Pre-LAB 4 Assignment Fundamentals of Circuits II: Currents ...

Name: 

Lab Partners: 

Date: 

Pre-LAB 4 Assignment 

Fundamentals of Circuits II: 

Currents & Circuits 

(Due at the beginning of lab) 

Question 1 Make a prediction for the relative brightness of the bulbs A, B and C in Fig. 1. 

Question 2 Consider the rankings you made in question 1. If the current is reversed by reversing 

the connections to the battery in Fig. 1, will your ranking change? If so, explain how. 

Question 3 What equipment will you use to measure and compare the currents at various 

points in the circuits in Fig. 1. 

Question 4 Define both series and parallel connections. Make a sketch of two light bulbs and 

a battery connected in series, and another sketch of two light bulbs and a battery connected in 

parallel. 

Question 5 Consider the circuit in Fig. 7. Predict how the brightness of bulb D will change 

when the switch is closed. 

Question 6 For the same circuit in Fig. 7, Predict how the current through the battery will 

change when the switch is closed. 

LAB 4 

Fundamentals of Circuits II 

PHYS-204:Physics II Laboratory 1

Fundamentals of Circuits II: Currents & Circuits v 0.3 

Objectives 

• To understand current in a circuit where a battery lights a bulb. 

• To understand series connections in an electric circuit. 

• To understand the relationship between the current in all parts of a series circuit. 

• To understand the meaning of parallel connections in an electric circuit. 

• To understand the relationship between the currents in all parts of a parallel circuit. 

• To begin to understand the concept of resistance. 

Overview 

In the previous lab you found that a bulb only lights when there is an electric current passing 

through the bulb. You also found that the bulb must be connected to a battery in a complete, 

continuous circuit in order for there to be a current. Specifically, there must be a continuous 

connection from the positive terminal of the battery, through the connecting wire to the bulb, 

through the bulb, through the connecting wire to the negative terminal of the battery and 

through the battery. 

Recall that you measured the current both as it entered current elements like a battery or 

a bulb and when it left. From these measurements you discovered that the electric current 

flowing out of a device is equal to the current flowing into it. These devices did not seem to 

“use up” any current which passed through. Current it seems doesn’t just disappear. This 

suggests that electrical current is the flow of some conserved quantity i.e., electric charge. In 

addition, by measuring voltage in circuits with one bulb and with two bulbs you observed that 

the potential difference (voltage) across a battery (if one can neglect its internal resistance) is 

fairly constant whether it is connected to one light bulb or two. In other words, the voltage of 

a battery is a property of the battery and not of the circuit. 

By contrast, you found that adding a second bulb in series to the first lead to a decrease in 

the current. Circuit elements like bulbs provide a resistance to electrical currents. The total 

resistance of a circuit determines the current when the circuit is connected to a battery. The 

larger the resistance, the smaller the current. 

In this lab we will consider more complicated circuits in order to extend and reinforce what 

we have learned previously. You will be comparing currents at different points in these circuits 

by making measurements using current probes and by comparing the brightness of bulbs. This 

lab has been adapted from the Real Time Physics Active Learning Laboratories [?]. The goals, 

guiding principles and procedures of this labs closely parallel the implementation found in the 

work of those authors [?, ?, ?]. 

Investigation 1: 

Current in Series Circuits 

In the spirit of scientific discovery, you will make a series of predictions about currents in 

various circuits. Afterwords, you will compare these predictions with measurements and observations. 

If you find that your experimental measurements and/or observations disagree with 

your predictions you should identify which if any of the assumptions which lead you to your 



initial prediction have been shown to be false. Subsequently, you should develop and adopt new 

concepts which are consistent with the results of your measurements. 

Our fist activity requires the following equipment: 

• Computer with Logger-Pro software 

• Lab Pro interface 

• two current probes 

• Two 1.5 V D cell battery with holder 

• connecting wires 

• alligator clips 

• Two #14 bulbs in sockets 

• contact switch 

Note: the battery holder, light bulbs and contact switch are part of the Pasco circuit board. 

