevitable energy loss (i.e., internal resistance)

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in a real power source. For deliverable 1, include one picture of your circuit with S1 OFF and another with S1 ON, both with your multimeter connected and the readings visible. Include labels that indicate which circuit has S1 ON/OFF. Set S1 to OFF and disassemble this circuit before continuing. LEDs are a very different type of light with unique properties that make them useful in many kinds of applications. We can investigate some of the properties of LEDs and how resistors and LEDs work together in a very basic circuit. Consider the circuit in Figure 2, which is similar to Project #7 in your circuits manual - it includes a battery pack B1, a red LED D1, resistor R1, and a slide ON/OFF switch S1. Before you connect any of the components together using the blue connectors, check to make sure that S1 is set to OFF. General note: due to the sensitive nature of electrical components, it is good general practice to assemble circuits in a way that keeps the power source disconnected from the rest of the circuit until you can verify that the connections are safe. This is similar to how an electrician operates when installing or repairing a circuit in your home - they will deactivate a circuit at your service panel to prevent current from flowing. If LEDs can be damaged by large current, this suggests that R1 in Figure 2 acts as a safety resistor to limit the current through the LED. For a component with resistance R experiencing a potential difference ΔV, the resulting current I is

.I = RΔV (1a)

Your textbook refers to this relationship as Ohm’s Law. What is the source of ΔV in your circuit? If ΔV were assumed constant in Equation 1a, what would you need to ensure when making your circuit if I must be small? Verify your connections using Figure 2, then activate your circuit by setting S1 to ON. Your red LED should now light. Measure ΔV across R1 (ΔVR1) by connecting your alligator clips to the R1 terminals, similar to how you did for the battery. We can think of R1 as a representation of a typical household lightbulb. If you’re getting ΔV < 0, what does that mean? How much current must be flowing through R1? Is this the same amount of current passing through the rest of the circuit? For deliverable 2, include a picture of your basic LED circuit with the LED on and show your calculations for the current IR1 passing through R1. In your image, include your connected multimeter and give your measured value of ΔVR1. One can rearrange Equation 1a to get ΔV = IR, which says that the ΔV required for a circuit element passing current I also depends on its resistance R. Now measure ΔV for your LED ΔVD = IDRD and compare this result to ΔVR1 = I R1R R1. How is ID related to I R1?

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What does this result mean about your RD vs. RR1? For deliverable 3, include a picture of your circuit setup with your multimeter setup to measure ΔVD and give your result for ΔVD. Include a statement about the sizes of RD vs. RR1 and explain your reasoning using your measurements for ΔVD and ΔVR1. Interesting side note: an isolated LED should have RD ≪ 100 Ω when on, and you may find that your results therefore suggest that your circuit kit LEDs are not truly isolated components. Activity 2a - Power Measurements Next, let’s quantify energy use by a circuit element. For the household batteries we’re using in this lab, the potential energy, and therefore the ability to do work on a charge moving through the circuit, comes from the conversion of chemical energy stored in the battery. The rate of this energy transfer in energy/unit time is electrical power P and is in units of Watts (1 Watt [W] = 1 Joule [J]/second [s]). We say that the energy supplied by the battery is dissipated in various circuit components via other forms of energy, such as heat, light, sound, or energy of motion. We can compute P for a circuit element in terms of current I, resistance R, and applied potential difference ΔV as

.R (ΔV ) P = I2 = I = RΔV 2

(2a)

Using your values of I (in Amps [A]), R (in Ohms [Ω]), and/or ΔV (in Volts [V]) for your LED and for your resistor R1, calculate the power dissipated in each component. For deliverable 4, show your calculations applying Equation 2a above separately for your R1 and LED, reporting your values in units of W. Which one uses more power? Can you think of any situations where you might want high P instead of low P or vice versa? Activity 3a - Applications of LEDs Return to your resistor/LED circuit, but now remove S1 to create an open circuit. You should have found that your values for current in Activity 1a were small (~ 0.01-0.03 A) even though the LED was on, which tells you that LEDs are quite useful detectors of small currents. We can use this circuit to test whether materials are good conductors of electricity, similar to Project #9. Your LED will light up if you complete the circuit with something that is a good conductor, but it won’t light up if the material is a poor conductor (insulator). Locate a few ordinary objects and see if you can find examples of both. Tip: if your test material doesn’t fit between the open portion of your circuit, make the connections with the red/black wire sets included in your kit. For deliverable 5, take a picture of your ‘testor’ circuit with labeled examples of at least 4 objects you found to be conductors vs. insulators. What would be your definition of a good conductor?

