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Name___________________ ID number_________________________

Date____________________ Lab partner_________________________

Lab CRN________________ Lab instructor_______________________

Ph 2306

Experiment 5: Direct Current (DC) circuits, Part 1

Objective

To learn about the concepts of current and potential difference (voltage drop) using simple circuits consisting of batteries, light bulbs, switches, and resistors. Required background reading Young and Freedman, sections 25.1, 25.2, 25.3 25.4

Introduction

In this lab and the next lab you will be working with direct-current (DC) circuits. DC circuits are those in which the direction of current does not change with time. This is in contrast to alternating current (AC) circuits, where the direction of current oscillates back and forth. In the next two labs, we will also usually restrict ourselves to the situation where the circuit properties are not varying with time (time independent). A typical electric circuit consists of various electrical elements (batteries, light bulbs, switches, wires, and resistors, for example) connected together such that the electrical path forms a closed loop or complete circuit. Electric charge flows steadily around such a loop; this is referred to as the electric current. Note that we can only have a steady current in a situation with a complete circuit; if the continuity of the circuit is broken, the current quickly drops to zero. The reason for this is explained on page 857 (12th edition) or page 955 (11th edition) of Young and Freedman. For each type of circuit element above, there is a change in electric potential energy of electric charges that pass through it. The circuit elements can be grouped into two basic categories – those for which there is a decrease in electric potential energy when charges pass through it and those for which there is an increase in electric potential energy. We consider each type of device in turn below. An example of a device where there is a potential energy gain (voltage increase) is a battery. A battery is a device that generates an electric potential difference from other forms of energy. You will be using familiar D-cell chemical batteries that convert internal chemical energy into electrical energy. As a result of the electric potential difference between the terminals of the battery, electric charge is repelled from one

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terminal of the battery and attracted to the other. When a battery is connected as part of a complete electrical circuit electric charge flows around the circuit. The potential energy increase that batteries give to electric charges is dissipated when the charge passes through circuit elements that provide resistance to the flow of electric charge. These circuit elements all cause a decrease in the electric potential energy when charges pass through them (voltage drop). For example, when a light bulb is connected to a battery, the flow of charge through the light bulb’s filament causes the light bulb to glow. In this case, the chemical energy from the battery is transformed into electric potential energy of the charges flowing in the circuit. When the charges pass through the the light bulb, they lose some of their electric potential energy, which is then transferred to thermal energy and the light that you observe. The behavior of dissipative elements like this is characterized by the quantity resistance:

V/IR = where R is the resistance of the element, I is the current flowing through it, and V is the voltage (electric potential) drop. Other examples of devices like this are resistors, which are circuit devices designed to have a specific value of resistance and conducting electrical wires. Conducting electrical wires usually have very low electrical resistance, so we will usually consider them as having “zero” resistance, which means there is no change in electric potential when a current passes through it. In this lab you will use simple circuits to investigate the physical measurements that characterize circuits – current, voltage (electric potential) changes, and resistance. The goal is for you to develop a qualitative intuition about these quantities before doing more quantitative work in the next lab. Another goal is for you to learn how to look at a “textbook perfect” circuit diagram and to actually build it with real wires, batteries and components.

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Name___________________ ID number________________________

Lab CRN number _________ TA name _________________________

Ph 2306 Experiment 5

Prelab assignment (complete and turn in at the beginning of your lab session)

Before answering the questions below, read the material on pages 5 – 10 of this writeup. 1. a) Which of the four models of current flow described in Figure 5 on page 9 do you believe to be correct? Explain your reasoning. b) To test each model proposed in Figure 5 on page 9, you will construct a circuit like that shown in Figure 6 on page 10. For each model, indicate what the current direction and relative magnitude of the two current probes (CP1 and CP2) will be in the table below: (Note: if the current in the current probe flows in the direction of the arrow on the probe, then we define it as positive current flow.) Please copy your answers from here to the table on page 11 for use during the lab).

Current

Probe Positive, negative,

or zero? CP1 > CP2,

CP1< CP2, or CP1=CP2?

CP1 Model A

CP2

CP1 Model B

CP2

CP1 Model C

CP2

CP1 Model D

CP2

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2. a) The two circuits drawn below consist of identical batteries and identical light bulbs A, B, and C. Rank the relative brightness of the bulbs using >, < , and = signs.

b) In the circuit shown below, D and E are identical light bulbs. How do you predict the brightness of bulb D will change when the switch is closed?

3. In this lab, you will observe that the voltage measured across a battery connected in a circuit with current flowing is lower than the voltage measured across a disconnected battery. Why is this true?