B 

+ 

− 

A 

+ 

− 

C 

(a) 

(b) 

Figure 1: Two different circuits: (a) a battery with a single bulb and (b) a battery with two 

bulbs. All three bulbs are identical. 

Prediction 1.1 Consider the two circuits shown in Fig. 1. Predict the relative brightness of 

the three bulbs shown in Figs 1(a) and 1(b) from brightest to dimmest. In making this prediction 

you should assuming that the batteries and bulbs have identical characteristics. (Recall that the 

voltage across a battery is essentially independent of the number of bulbs it is connected to.) In 

your prediction you should identify which bulbs, if any, are equal in brightness. Hint: Helpful 

symbols are > ”is greater than”, < ”is less than”, = ”is equal to,” for example B > C = A. 



Transistor Socket 

Batteries Holders 

Switch 

Bulb Sockets 

Potentiometer 

Inductor 

+ 

A B C 

L = 8mH 

− 

E 

B 

C 

+ 

− 

Figure 2: One possible way of wiring the circuit in Fig. ?? on the Pasco circuit board. An 

Annotated picture of the Pasco circuit board can be found in the appendix of this lab handout. 

Activity 1.1: The Relative Brightness of Bulbs 

Step 1: Connect the circuits in Fig. 1, observe the relative brightness of the bulbs, and rank 

the bulbs in order of the brightness you actually observed. 

Comment from Real-Time Physics: 

“These activities assume identical bulbs. Differences in brightness may arise if the bulbs 

are not exactly identical. In this and later activities, to determine whether a difference in 

brightness is caused by a difference in the currents through the bulbs or by a difference 

in the bulbs, you should exchange the bulbs. Sometimes a bulb will not light noticeably, 

even if there is a small but significant current through it. If a bulb is really off, that is, 

if there is no current through it, then unscrewing the bulb will not affect the rest of the 

circuit. To verify whether a non-glowing bulb actually has a current through it, unscrew 

the bulb and see if anything else in the circuit changes.” 

Observed ranking of the brightness of the bulbs: 

Question 1.1 Do your observations and your predictions agree? If not, identify any 

assumption(s) that you have now shown to be false. 



Prediction 1.2 Consider reversing the batteries in the circuits shown in Fig. 1. What 

will happen to the brightness of bulbs A, B and C if the positive and negative terminals of 

the battery are reversed? Explain the reason(s) for your prediction. 

Step 2: Test your prediction by reversing the terminals of the battery in each of your circuits. 

Question 1.2 Did your observation agree with your prediction? Explain. Identify any assumptions(s) 

which you have now falsified. 

Question 1.3 Considering the observations you have made above, without knowing how the 

battery is hooked up, does the brightness of the bulbs tell you what the direction of the current 

is? Explain. 

Activity 1.2: Current in a Simple Circuit with Bulbs 

This activity is designed to further investigate how a bulb and a series of bulbs affects the 

current in a circuit. 

Prediction 1.3 How do you predict the currents going through each of the bulbs in Fig. 1 will 

compare? Write down your predicted rankings of the currents through bulbs A, B and C, and 

explain your reasoning. 

You will test your prediction by using current probes. We want the current probe to measure 

the current which goes through a bulb. Therefore we must connect the current probes so that 

the current which passes through a bulb also passes through the current probe. Study Fig. 3 

and convince yourself that the the current probes there measure the currents as described in 

the figure caption. 

Real-Time Physics Comment: “In carrying out your measurements, it is important to realize 

that the measurements made by the current probes are only as good as their calibrations. Small 

differences in calibration can result in small differences in readings.” 

Step 1: Open the file L2A1-2 (Two Currents). You should see two sets of current axes similar 

to the picture below. 

Step 2: Zero the probes with them disconnected from the circuit. 