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Next, add S1 back into your circuit, but this time reverse the polarity (i.e., direction) of the LED and reassemble your circuit. You should observe that the LED does not turn

Figure 3: LED current direction indicator circuit with motor on when S1 is on. This demonstrates another property of LEDs - namely, they only allow current to pass through them in one direction! This is used widely in many types of circuits, and it can even be a safety feature used to shield devices that can be damaged by current passing through them in the wrong direction. As another example, consider the circuit from Project #276: LED Fan Rotation Indicator, which is shown in Figure 3. Locate the components and assemble the circuit with S1 set to OFF (don’t add the fan attachment to the motor M1 just yet). Note: the fan blades will rotate during part of this experiment, so keep clear of the motor while it is running. Pay careful attention to the orientation of the LEDs (D1, D2) and battery packs B1. Once you have assembled the circuit, slide S1 to ON and you should see your green LED activate while the motor spins clockwise (if not, check the orientation of all components before proceeding further; we had to push down on the L2 connections to make good electrical contact). At this point, L1 and L2 should not appear to be very bright. Based on what you learned about LEDs above, what does the LED tell you about the direction of current flow through R1? Which power source is running the motor? Next, slide S1 to OFF and now press the momentary switch S2 (we had to press hard), which should activate the red LED with the motor spinning counterclockwise. Has the current direction through R1 changed? Which power source is running the motor now? The two sides of the circuit drive the motor in opposite directions, depending on which switch (S1 or S2) is activated and therefore which side

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of a battery is connected to the positive ‘+’ side of M1. The LEDs are acting as indicators for which direction current is flowing. Next, try adding the fan to the motor and try S1 or S2 (but not both at the same time). You should see that now either L1 or L2 light up more, but the LEDs do not activate like they did before without the fan installed. In this case, a lamp lights up regardless of the direction of current, so this tells us that ordinary lamps don’t have a polarity (unlike LEDs). For deliverable 6, include pictures of your working circuit with red LED on, green LED on, and one with your fan installed and one lamp (either L1 or L2) on. Indicate which battery pack is connected and the direction of current flow through R1 in each image. The lamps light up more when the current is high due to the increased load on the motor when the fan is installed, and the LEDs light up when the current is low without the fan. In this case, the lamps also prevent a short-circuit (i.e., direct path between the ‘+’ and ‘-’ sides of a battery) due to their relatively high resistance. What do you think will happen if you leave one of the switches ON for a while? Activity 1b - Introduction to Resistors The goal is this first activity is to understand how voltage, current, and resistance can be related to predict the behavior of various circuit elements. For a component with resistance R experiencing a potential difference ΔV, the resulting current I is

.I = RΔV (1b)

Your textbook refers to this relationship as Ohm’s Law. Let’s start with some predictions based on Equation 1b. Consider if ΔV were assumed constant - how could you control the resulting I? How would you achieve small I vs. large I? Our various devices all plug into a 120 V outlet, but each has different requirements to operate, so you can see already how useful Equation 1b is if you need a different I from only one available value of ΔV. Let’s explore these relationships further with the Ohm’s Law simulation, which is also shown in Figure 4. All current passes through the red cylinder representing a fixed resistor, and the slider controls allow smooth adjustments in the value of ΔV and the resistance R in Ohms Ω. The reported current I is displayed in milliamps (1 mA = 10-3 A). Explore these features and evaluate your understanding of Equation 1b. What is the source of ΔV in your circuit? What happens if you make ΔV larger or smaller? What do the direction and size of the arrows represent?

