Complex Electrical Circuits



Lab 6: Complex Electrical Circuits, Part I

|Introduction |In this laboratory you will connect electric lamps together in a variety of circuits. The purpose of these|

| |exercises is to extend your understanding of the “physics” of simple electrical circuits. That is, the |

| |purpose is for you to learn qualitative properties of circuits. There is an additional purpose, to help |

| |you learn the skills of being able to predict what you expect to happen, observe what happens, and correct |

| |the original misunderstanding, if necessary. What you learn in assembling, observing, and measuring the |

| |properties of these circuits will be applicable to the real-world electrical circuits you will meet on the |

| |job and in everyday life. |

| |Note that, as in the last lab, you will be asked to make predictions, then measurements, and then to |

| |discuss how the predictions and measurements agreed. You will not be graded on the predictions, but rather|

| |on your measurements, observations, and discussion. |

1. Series connections

a. The effect of an ammeter

|Introduction |In an ideal world, a meter should have no effect on the circuit it’s measuring. Before making measurements,|

| |then we should find out if our DMM used as an ammeter changes the circuit in which it is used. To make this|

| |test, you will need two meters, so find another pair of students and do this part together. |

|Procedure |• Make a single-bulb circuit with a socket using the #502 bulb. |

| |• Set the power supply at 5 volts. Measure the current through the bulb first with the meter on the 300 mA|

| |range, and then with it on the 10 A range (remember that you have to plug the red lead into a different |

| |terminal). Are the currents the same? Is the bulb the same brightness? What changed? |

| |• Use the second DMM as a voltmeter to measure the potential drop across the ammeter. An ideal ammeter |

| |would have no voltage across it. Is the DMM ideal? Does the voltage drop depend on whether it is used on |

| |the 300 mA or the 10 A range? |

| |• Explain why the current through the bulb depends on the ammeter range. |

b. The second type of bulb

|Introduction |This week you will use two different types of bulbs. One has the number 425 on the base, the other 502. |

| |The first task is to find out how these two bulbs differ. |

|Procedure |• Return to the single-bulb circuit with the power supply set at 5 volts. Screw first one bulb into the |

| |socket and then the second type. Compare their brightnesses. |

| |• Predict which bulb will draw more current. Predict which will have the higher resistance. |

| |• Now let’s make a measurement and see if you were correct. Use the DMM to measure the voltage of the |

| |power supply and adjust it to be exactly 5 volts. |

| |• Measure the current in the circuit and record your result on the worksheet. Hint: in view of your |

| |results in part a, can you measure the current through one bulb on the 300 mA range and the other on the |

| |10 A range? |

| |• Measure the current through the second bulb. |

| |• Calculate the resistances of the two bulbs when each has 5 volts across it. |

c. Two different bulbs in series

|Introduction and prediction |You now know which bulb is brighter, which draws more current, and which has a higher resistance. Predict |

| |which bulb will be brighter when the two lamps are connected in series. Write your prediction on the |

| |worksheet now! |

[pic]

Figure 1. Series circuit

|Procedure |• Now try the experiment. Wire the circuit shown in Figure 1. Again, set the power supply at 5 volts. |

| |Remember to put a 425 bulb in one socket and a 502 into the other. |

| |• Discuss any discrepancies with your predictions. |

| |• There is a mystery here. Why doesn’t the one bulb glow? Perhaps no current is going through it. See |

| |what happens if you unscrew the bulb. Explain as fully as you can why it doesn’t glow when it is in |

| |series with the other bulb. |

d. The equivalent resistance of a series combination

|Introduction |Each bulb acts as an obstacle to the flow of charge and develops a potential drop across it. You can also |

| |consider the two bulbs in series as a single obstacle. There is a current through “it” and a potential |

| |difference across “it,” so “it” has a resistance. How does the resistance of this series combination |

| |compare with the resistances of each bulb? |

|Procedure |• With the two bulbs in series, determine the resistance of each bulb. Remember that because the |

| |resistance of a bulb depends on the current through it, you can’t use the ohmmeter or the measurements you|

| |made above to find the resistances. Make one measurement of the current through the bulbs (Question: |

| |Which ammeter range should you use so that it has the least effect on your results?) Measure the potential|

| |differences across each bulb. The worksheet will guide you. |

| |• Do you need to make further measurements to find the resistance of the two bulbs in series? If not, |

| |which previously tested “law” of circuit analysis do you need to use? If you want to make another |

| |measurement, go ahead. Then calculate the resistance. |

| |• Compare the resistance of the series combination to that of the individual bulbs. |