S 

S 

+ 

CP1 

− 

B 

+ 

CP1 

− 

+ 

− 

A 

+ 

− 

+ 

CP2 

− 

+ 

CP2 

− 

C 

(a) 

(b) 

Figure 3: Current probes connected to measure the current through bulbs. In circuit (a), CP1 

measures the current into bulb A, and CP2 measures the current out of bulb A. In circuit (b), 

CP1 measures the current into bulb B while CP2 measures the current out of bulb B and the 

current into bulb C. 

Step 3: Connect circuit shown in Fig. 3(a). 

Step 4: Begin graphing. Close the switch for a second or so, open it for a second or so and 

then close it again. Sketch your graphs on the axes provided below. 

Current 2 (A) 

Current 1(A) 

Time (s) 

Step 5: Use the analysis feature of the software to measure the currents into and out of bulb 

A when the switch is closed: Measure the current at points where the current is steady, 

not at the spike that appears at the moment the switch is closed. 

Current into bulb A: Current out of bulb A: 

Question 1.4 How does the current into bulb A compare to the current which comes out 

of bulb A? Are they equal, or do they differ by an amount which is larger than can be 

explained by the calibration of the current probes. What can you say about the direction 

of the current on either side of the bulb? Is this what you expected? Explain. 



Step 6: Connect circuit shown in Fig. ??(b). Begin graphing current as above, and record the 

measured values of the currents. 

Current through bulb B: Current through bulb C: 

Step 7: Sketch the graphs on the axes provided: 

Current 2 (A) 

Current 1(A) 

Time (s) 

Question 1.5 Consider your observation of the circuit shown in Fig. 3(b). Was any current 

”used up” in the first bulb or is it the same in both bulbs? Explain, based on your observations. 

Question 1.6 Do the measured values of the currents in bulbs A, B and C agree with your 

predicted ranking? If not, explain what assumption(s), if any, you have now shown to be false. 

Question 1.7 Based on your measurements and observations, how does the brightness of a 

bulb depend on the current passing through it? 

Question 1.8 Compare the current produced by the battery when there is a single bulb in the 

circuit,as in Fig 1(a), to the current produced by the battery when there are two bulbs,as in Fig 

1(b). How did the addition of a second bulb affect the current through in the circuit? Explain. 

Question 1.9 These observations suggest that we think of the bulb as providing a resistance 



to the current in a circuit as opposed to something that uses up current. If each bulb provides 

its own resistance, how is the total resistance of a circuit is affected by the addition of bulbs as 

in Fig. 1(b)? 

Question 1.10 Create a qualitative rule which can be used to predict whether current increases 

or decreases as the total resistance of a circuit is increased. 

Real-Time Physics Comment: “The rule you have formulated based on your observations 

with bulbs may be qualitatively correct–correctly predicting an increase or decrease in current– 

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 how large the current through it. In the next 

activity you will reformulate your rule using resistors.” 

Prediction 1.4 Fig. 4 shows circuit diagrams where the light bulbs of Fig. 1 have been replaced 

by identical resistors. Consider the amount of current going through each resistor in Figures 

4(a) and 4(b)? Rank the currents through each resistor resistors A, B and C. Explain your 

reasoning. (You may consider these resistors as ideal resistors: an ideal resistor has a constant 

resistance independent of the current passing through it.) 

To test your predictions, in addition to the equipment you have been using, you will need 

• Two 10-ohm resistors 

Activity 1.3: Current in a simple circuit with resistors 

Step 1: Continue to use the experiment file L2A1-2 (Two Currents ). 


Step 3: Connect circuit shown in Fig. 5(a). 

Step 4: Withthecurrentprobes, usetheanalysisfeatureinthesoftwaretomeasurethecurrent 

through resistor A in circuit 5(a) and the currents through resistors B and C in circuit 

5(b). 



R B 

+ 

− 

R A 

+ 

− 

R C 

(a) 

(b) 

Figure 4: Two different circuits: (a) a battery with a single resistor, and (b) a battery with two 

resistors identical to the one in (a). 

Current through resistor A: 

Current through resistor B: 

Current through resistor C: 

Question 1.11 Do the measured values of the currents in resistors A, B and C agree with the 

ranking that you predicted? If not, can you identify what assumption(s), if any, you have now 

shown to be false? 