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Figure 4: Ohm’s Law simulation window Set ΔV = 3.0 V (a typical household value coming from 2 AA batteries end-to-end). If a device would be damaged by I > 50 mA, what does Equation 1b predict for the acceptable range of R values? If a machine requires I > 1 A to run, can you achieve this with 3 V? Why or why not? For deliverable 1, show your calculations for I = 50 mA and I = 1 A, and include screenshots of the simulation that support your answers. Also report your corresponding ranges of acceptable R values. If we rearrange Equation 1b above, we can solve for R = ΔV/I. This implies a useful way to characterize an unknown circuit element if we can apply ΔV and measure I. If a particular electronic circuit element has I = 100 mA when we apply ΔV = 9 V, use this expression to predict the value of R and check your result using the simulation. Activity 2b - Circuit Resistance In this set of experiments you will apply Equation 1a to a few simple circuits. Open the circuit construction simulation and build a circuit consisting of a 9 V battery, a single R1 = 10 Ω resistor (default values), and include a switch (e.g., Figure 5). Have ‘Values’ and ‘Labels’ checked in the options in the upper right. What do you expect to measure for ΔV1 across the resistor with the switch open? How about when the switch is closed? Test your predictions by connecting the voltmeter to the circuit at both ends of the resistor. If you measure ΔV1 < 0, what does that mean? Figure 5: ex. battery + resistor circuit Now change the resistor to R2 = 20 Ω and measure ΔV2. Is your ΔV2 value different from ΔV1? For deliverable 2, include screenshots of your R = 10 Ω and R = 20 Ω

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circuits and give your readings for ΔV1 and ΔV2. Include an explanation for why the voltages are (or are not) the same. Consider the case where ΔV = 12 V, what would your new values be for ΔV1 and ΔV2 for your resistors?. Now go back to R1 = 10 Ω, and adjust your battery to 12 V by clicking on it and adjusting the slider at the bottom. Use Equation 1b to calculate your predicted value of current I in Amps (A) in this circuit. Remove one of the wires and replace it with the ammeter (used to measure current) from the tools on the right side, connected between the same two points and check your answer. For deliverable 3, include a screenshot of your circuit with the ammeter added and show your calculations for the predicted I compared to your ammeter measurement. Make sure you still have ‘Values’ checked. If the battery polarity (i.e., orientation of + and - sides) were reversed, do you predict anything in the circuit will change? Click the battery and change its orientation using the button in the edit window that appears at the bottom of the simulation to the left of the voltage slider. Check ‘Show Current’ on the upper right options window and observe the flow of charges. Return the battery to its original polarity and observe the current again. Do you notice any differences? For deliverable 4, include screenshots of your circuit for both battery orientations and label each with the direction(s) of current and an explanation of your results (hint: recall Lab 3). Figure 6: light bulb circuit Now create a new circuit by replacing the resistor

with a light bulb, similar to the Figure 6. Can you make your bulb light? Can you use your intuition from the previous set of experiments to predict what will happen (if anything) to the bulb brightness when the battery polarity and/or voltage is changed? Test your predictions by selecting the battery to (1) vary the polarity and (2) the battery V, and record your observations. Were your predictions correct? For deliverable 5, include screenshots of your circuit at two values of battery V and another with the battery polarity flipped and give an explanation as to why

your bulb brightness changed. Did the light bulb seem to care which way the current was flowing? Label the current directions for each circuit.

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Activity 3b - Power Measurements For real batteries, the potential energy, and therefore the ability to do work moving charge around the circuit, comes from the conversion of chemical energy stored in the battery. The rate of this energy transfer in energy/unit time is electrical power P and is in units of Watts (1 Watt [W] = 1 Joule [J]/second [s]). We say that the energy supplied by the battery is dissipated in various circuit components via other forms of energy, such as heat, light, sound, or energy of motion. We can compute P for a circuit element in terms of current I, resistance R, and applied potential difference ΔV as

.R (ΔV ) P = I2 = I = RΔV 2

(2b)

For your light bulb circuit in Activity 2b, calculate the power dissipated in your light bulb by assuming that it carries a current I = 0.3 A when connected to a battery with ΔV = 4 V. What is the value of your light bulb resistance R B1? Consider another light bulb with a resistance RB2 = 2R B1. Which one uses more energy in 1 second? For deliverable 6, show your calculations applying Equation 2b above separately for RB1 and R B2, reporting your values in units of W. Can you think of any situations where you might want a high P instead of low P bulb or vice versa? Activity 4a - Variable Resistance

In everyday electronics it is often desirable to have a variable resistor. From our experiments so far, you hopefully understand that a variable R can result in a variable I for fixed ΔV. Consider the volume control on your stereo, for example, which acts similarly. When you want more volume, you need to supply more current to the speakers, which you can accomplish by decreasing R. If you want less, you increase R to achieve the opposite effect. Locate your Snap Circuits variable resistor component ‘RV’ and you will notice it has three leads. It consists of a normal 50 kΩ resistor with an additional third contact at the base of the arrow which permits you to control how much of that resistor you’re using with the slider. Locate the components and assemble Project #172 as shown in Figure 7.