2. Parallel Connections

a. Two bulbs in parallel

|Introduction and prediction |Imagine the two circuits shown in Figure 2. Bulb C in the circuit on the right is “in parallel” with the |

| |bulb B. Now, make a prediction. What will be the order of brightness of the three bulbs, A, B, and C? In|

| |other words, which will be brightest, second brightest, and least brightest. You may also say that you |

| |think two or more bulbs will have the same brightness. Write down your prediction on the worksheet. |

[pic]

Figure 2. Single bulb and parallel bulb circuits

|Procedure |• Build the circuit, using #502 bulbs, and see. A good way to compare brightnesses is to watch the |

| |brigtness of B as you quickly connect and disconnect C. Any surprises? What can you conclude about the |

| |current in each of the parallel bulbs in comparison with the current through the single bulb? |

| |• Let’s check this conclusion by making some current measurements. To help you make these measurements, |

| |change the wiring slightly to that shown in Figure 3 below. Use one clip lead or wire between points B |

| |and C and another between B and G. Then use a third one to connect to the power supply at A. Do the same|

| |for DE, EH, and EF. Make sure both bulbs light. |

[pic]

Figure 3. Parallel Bulbs

|• Before making a measurement, make a quantitative prediction. Suppose the current through a single bulb|

|is i1 On the circuit diagram on the worksheet, write your predictions in the boxes for the current in |

|every wire in terms of the current i1. |

|Discuss the predictions carefully with your partner. |

|• Measure the actual currents in the various branches of the circuit, and write down the results in the |

|boxes on the answer sheet in terms of the current through the #502 bulb that you measured in part 1 of |

|this laboratory. Note that you will have to think carefully about making this measurement. To measure |

|the current in wire AB you have to replace the wire with the ammeter. The same is true for the wire BC. |

|Be sure to use the ammeter range that will have the least effect on your results. |

|• Discuss any discrepancies between your predictions and your measurements. |

|• There is another important fact about parallel combinations. Measure the potential difference across |

|bulb #1 and then across bulb #2. How do they compare? How do they compare with the EMF produced by the |

|power supply? |

b. Current in the branches

|Introduction and prediction |In the case of two identical bulbs the currents through the two bulbs were equal. The sum of these |

| |currents was equal to the current from the power supply. How does the brightness of one bulb depend on the|

| |second bulb? What does that mean about the current? Predict what would happen to the current through #1 |

| |if you were to remove bulb #2. Predict what would happen if you replaced #2 with a 425 bulb. Write these |

| |predictions on the worksheet. |

|Procedure |• Try it. Unscrew the bulb #2. What do you see? Does what you see agree with your prediction? |

| |• Measure the current through bulb #1 now. |

| |• While measuring the current through #1 screw a 425 bulb into the #2 socket. What happens to the current|

| |through bulb #1? |

| |• Measure the current through bulb #2 and the current from the power supply (wire AB). Are the currents |

| |#1 and #2 equal? Is their sum equal to the current from the supply? In other words, is Kirchhoff’s node |

| |law satisfied? |

|Prediction |What would happen if a third bulb were put in parallel with the other two? Pat, Leslie, and Robin disagreed|

| |again. Pat said that the first two bulbs would get dimmer, because the current through them would decrease|

| |in order to satisfy Kirchhoff’s node law. Leslie said that one-third of the current would go through the |

