Lightbulbs and Dimmer Switches: DC Circuits

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1 Introduction It is truly amazing how much we rely on electricity, and especially on devices operated off of DC current. Your PDA, cell phone, laptop computer and calculator are all examples of DC electronics. In fact, any electronic device you use that has a transformer (one of those big bricks that makes it hard to use all 5 sockets on a power strip) is run off DC. In this lab you will be experimenting with basic DC circuits. You will qualitatively and quantitatively explore how current functions in parallel and in series. You will also learn how to use basic electronic measuring devices, including a digital multi-meter and an ampmeter. One of the keys to getting any electronic device working in an efficient manner is being organized. In this lab, it is very easy to end up with a tangle of wires and switches with light bulbs haphazardly poking out of the mess at odd angles. To avoid this electronic spaghetti, you should neatly arrange your circuits on the tabletop. Don t be afraid to spread out and build a circuit that looks identical to the diagrams in this lab handout. Inventory V Battery 1Battery Charger 1 Digital multimeter 1 Simpson Ampmeter 3 Light bulbs 1 Potentiometer 4 pair of alligator clip wires Ω Resistor Ω Resistor Ω Resistor 1/6

2 Ohm s and Kirchoff s Laws When you are faced with any given circuit you have just three quantities to worry about: Voltage, V, across the circuit; current, I, through the circuit; and resistance, R, in the circuit. These three quantities are related by Ohm s Law: V = I R. In a closed system for any complete loop in a circuit the total voltage will stay constant. This means that as a circuit forks into multiple pieces that come back together, the voltage in each branch will be the same as the voltage drop between the start and end points of the branch. This means that the sum of voltage drops across all the resistors in a single branch in the circuit is equal to the voltage across the The current in a circuit, however, isn t the same in different branches in a circuit. At any junction, the current that flows in will equal the current that flows out. The amount of current in each branch will be directly related to the total resistance of each branch, where the current will be higher in the branch that offers lower resistance. In order to figure out the current anywhere in a closed circuit, all you need to know is the voltage supplied to the circuit and the resistance. Mathematically, the total resistance from N resistors in series is just the sum of their resistances (Remembering that sum of the voltages across each resistor is the same as the total voltage across the circuit, and current is constant in a closed loop in the circuit): V = V 1 + V V N = IR 1 + IR IR N = I (R 1 + R R N ) The total resistance of several resistors in different, parallel branches of a circuit, however is related to the sums of the currents (remembering that V is the same across each branch): I in = I 1 + I I N = V/R 1 + V/R V/R N Resistors come in a couple of different forms. Ohmic devices have the same resistance no matter how much current you run through them (assuming you don t blow them up). Non- Ohmic devices have resistances that can be a function of many different things: temperature, current, pressure, etc. Light bulbs are one example of a non-ohmic device; the filament undergoes huge changes in temperature when current passes through it. Therefore, the resistance of the filament is not constant; rather, it increases with current. These rules were summarized by G.R. Kirchhoff, who stated: 1) At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction 2) The sum of the changes in potential around any closed path of a circuit must be zero. 2/6

3 Before you start This section should be completed prior to attending lab class. 1. In lab we will be working with light bulbs. Before using them as an element in a circuit, you should work to understand how they work. Find a standard incandescent light bulb and exam it closely. How many conductors and insulators are in the light bulb? How many wires connect to the filament and where do they connect in the base? How does the base work? Sketch the bulb and carefully label all the components. 2. You will be connecting your light bulb into the circuit using a light bulb socket. This is another element you should work to understand. Exam a light bulb socket closely. How does the socket work? Sketch the bulb and carefully label all the components. 3. Some electronic components only work if the positive terminal on a battery is connected to them in a specific way. What will happen if you try connecting the light bulb to the battery in different ways, reversing how the current flows through the light bulb. In class you will be building 7 different circuits. Prior to coming to class we re going to have you predict what you expect will happen in each circuit. As well as writing up the questions below in your pre-lab, please prepare a worksheet for use in lab that contains your pre-lab predictions and spaces for your in class results. Circuit 1 & 2: two identical light bulbs are wired in series with a 6.5V battery. 4. Given that the bulbs and batteries in both circuits 1 and 2 are identical, rank what you would expect the order of brightness of the bulbs to be. That is, relate the brightness of A, B, and C with >, <, or = signs. Explain your reasoning. 5. What happens if you unscrew one of the bulbs? What would happen if you short out one of the bulbs? How does the brightness of the unshorted bulb compare to that of the single bulb C in circuit 1? 6. What would you predict for the brightness of the bulbs if a third bulb were added in series with A and B? (This is Circuit 3) 7. For a given battery, does the same amount of current always flow or does it depend on the particular circuit connected to it? 3/6