Figure 7: variable resistor circuit

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Set both S1 and S2 switches to ON. What do you observe as you adjust RV by moving the slider back and forth? You should find that the LEDs vary in brightness. Based on your results, can you identify which slider position corresponds to low R for the LED on that side of your circuit? What is the purpose of the 1 kΩ resistor in the circuit? For deliverable 7, include two pictures of your completed circuit with your LEDs operating on each side. For each image, label which side of the circuit (the red LED side or the green LED side) had the lowest resistance and explain your reasoning.

There are other types of resistors as well. Your kit includes thermistors, which help limit high currents and prevent damage to equipment by increasing their resistance in response to the increase in temperatures. If you carefully examine your battery packs, you will see them hidden inside, and they are intended to prevent high currents from damaging your batteries and/or components in the event of an accidental short circuit. There are also photoresistors, which are another kind of variable resistor that responds to light (e.g., streetlights or nightlights and automatic headlights are example applications). If you are interested in an example of photoresistors, you can explore one after lab on your own in Project #272. A microphone resistor exploration is in Project #273. Activity 5a - Resistance of Water In this activity, you will explore the conductivity of water vs. saltwater. To show how water can be a conductor of electricity, locate and construct Project #166 (Figure 8). You’ll also need a small cup of water and your wire connectors from your circuit kit. Note: be careful not to spill your water on your circuit components, since you will likely

Figure 8: water conductivity test circuit

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damage them and you may be injured by your battery pack. After confirming your circuit connections, test your circuit by directly connecting the red and black leads together. If your circuit is working correctly your LED should be fairly bright.

Now separate the red/black leads and place them in a cup of tap water about ½ full. You should notice your LED light up slightly. What can you conclude about the conductivity of your tap water? This experiment will not work well if you use distilled water, so you want to find a non-purified source. The brightness will depend on the amount of minerals and other contaminants in your water. Can you predict what will happen (if anything) if you were to add more of the same water to your cup? Test your prediction by observing your LED as you fill the remaining space in your cup. Pure water has very high resistance, so this limits the flow of current available to your LED. Impurities will lower the resistance and allow more current to flow. If you have salt available, repeat your experiment with some dissolved in your water. Did your LED get brighter? What would happen if you added even more salt? For deliverable 8, include a picture of your working water detector (either with or without salt). For our bodies, the conducting properties of water are a key reason why we need to stay hydrated for proper nutrient circulation, organ function, and muscle/nervous system control. Various types of ions of Potassium, Calcium, Magnesium, and others are referred to as electrolytes, and having high enough concentrations of them in our bodies helps them to function properly. In general, it’s important to remember that water is an excellent conductor, so you should always be mindful that water is very hazardous near sources of electricity due to the high currents it allows to flow. Activity 4b - Ohm’s Law in Resistors vs. Diodes vs. Light Bulbs In this activity, our goal is to better understand how different circuit elements may or may not obey Equation 1b, which describes a linear relationship between the applied voltage and the resulting current. Those circuit elements that do not obey Equation 1b are referred to as non-Ohmic. To explore this variation in behavior, navigate to the Ohm’s Law illustration, and select ‘Resistor’ in the list of elements below the description to pull up a graph/circuit display similar to Figure 9. Here, the rectangle labeled with ‘R’ in the circuit diagram at left is the resistor being studied, ‘Vac’ is the voltage source, ‘V’ represents the voltmeter, and ‘A’ represents the ammeter to measure current (negligible current passes through the voltmeter, so we will assume going forward that all current passes through R). This graph is current I (in mA) vs. voltage V (in V), and the red line represents the set of data points generated by varying V and measuring I. Verify this by adjusting the V slider back and forth to trace the graph. Click and hold on a point on the line to read off specific data points (x, y) = (V, A). For deliverable 7, measure three

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Figure 9: Ohm’s Law analysis for resistor R sets of data points and show your work to calculate R for each measurement, and report the average value of R in Ohms. Note: be sure to sample a wide range of I values since the values only change by increments of 10 mA. Next, select ‘Diode’ in the list below the graph description, and the circuit & graph will update to show the I-V relationship for the diode. Do you notice any differences? Is this relationship linear? This illustrates an example of a non-Ohmic circuit element. Note that as you move the voltage slider to trace the plot, there is a significant range of V values where I ≅ 0, then the I value seems to increase rapidly. Use the graphing tool to find values of voltage V1 where I1 = 0.5 A, V2 where I 2 = 1.0 A, and V3 where I 3 = 2.0 A. For deliverable 8, report your corresponding voltage/current values, and calculate the corresponding values of resistance R1, R2, and R3. Interpret your results and compare/contrast this behavior with the fixed resistor you analyzed in Figure 9.