Question 1.12 Compare the amount of current produced by a battery which is connected to a 

single resistor, as in Fig. 4(a), to the amount of current produced when the battery is connected 

to two resistors, as in Fig. 4(b). How does the presence of the second resistor affect the current 

in the circuit? Explain. 

Question 1.13 How did your observations with resistors differ from your observations with 

bulbs in Activity 1.2. 

Question 1.14 Extrapolating from your observations, write down a quantitative formula which 

predicts how the current in the circuit decreases as of resistors connected in series, as shown in 



S 

S 

+ 

CP1 

− 

+ 

CP1 

− 

R B 

+ 

− 

R A 

+ 

− 

+ 

CP2 

− 

+ 

CP2 

− 

R C 

(a) 

(b) 

Figure 5: Current probes connected to measure the current through resistors. In circuit (a), 

CP1 measures the current into resistor A, and measures the current out of resistor A. In circuit 

(b), CP1 measures the current into resistor B while CP2 measures the current out of resistor 

B and into resistor C. 

Fig. 4(b), increases. 

Question 1.15 Does the quantitative rule you formulated work for bulbs which are connected 

as in Figures 1 (a) and (b)? Explain. 


Current in Parallel Circuits 

There are two distinct ways to connect a pair of resistors, bulbs or other elements in a circuit: 

series and parallel. In the circuits you connected so far in this lab, the bulbs and resistors 

have all been connected in series. Below we summarize a more formal definition of series and 

parallel. 



x 

y 

x 

Two resistors or bulbs are in series if they are connected so that the 

same current that passes through each. In the series connection pictured 

to the left, there is only one path available for a current which 

runs from x to y. 

y 

Two 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, as at the junctions above 

and below the bulbs at the left. 

It is important to keep in mind that in more complex circuits, say with three or more 

elements, not every element is necessarily connected in series or parallel with other elements. 

Let’s compare the behavior of a circuit with two bulbs wired in parallel to the behavior of 

a circuit with a single bulb. (See Fig. 6.) 

+ 

− 

A 

+ 

− 

D 

E 

(a) 

(b) 

Figure 6: Two different circuits: (a) a single bulb circuit and (b) a circuit with two bulbs 

identical to the one in (a) connected in parallel to each other and in parallel to the battery. 

Note that if bulbs A, D and E are identical, then the circuit in Fig. 7 is equivalent to circuit 

6(a) when the switch is open (as shown) and equivalent to circuit 6(b) when the switch is closed. 

Question 2.1 Explain how you know that the caption of Fig. 7 correctly describes the circuit. 

Prediction 2.1 Predict how the brightness of bulbs D and E in the parallel circuit of Fig. 6 (b) 

will compare to bulb A in the single bulb circuit of Fig. 6 (a). How will D and E compare with 

each other? Rank the brightness of all three bulbs. Explain the reasons for your predictions. 



S 

+ 

− 

D 

E 

Figure 7: When the switch is open, only bulb D is connected to the battery. When the switch 

is closed, bulbs D and E are connected in parallel to each other and in parallel to the battery. 

To test this and other predictions, you will need: 

• Computer with Logger-Pro software 

• Lab Pro interface 

• Two current probes 

• 1.5 V D cell battery with holder 

• connecting wires 

• alligator clips 

• Three #14 bulbs in sockets 

• contact switch 

Note: the battery holder, light bulbs and contact switch are part of the Pasco circuit board. 

Activity 2.1: Brightness of Bulbs in a Parallel Circuit 

Set up the circuit in Fig. 7, and describe your observed rankings for the brightness of bulb D 

with the switch open, and D and F with the switch closed. 

Question 2.2 Did the observed rankings agree with your prediction? If not, can you explain 

what assumptions you were making that now seem false? 

Prediction 2.2 Based on your observations of brightness, what do you predict about the relative 

amount of current through each bulb in a parallel connection, i.e., bulbs D and E in Fig. 6 

(b)? 