4 Circuit 3: Two identical light bulbs are wired in parallel with a 6.5V battery. 8. Rank the brightness of bulbs C, D and E using >, <, or = signs. What is your reasoning? 9. Would removing one bulb change the brightness of the other bulb? Why? 10. Will the total current through the battery change when you unscrew one bulb? 11. Suppose we were to increase the number of bulbs to three. What would happen to the brightness of each bulb? Circuit 4: Now you can combine the knowledge gained in the one and two bulbs circuits. Consider circuit 4. With the switch 2 open and switch 1 closed, this circuit is identical to Circuit 2. However, a third bulb F can be added in parallel with bulb B by closing switch Rank the brightness of bulbs A, B and F using >, <, or = signs. What is your reasoning? 13. The brightness of bulb A indicates the total current in the circuit. Will the total current in the circuit increase or decrease when bulb F is added? 14. Did the total resistance of the circuit increase, decrease, or remain the same when F was added? 15. What happens to the current at the point marked a? Circuit 5: In this circuit you will be replacing the light bulb in circuit 1 with a standard resistor. In class you will be measuring the current and voltage with instruments discussed in the next section of the lab. For the prelab, please ignore the symbols in the diagram. 16. What is the current through this circuit? V A Circuit 5 R Ω V 17. What is the voltage across the resistor? 18. Calculate the power dissipated 4/6

5 Circuit 6: As above, ignore the symbols for an ampmeter and digital voltmeter in the diagram. This circuit replaces the two light bulbs in circuit 2 with 200 Ω and 300 Ω resistors. 19. Using the component values shown in the figure, predict how much current will flow through the ammeter. Compare this with the current predicted for Circuit 5. Explain. 20. Predict the voltage drop across R 2 (V ab ). 21. Predict the voltage drop across R 3 (V bc ). 22. Using Kirchoff s Loop Rule, predict the relationship between V ab, V bc, and V bat, the voltage across the battery. Circuit 7: As above, ignore the symbols for an amp-meter and digital voltmeter in the diagram. This circuit replaces the light bulbs in circuit 3 with 300 Ω and 510 Ω resistors. 23. Predict the current through R 4 (I 4 ). 24. Predict the current through R 5 (I 5 ). 25. Predict the current flow through the battery (I total ). 26. Using Kirchoff s Junction Rule, predict the relationship between I 4, I 5 and I total. 27. Using resistor-addition rules, calculate the equivalent resistance for R 4 in parallel with R 5. 5/6

6 Circuit 8: In this circuit, the potentiometer, R v, behaves as a variable voltage divider. The resistance between points d and f is adjusted by changing the location of the center contact. The resistance between points d and e is fixed at 25 ohms, while the resistance between d and f varies from 0 to 25 ohms. If a voltage is applied across the potentiometer, any percentage of that voltage will be applied to the center terminal f. That is, if V de = 6.5 volts, then V fe can be set to any voltage between 0 and 6.5 volts. The volume control on your radio is an example of a potentiometer. 28. Predict the maximum current flow through R 6 (I 6 ). Prepare a sheet of graph paper to show how I 6 varies with V fe and make a plot showing I and V for R 6 = 0, 5, 10, 15, 20 and 25 Ω. In Class Circuitry When you come to class build all of the above circuits and compare your prelab values to your experimental values. There will be some discrepancies between your predicted and actual values. Taking the gold/silver bands on the resistors into consideration, discuss if your discrepancies make sense, and where they are coming from. Submission Check List Answer all the questions above in class. Turn in your pre-prepared worksheet with pre-lab values. 6/6