Figure 10: Ohm’s Law analysis of realistic light bulb filament

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Now select the ‘Real lightbulb’ circuit and explore the shape of this graph (Figure 10), taking note of any differences in the behavior of this circuit element. What does the slope of the line represent (noting the change in current units)? Check your understanding by reading current values I1 at V1 = 1.0 V and I2 at V 2 = 4.0 V and then computing the lightbulb resistance values. Does R1 = R2? For deliverable 9, include your data points and R values and a snapshot of your simulation set to one of your voltage values. Decide if the bulb is Ohmic or non-Ohmic and explain your reasoning. Activity 5b - Applications of Resistance Why do some things conduct electricity while others do not? Another way to learn about the electrical properties of everyday objects is to see which can be used to complete a circuit. Navigate back to the virtual circuits simulation. Create a new circuit with only a 1.5 V battery (similar to a D cell common in flashlights) and wires to light up a single bulb, using the items in the list below to create a closed circuit (e.g., Figure 11). Scroll through the circuit elements list on the left of the simulation to find each item.

Figure 11: testing a paper clip to complete a light bulb circuit Record your results to complete your own version of the list below.

Item Bulb turned on Bulb did not turn on paper clip pencil dog hand dollar coin eraser

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Which items could be used to turn on the light bulb? Do they have anything in common? Do you think you could turn on the light bulb using two items from your home (not in the list above)? Why did you choose your items? Objects that allow electricity to flow through them are conductors and those that do not are called insulators. For deliverable 10, include your answers to the questions above and make a table of all of the objects (including any of your own) that classifies them as conductors vs. insulators. 3. Deliverables For full credit please include the following in your lab report. Follow the template provided on the Weebly Lab 4 page and include one deliverable per Google Slide in the order that they are presented for your set of activities below. Always label your images.

1. Pictures of your circuit with S1 OFF/ON, including multimeter with readings. 2. A picture of your activated LED circuit and your calculations for IR1. Include your

connected multimeter and give your measured value of ΔVR1. 3. A picture of your circuit and multimeter to measure ΔVD and its value. Compare

RD vs. RR1 and explain your reasoning using ΔVD and ΔV R1. 4. Show your work applying Equation 2a for R1 and LED, reporting values in W.

Indicate which uses more P and a few example situations for high P vs. low P. 5. A picture of your ‘testor’ circuit with examples of ≥ 4 objects you found to be

conductors vs. insulators. 6. Three pictures of your circuits: (1) red LED on, (2) green LED on, (3) fan installed

and L1/L2 on. Indicate which battery pack is connected and the direction of current flow through R1 in each image.

7. Pictures of your circuit with LEDs operating on each side. Label which side (red/green LED) had the lowest resistance and explain your reasoning.

8. A picture of your working water detector (either with or without salt).

1. Your calculations for I = 50 mA and I = 1 A; simulation screenshots that support your answers; corresponding ranges of acceptable R values.

2. Screenshots of your R = 10 Ω and R = 20 Ω circuits and readings for ΔV1 and ΔV2. Include an explanation for why the voltages are (or are not) the same.

3. A circuit screenshot with the ammeter added and your calculations for the predicted I compared to your ammeter measurement.

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4. Screenshots of your circuit for both battery orientations and label each with the direction(s) of current and an explanation of your results.

5. Screenshots at two values of V and another with battery polarity flipped; explanation as to why your bulb brightness changed. Did the light bulb seem to care which way the current was flowing? Label the current directions.

6. Show your work applying Equation 2b for RB1 and RB2, reporting your values in W. Indicate which uses more P and example situations for high P vs. low P bulb.

7. Three sets of data points and show your work to calculate R for each and the average R in Ohms.

8. Report your V and I values, calculate the values of resistance R1, R2, and R3. Interpret your results and compare/contrast this behavior with the fixed resistor.

9. Data points and R values and simulation snapshot set to one of your voltage values. Say if the lightbulb is Ohmic or non-Ohmic and explain your reasoning.

10.Which items could be used to turn on the light bulb? Do they have anything in common? Do you think you could turn on the light bulb using two items from your home? Why did you choose your items? A table of all objects (including your own) that classifies them as conductors vs. insulators.

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