+ _ A +

_

B

C

+ _ D E

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Equipment

You will use the following equipment: • Pasco EM-8656 AC/DC Electronics Laboratory • 2 D cell (1.5 V) batteries • Seven 10 cm long wires (in Ziploc bag) • Six 25 cm long wires (in Ziploc bag) • Two 33 ohm resistors (in Ziploc bag) • Two Pasco CI-6556 current sensors (plugged into channels A and B of the Science

Workshop 750 Interface) • One Pasco voltage sensor (plugged into channel C of the Science Workshop

interface) • Two red and two black banana plug – banana plug cables (these are probably already

connected to the two current sensors • Three red and three black banana plug to alligator clip adapters (some of these may

already be connected to the cables attached to the current and voltage sensor) Since this lab involves many components, you should take a minute to see if everything in the above list is present at your station. If you are missing something, consult with your TA. First, take a chance to get familiar with some of the equipment you will be using and how the different pieces connect together. The heart of the activity will be the Pasco EM-8656 “breadboard” (shown in Figure 1) that you will use as the backbone for building your circuits. Connections are made on this board by using the pieces of wire with white insulation on them. To make a connection, push the stripped end of the wire into the spring. For maximum effect, the stripped part of the wire should extend so that it passes completely across the spring, making contact with the spring at four points. See the figure below. Components (like resistors) with stripped wire ends can also be connected in this way.

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Figure 1: Picture of Pasco EM-8656 Electronics laboratory along with schematic diagram pointing out some of the important components. Only the circled components will be used in this lab. For the springs in the lower half of the board, it is important to realize that each of the two springs is electrically connected to the other (as indicated with the white line connecting the two springs on the circuit board).

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Also locate the two current sensors (Figure 2a) and the voltage sensor (Figure 2b). The two current sensors should be plugged in to channels A and B of the Science Workshop 750 Interface. The voltage sensor should be plugged in to channel C of the ScienceWorkshop 750 interface. Both the current and voltage sensor should have cables with banana plugs attached. The male banana plugs are useful for connecting to female banana plug connectors like those on the lower right hand of the Pasco CM-8656 board. You are also provided with adapters for converting the banana plug ends to alligator clips; these are useful for attaching directly to the spring connectors on the CM-8656 board.

Figure 2: a) Pasco CI-6556 current sensor; the arrow on top indicates the direction of flow of positive charge; your current sensor should also have cables with banana plugs attached b) Pasco Voltage sensor; the cables at the end are banana plugs; they can be converted to alligator clips with the banana plug to alligator clip adapters provided.

In this lab, you will be constructing circuits based on circuit diagrams. Circuit diagrams use some standard symbols to represent circuit components. Some typical examples are shown in the figure below:

wire battery switch bulb resistor capacitor+

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Important note about building the circuits in this lab: We realize this may be the first time following circuit diagrams and setting up circuits for many of you. If you are having any trouble setting up the first several circuits, we have placed photographs of a correct setup for each in the file ph2306_lab5_circuit_examples.pdf in the Class Notes directory for this lab. Please give yourself a chance to try setting up the circuits on your own before referring to these pictures, but if you are really struggling take a look there for some help. To get started with this equipment, first construct the simple circuit shown below in Figure 3. You should install a D cell (1.5 volt) battery in your upper battery holder if there is not already one there. To construct a circuit from a diagram, start with one element, identifying one of the leads with its counterpart in the circuit. Follow the “wire” in the diagram to the lead of the next circuit element. Find the corresponding lead and connect it to the first one. Proceed around the circuit until it is complete. Although an actual circuit may look a bit more chaotic than the neat circuit diagrams, it will make sense if you use a methodical step-by-step approach.

Figure 3a) An“ideal” circuit diagram consisting of a battery, light bulb, and a switch. b) A diagram showing how to make the connections on your Pasco EM-8656 to construct the circuit shown on the left. (Note: there is a photograph of a properly setup circuit of this type in the ph2306_lab5_circuit_examples.pdf document in the Class Notes folder).

After you have made the connections in Figure 3, push the push button to make sure light bulb A lights. If it doesn’t light, consult with your TA. Leave this circuit connected up. You will now begin your investigations of current, voltage differences, and resistance.

Investigation 1: Models Describing Current

Physics education research has shown that some students have an incorrect notion of how current flows in electric circuits. The goal of this investigation is to make sure your understanding of how current flows in a circuit is correct. The set-up will be very simple. You will use the light bulb and battery that you have already connected up. Figure 4

+ _

9

shows the detail of the inside of a light bulb. It is simply a filament that lights up visibly when enough current passes through it. Each end of the filament is connected to a metallic contact on the outside of the bulb.