Prediction 2.3 Based on your observations of brightness, how do you think that closing the 

switch in Fig. 7 affects the current through bulb D? 

Activity 2.2: Current in Parallel Branches 

You can test Predictions 2.2 and 2.3 by connecting current probes to measure the currents 

through bulbs D and B. 

Step 1: Open the experiment file called L2A1-2 (Two Currents), if it is not already opened. 


Step 3: Connect the circuit shown below in Fig. 8. 

S 

+ 

− 

D 

+ 

CP1 

− 

E 

+ 

CP2 

− 

Figure 8: Current probes connected to measure the current through bulb D and the current 

through bulb E. 

Step 4: Begin graphing the currents through both probes then close the switch for a second 

or so, open it for a second or so, and then close it again. 

Step 5: Sketch the graphs on the axes below. 



Current 2 (A) 

Current 1(A) 

Time (s) 

Step 6: Use the analysis feature of the software to measure both currents. Record the results 

below: 

Switch open: Current through bulb D: Current through bulb E: 

Switch closed: Current through bulb D: Current through bulb E: 

Question 2.3 Based on your graphs and measurements, were the currents through bulbs D and 

E what you predicted based on their brightness? If not, can you now explain why your prediction 

was incorrect? 

Question 2.4 Did closing the switch and connecting bulb E in parallel with bulb D significantly 

affect the current through bulb D? How do you know? (Note: you are making a very significant 

change in the circuit. Think about whether the new current through D when the switch is closed 

reflects this,m or if it only a few per cent.) 

You have already seen in Lab 2 that the voltage maintained by a battery doesn’t change 

appreciablynomatterwhatisconnectedtoit. Butwhataboutthecurrentthroughthebattery? 

Is it always the same no matter what is connected to it, or does it change depending on the 

circuit? (Is the current through the battery the same whether the switch in Fig. 8 is open or 

closed?) This is what you will investigate next. 

Prediction 2.4 Based on your observations of the brightness of bulbs D and E in Activity 2.2, 

what do you predict about the amount of current through the battery in the parallel bulb circuit, 

Fig. 6 (b), compared to that through the single bulb circuit, Fig. 6 (a)? Explain. 



Activity 2.3: Current Through the Battery 

Step 1: Test your prediction. Connect the circuit with the current probes positioned to measure 

the current through the battery and through light bulb D, shown in Fig. 9. Use the 

same experiment file, L2A1-2 (Two Currents), as in the previous activities. 

S 

− 

CP2 

+ 

+ 

CP1 

− 

+ 

− 

D 

E 

Figure 9: Current probes connected to measure the current through the battery (CP2) and the 

current through bulb D (CP1). 

Step 2: Begin graphing while closing and opening the switch as before. Sketch your graphs on 

the axes that follow. Label on your graphs when the switch is open and when it is closed. 

Current 2 (A) 

Current 1(A) 

Time (s) 

Step 3: MeasurethecurrentsthroughthebatteryandthroughbulbD.Recordtheresultbelow 

Switch open: Current through battery: Current through bulb D: 

Switch closed: Current through battery: Current through bulb D: 

Question 2.5 Describe how the connection of current probes in Fig. 9 differs from that in 

Fig. 8. How do you know that probe 2 is measuring the current through the battery? 



Question 2.6 Use your observations to formulate a rule to predict how the current through 

a battery will change as the number of bulbs connected in parallel increases. Can you explain 

why? 

Question 2.7 Comparing your rule in Question 2.6 to the rule you stated in Questions 1.10 

and 1.14 relating the current through the battery to the total resistance of the circuit, does the 

addition of more bulbs in parallel increase, decrease, or not change the total resistance of the 

circuit? Explain. 

Question 2.8 Can you explain your answer to Question 2.7 in terms of the number of paths 

for current available in the circuit? Explain. 

Question 2.9 Considering your experiences with series and parallel circuits in Investigations 

1 and 2, does the current through the battery only depend on the number of bulbs or resistors 

in the circuit, or does the arrangement of the circuit elements matter? 