| |new bulb, so the currents through the other bulbs would adjust. Robin said that the brightness of the first|

| |two bulbs wouldn’t change, because the power supply would supply whatever current was needed. With whom do|

| |you agree? Use the knowledge you gained in the experiment above to write brief explanations to the three |

| |students. |

c. The equivalent resistance of a parallel combination

|Introduction |Each bulb acts as an obstacle to the flow of charge and develops a potential drop across it. You can also |

| |consider the two bulbs in parallel as a single obstacle. There is a current through “it” and a potential |

| |drop across “it,“ so “it” has a resistance. How does the resistance of this parallel combination compare |

| |with the resistances of each bulb? |

|Procedure |• Determine the resistance of each bulb. You have measured the currents in the preceding part. In case |

| |you have changed the power supply setting, you should probably recheck the EMF. |

| |• Do you need to make further measurements to find the resistance of the two bulbs in parallel? If not, |

| |go ahead and calculate the resistance. |

| |• Compare the resistance of the parallel combination to that of the individual bulbs. |

Names

Section

Complex Electrical Circuits, Part I

1. Series connections

a. The effect of ammeters

|Range |300 mA |10 A |

|Current (A) | | |

|Potential drop across ammeter | | |

|(V) | | |

Why does current through the bulb depend on the ammeter range? Use Kirchhoff’s loop law in your explanation.

b. The second type of bulb

Prediction: which bulb will draw more current: 425 or 502? Justify your answer please.

Prediction: which bulb will have more resistance: 425 or 502? Justify your answer please.

|V = (volts) | 425 bulb |I = (amps) |R = Ω |

|V = (volts) | 502 bulb |I = (amps) |R = Ω |

Comments on the correctness of your predictions.

c. Two different bulbs in series

Prediction: When the 425 and 502 are connected in series, how bright will each be compared with their brightness when each is the only bulb in the circuit.

Explain your reasoning.

Observed brightnesses:

Comment on agreement or disagreement:

Why doesn’t the one bulb glow yet the other one does? Explanation.

d. The equivalent resistance of a series combination

Resistance of 425 when in series circuit: R = _____ (V)/_____(A) = (Ω)

Resistance of 502 when in series circuit: R =_____ (V)/_____(A) = (Ω)

Resistance of two in series: R = _____ (V)/_____(A) = (Ω)

Compare resistance of series combination to resistance of any one of the two devices. (i.e. it is smaller, equal to, or larger than a single bulb.)

2. Parallel Connections

a. Two bulbs in parallel

Predict the order of brightness of A, B, and C.

Observed order of brightness:

Comments:

How do currents in parallel bulbs compare with the current in a single bulb?

Predicted currents in terms of I1.

[pic]

Measured currents

[pic]

Comments on agreement or disagreement:

Bulb #1 V = Bulb #2 V =

b. Current in the branches

Prediction: What would happen to current through #1 if Bulb #2 were removed? Justify your reasoning:

Prediction: What would happen to current through #1 if second type of bulb were put into #2 socket? Justify your reasoning:

Observation: What happened when #2 was removed:

Measurement: current through #1. Compare with previous measurement:

Observation: What happened when second type of bulb was used in #2 socket:

Measurement: current through #1. Compare with previous measurement:

Is current through #1 equal to current through #2?

Measurement: current from power supply:

Does current from supply equal to sum of currents in bulbs? Show the work that supports your conclusion.

Summarize what you have learned about parallel combinations:

Describe how the currents in the branches compare with the total current.

Describe how the current in one branch depends on the current in the other branch.

Describe how the voltages across the two branches compare with each other and with the voltage produced by the power supply.

Regarding the question of putting a third bulb were put in parallel with the other two. Whose prediction do you support?

Comments to Pat

Leslie

Robin

c. The equivalent resistance of a parallel combination

502 bulb R = (Ω) 425 bulb R = (Ω)

Parallel combination (make sure you show your method)

R = (Ω)

Compare resistance of parallel combination to that of any of the individual bulbs.

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