Figure 4: Diagram of the wiring inside a light bulb.

You will use the light bulb and battery to explore models for current in a simple circuit. The diagrams in Figure 5 show several models of current that people often propose. Only one is correct. You will do investigations to verify which one.

Figure 5: Four alternative models for how current flows in a simple circuit.

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Prediction 1-1: Which of the four models to you believe to be correct? Explain your reasoning; discussion with your partner about this before proceeding is encouraged.

To study the different models of current proposed above in Figure 5, you will construct a circuit like that shown in Figure 6. This is the same circuit that you already built, except that two current probes are inserted to measure the current. To measure the current through a part of the circuit, you must break open the circuit at the point where you want to measure the current and insert the current probe. Note that the current probe measures both the magnitude and direction of the current. All you need to do to set up this circuit is to remove the wires that connect the light bulb to the battery and replace them by the current probe.

+ _

+

+

CP1

CP2

Figure 6: Circuit for studying the current models proposed in Figure 5. Two current probes are used to measure the currents in “wire 1” and “wire 2”. The symbols for the current probe indicate that they have a definite direction that they should be hooked up. When current flows in the direction of the arrow shown for a given CP, it will give a positive reading. (CP1 should be hooked to the Science Workshop 750 analog channel A; CP2 should be hooked to the Science Workshop 750 analog channel B). (Note: there is a photograph of a properly setup circuit of this type in the ph2306_lab5_circuit_examples.pdf document in the Class Notes folder).

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Prediction 1-2: You will use the circuit in Figure 6 to test each of the models proposed in Figure 5. For each model, indicate what the current direction and magnitude of the two current probes (CP1 and CP2) will be in the table below. If you already worked this out for the prelab, then copy your answers in here before turning in your prelab assignment.

Current Probe

Positive, negative, or zero?

CP1 > CP2, CP1< CP2, or

CP1=CP2? CP1 Model A

CP2

CP1 Model B

CP2

CP1 Model C

CP2

CP1 Model D

CP2

Open the DataStudio file Current_Model from the ClassNotes folder on the desktop. Set up your circuit as shown in Figure 6. Your screen should have a graph for each current probe along with a digital display for each. Start data-taking; close the switch for a couple of seconds, then open it for a couple seconds, and then leave it closed until time runs out (at 10 seconds). Note down the final currents observed for each probe: CP1 current = ________________________ CP2 current = ________________________ Question 1-3: Did you observe a significant difference (> 5%) in the currents at these two locations in the circuit, or was the current the same?

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Question 1-4: Based on your observations, which model seems to correctly describe the behavior of the current in your circuit. Explain carefully based on your observations.

Investigation 2: Current and Potential Difference

In addition to current, another important quantity used to describe circuits is the electric potential difference across devices in the circuit. We will usually refer to this as the voltage. In this investigation you will measure both the voltage and current in a simple circuit. To start, replace the current probes from your previous circuit with wires. The circuit should look as shown in Figure 7. The two circuit symbols with V’s in them stand for voltage probes (see Figure 2b). (Note: you only have one voltage probe; you will move it between the two points in the circuit).

Figure 7: A simple circuit with a battery and a light bulb. Also shown are two locations to which you will attach a voltage probe. (You are only provided with one voltage probe, so you will make separate measurements with it at the two points shown.)

+ _ V V

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Prediction 2-1: In the circuit shown in Figure 7, what do you expect the voltage across the battery to be with the switch open and closed? What do you expect the voltage across the bulb to be with the switch open and closed? Connect the voltage probe across the battery by connecting its ends (with alligator clips on them) to the springs nearest to the battery. Open the DataStudio file Voltage from the ClassNotes folder. Start data-taking and write down the battery voltage with the switch open and closed: Battery voltage, switch open = __________________________ Battery voltage, switch closed = _________________________ Question 2-2: Does the voltage across the battery change as the switch is open and closed? What is the “open circuit” battery voltage, and what is the battery voltage with a “load” on it (ie. when it’s powering the light bulb)? How do you think the “open circuit” and “load” voltage would compare for a perfect (ideal) battery? Why is there a difference between the open circuit voltage and the “load” voltage for this real battery? Now connect the voltage probe across the light bulb by connecting its ends to the springs nearest to the light bulb. Start data-taking and write down the voltage across the bulb with the switch open and closed: Bulb voltage, switch open = __________________________ Bulb voltage, switch closed = _________________________ Question 2-3: How do your observations of the battery and bulb voltage compare to your predictions from Question 2-1?