Question 2.10 Since current and resistance are related, does the resistance just depend on the 

number of bulbs or resistors, or does it depend on the arrangement of the circuit elements as 

well? Explain. 


More Complex Series and Parallel Circuits 

Now you can apply your knowledge to some more complex circuits. Consider the circuit consisting 

of a battery and two bulbs, A and B, in series shown in Fig. 10(a). What will happen 

if you add a third bulb, C, in parallel with bulb B as shown in Fig. 10(b)? You should be able 

to predict the relative brightness of A, B, and C based on previous observations. The tough 

question is: how does the brightness of A change when C is connected in parallel to B? 

Question 3.1 In Fig. 10 (b) is bulb A in series with bulb B, with bulb C, or with a combination 



A 

A 

+ 

− 

+ 

− 

B 

B 

C 

(a) 

(b) 

Figure 10: Two circuits with different arrangements of identical light bulbs 

of bulbs B and C? (You may want to go back to the definitions of series and parallel connections 

at the beginning of Investigation 2.) 

Question 3.2 In Fig. 10 (b) are bulbs B and C connected in series or in parallel with each 

other, or neither? Explain. 

Question 3.3 Is the resistance of the combination of bulbs B and C larger than, smaller than, 

or the same as bulb B alone? Explain. 

Question 3.4 Is the resistance of the combination A, B and C in Fig. 10 (b) larger than, 

smaller than or the same as the combination of A and B in Fig. 10 (a)? Explain. 

Prediction 3.1 Predict how the current through bulb A will change, if at all, when circuit 

10(a) is changed to 10(b) (when bulb C is added in parallel to bulb B). What will happen to the 

brightness of bulb A? Explain the reasons for your predictions. 



Prediction 3.2 Predict how the current through bulb B will change, if at all, when circuit 

10(a) is changed to 10(b) (when bulb C is added in parallel to bulb B). What will happen to the 

brightness of bulb B? Explain the reasons for your predictions. 

Prediction 3.3 Also predict the relative rankings of brightness for all the bulbs, A, B, and C, 

after bulb C is in the circuit. Explain the reasons for your predictions. 

Activity 3.1: A More Complex Circuit 

Step 1: Set up the circuit shown in Fig. 11(a). Convince yourself that this circuit is identical 

to Fig. 10(a) when the switch, S, is open, and to Fig. 10(b) when the switch is closed. 

+ − 

CP1 

A 

A 

+ 

− 

S 

+ 

− 

+ 

CP2 

− 

S 

B 

C 

B 

C 

(a) 

(a) 

Figure 11: (a) Circuit equivalent to Fig. 10(a) when the switch, S, is open and to Fig. 10(b) 

when the switch is closed. (b) The same circuit with current probes connected to measure the 

current through bulb A (CP1) and the current through bulb B (CP2). 

Step 2: Observe the brightness of bulbs A and B when the switch is open, and then the 

brightness of the three bulbs when the switch is closed. Compare the brightness of bulb 

A with the switch open and closed, and rank the brightness of bulbs A, B, and C with 

the switch closed. 

Question 3.5: If you did not observe what you predicted about the brightness, what 

changes do you need to make in your original reasoning? Explain. 

Step 3: Connect the two current probes as shown in Fig. 11(b). Use same the experiment file 

L2A1-2 (Two Currents) that you have used in the previous activities. 



Step 4: Begin graphing and observe what happens to the current through bulb A (through 

the battery) and the current through bulb B when the switch is opened and closed. 

Question 3.5 What happens to the current through the battery and through bulbs A and 

B when bulb C is added in parallel with bulb B? What do you conclude happens to the 

total resistance in the circuit? Explain. 

This laboratory exercise closely follows the references below. 



Appendix: The Pasco Circuit Board 

Batteries & Holders 

Switch 

Bulb Sockets 

Transistor Socket 

Potentiometer 

Inductor 

+ 

A B C 

L = 8mH 

− 

E 

B 

C 

+ 

−

Pre-LAB 4 Assignment Fundamentals of Circuits II: Currents ...

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