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Now connect the circuit up so that you can measure the voltage across the battery and the current passing through it at the same time, as shown in Figure 8.

+ _ V

+

CP1

Figure 8: A simple circuit with a battery and a light bulb. The voltage probe measures the voltage drop across the battery while the current probe CP1 measures the current passing through it. (Note: CP1 is the current probe that is attached to Port A of the Science Workshop 750 Interface).

Open the DataStudio file called Current_and_Voltage from the ClassNotes folder. It has displays for the voltage probe and current probe outputs versus time. Start data-taking, and close and open the switch several times. Question 2-4: Explain the appearance of your current and voltage graphs. What happens to the current through the battery as the switch is opened and closed? What happens to the voltage across the battery? Find the voltage across the battery and the current through the battery when the switch is closed: Voltage across battery, switch closed = _________________________ Current through battery, switch closed = ________________________

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Prediction 2-5: Now suppose you connect a second identical bulb in series as shown in Figure 9. Do you think the voltage across the battery will change significantly compared to that with only one bulb? What about the current in the circuit and the brightness of the bulbs (as compared to the case with one bulb)?

+

CP1

+ _ V

Figure 9: A simple circuit with a battery and 2 light bulbs. The voltage probe measures the voltage drop across the battery while the current probe CP1 measures the current passing through it. (Note: there is a photograph of a properly setup circuit of this type in the ph2306_lab5_circuit_examples.pdf document in the Class Notes folder).

Connect the circuit with the two bulbs shown in Figure 9. Find the voltage across the battery and the current through the battery when the switch is closed: Voltage across battery, switch closed = _________________________ Current through battery, switch closed = ________________________ Question 2-5: Did the current through the battery change significantly when you added the second bulb to the circuit (by more than 5%)?

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Question 2-6: Did the voltage across the battery change significantly when you added the second bulb to the circuit (by more than 5%)? Question 2-7: Does the battery appear to be a source of constant current, constant voltage, or neither when different elements are added to the circuit?

Investigation 3: Current in Series Circuits

In this set of activities, you will be asked to make a number of predictions about the behavior of current in series circuits, and then you will compare your predictions with actual observations. Prediction 3-1: With the switch closed, what would you predict about the relative amount of current going through each bulb in Figures 10 (a) and (b)? Write down your predicted order of the amount of current passing through bulbs A, B, C (from greatest current to least current).

Figure 10a and 10b): Simple circuits with batteries and one or two light bulbs.

+ _ A

CP2

CP1

+ _ B

C

CP1

CP2

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Set up the circuit shown in Figure 10a. In this circuit, current probe CP1 measures the current into bulb A, while CP2 measures the current out of bulb A. Open the DataStudio file Two_Currents from the ClassNotes folder. Start data-taking and note down the currents in CP1 and CP2 after closing the switch: Current into bulb A (CP1) = ________________________________ Current out of bulb A (CP2) = ______________________________ Question 3-2: Are the currents into and out of bulb A equal or is one significantly larger (do they differ by more than a few percent)? What about the directions of the currents? Is this what you expected? Connect the circuit shown in Figure 10b. In this circuit, current probe CP1 measures the current into bulb B, while current probe CP2 measures the current out of bulb B and into bulb C. Start data-taking and note down the currents in CP1 and CP2 after closing the switch. Current into bulb B (CP1) = ________________________________ Current out of bulb B (CP2) = ______________________________ Question 3-3: Consider your observation above; is the current “used up” in the first bulb or is the same in both bulbs? Question 3-4: Is the ranking of the current in the bulbs A, B, and C what you predicted? If not, can you explain what assumptions your were making that now seem false? Question 3-5: Based on your observations, how is the brightness of a bulb related to the current through it?

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Question 3-6: Formulate a qualitative rule (in words, not an equation) for predicting whether current increases or decreases as the total resistance of the circuit is increased? The rule you have formulated based on your observations with bulbs may be qualitatively correct, but it won’t be quantitatively correct. That is, it won’t allow you to predict the exact sizes of the currents correctly. This is because the resistance of a bulb to current changes as the current through the bulb changes. Another common circuit element is a resistor. A resistor has a constant resistance to current regardless of the current through it. In the next activity you will reformulate your rule using resistors. Prediction 3-7: Consider the circuit diagrams in Figure 11, which have the light bulbs from your previous circuits replaced by identical resistors. Write down your predicted rankings of the relative current in the resistors A, B, and C (from greatest current to least current). (Remember that a resistor has constant resistance to current regardless of the current through it.

Figure 11 a) and b): Circuits with resistors instead of light bulbs.

+ _ A +

_

C

B

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Figure 12 a) and 12 b): Simple circuits with batteries and one or two resistors. (Note: there is a photograph of a properly setup circuit of this type for Fig. 12a in the ph2306_lab5_circuit_examples.pdf document in the Class Notes folder).

Continue to use the DataStudio file Two_Currents. Set up the circuit shown in Figure 12 a) using one of the 33 Ω resistors in your Ziploc bag. Determine the current through resistor A. Then add a second resistor and set up the circuit shown in Figure 12 b). Record your currents below: Current through resistor A in Figure 12 a) = ___________________________________ Current through resistor B in Figure 12 b) = ___________________________________ Current through resistor C in Figure 12 b) = ___________________________________ Question 3-8: Is the ranking of the currents in resistors A, B, and C what you predicted? If not, can you explain what assumptions you were making that now seem false? Question 3-9: How does the amount of current produced by the battery in the single resistor circuit compare to that produced by the battery with two resistors? Does the addition of the second resistor affect the current through the original resistor? Explain.

+ _ A

CP2

CP1

+ _ B

C

CP1

CP2

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Question 3-10: Re-formulate a more quantitative rule for predicting how the current supplied by a battery decreases as more resistors are connected in a circuit.

Investigation 4: Current in Parallel Circuits

There are two basic ways to connect resistors, bulbs or other elements in a circuit – series or parallel. So far you have been connecting bulbs and resistors in series. Two resistors or bulbs are in series if they are connected so that the same current that passes through one resistor or bulb passes through the other. Resistors or bulbs are in parallel if their terminals are connected together such that at each junction one end of a resistor or bulb is directly connected to one end of the other resistor or bulb. Similarly, the other ends are connected together. Resistors or bulbs in parallel have the same voltage drop. Prediction 4-1: Consider the circuit shown in Figure 13. When switch S1 is closed, how do you expect the current through the identical light bulbs A and B to compare?

+ _

CP2 CP1

S1

B A

Figure 13: A circuit with two identical light bulbs connected in parallel.

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Prediction 4-2: In Figure 13, how does the current flowing through light bulb A compare with S1 open and closed? Test your predictions in Questions 4-1 and 4-2 by connecting a circuit as shown in Figure 13. For switch S1 use the same push button switch that you used in the previous circuits. Open the DataStudio file Two_Currents (it may still already be open). Start data-taking. First measure the current in CP1 and CP2 with S1 open. Then close S1 and measure the currents in CP1 and CP2. Switch S1 open: Current through bulb A (CP1) = _______ Current through bulb B (CP2) = ___________ Switch S1 closed: Current through bulb A (CP1) = _______ Current through bulb B (CP2) = ___________ Question 4-3: Did closing the switch S1 and connecting bulb B in parallel with bulb A significantly affect the current through bulb A? You have already seen earlier in this lab that the voltage maintained by a battery doesn’t change appreciably no matter what is connected to it (ie. an ideal battery is a constant voltage source). But what about the current through the battery? Is it always the same no matter what is connected to it, or does it change depending on the circuit? That is what you will investigate next. Prediction 4-4: What do you predict about the amount of current flowing through the battery in Figure 13 with S1 open and closed? Test your prediction with the circuit shown in Figure 14.

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+ _

CP2

CP1

S1

A B

Figure 14: A circuit with two identical light bulbs connected in parallel set up so that the current through the battery can be measured.

Set up the circuit shown in Figure 14; you only need to change the position of one of the current probes relative to how you had the Figure 13 circuit set up. Start data-taking; measure the currents through the battery and bulb A. Switch S1 open: Current through bulb A (CP1) = _______ Current through battery (CP2) = ___________ Switch S1 closed: Current through bulb A (CP1) = _______ Current through battery (CP2) = ___________ Question 4-5: Does the current through the battery change as you predicted? If not, why not? Question 4-6: Does the addition of more bulbs in parallel increase, decrease or not change the total resistance of the circuit?

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When you are done with the lab (or if there are only 10 minutes left in the period) answer the postlab question on the next page. Also, do the following before you leave the lab: CLEAN UP YOUR STATION: This means: 1. Put away your 33 Ω resistors in their proper plastic bag. 2. Put away all of your white insulated wires in their proper plastic bag. 3. Disconnect all connections from the electronics circuit board (but leave the two current sensors and the voltage sensor connected to their proper channels on the Science Workshop 750 Interface). 4. Collect your red and black cables into a neat pile. I have told your TAs that they can take points off your lab score if you leave a very messy station; it is rude to the following group of students to do so.

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If there are “ties” put them on the same space. You won’t necessarily fill in all 6 slots.