dc circuits - penn...b. dc circuits: • loop rule: sum up the changes in potential that you...

Penn State University Created by nitin samarth Physics Lab 202P-7 of 16

Physics Lab 202P-7

DC Circuits

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Physics Lab 202P-7

Software listN/A

Equipment List (all items marked with * are in the student kit, others are supplied at the time ofthe lab)

Digital multimeter + probes*Two 1.5 V batteries + a battery holder*Flashlight bulbs (2 round, 2 long) + bulb holders*Hookup wires with alligator clips*Compass


Physics Pre-lab 202P-7

DC Circuits

Name:__________________________Section:_____ Date:__________

(Read this & answer the questions before coming to lab. Note that this prelabrequires the use of your experimental kit.)

Summary of relevant concepts:A. EMF devices:• An ideal EMF device maintains a constant potential difference across its terminals; we

denote the EMF E by an arrow pointing from the negative to positive terminals;• Real (non-ideal) EMF devices are modeled as an ideal EMF with an internal resistance in

series; the potential difference across the terminals of such a device depends on theresistance of the external circuit (or "load");

B. DC circuits:• LOOP RULE: sum up the changes in potential that you encounter as you go around the

loop; this sum should be zero.• RESISTANCE RULE: if you go thru' a resistance in the direction of current flow, the

change in potential is -iR• EMF RULE: if you go thru' an EMF device from the - to + terminal i.e. in the direction

of the EMF arrow, the change in potential is +E.• JUNCTION RULE: the sum of the currents entering a junction must equal the sum of

the currents leaving the junction

C. Resistances in series:• Resistors are said to be in series with each other when the total potential

difference across the combination = sum of potential differences across individualresistances; the resistors all carry the same current.

• The combination of resistances in series = sum of resistancesD. Resistances in parallel:

• Resistors are said to be in parallel when the total current through the combination= sum of currents through each resistor; the resistors all have the same potentialdifference across their ends.

• The reciprocal of the equivalent parallel resistance = sum of individual reciprocalresistances.

Prelab checkbox:SatisfactoryUnsatisfactory


Pre-lab Questions:

These prelab questions require you to carry out some simple experiments. It may be convenientto enlist the help of a friend for an extra pair of hands… .

Use the components in your lab kit to set up each of the circuits shown below. Observe thebrightness of the bulbs in each case, and also measure the current in the circuit using yourcompass as an ammeter. Then, answer Q1.

Q1. Assume that each battery is IDEAL with an EMF of 1.5 V and that each bulb has aresistance of 50Ω when it lights. (Note that -- in actuality -- the resistance of each bulb willchange, depending on how "bright" it is. Why?) Use these assumptions to explain the differencesthat you observed in the two circuits shown above.

++

-- Round

Bulb++

-- 2 Round

Bulbs inseries


Use the components in your lab kit to set up the circuit shown below.• Observe the brightness of each bulb, and also measure the current in the branch AB using

your compass as an ammeter.• Unscrew one of the bulbs from its socket. Carefully note any changes in the brightness of

the remaining bulb, and also any changes in the current in the branch AB using yourcompass as an ammeter.

• Then, answer Q2-Q5, assuming that each battery is IDEAL and has an EMF of 1.5 V.Also, assume that each bulb has a resistance of 50 Ω when it is lit.

Q2. When both bulbs are in parallel, what is the current in each bulb? What is the total currentdelivered by the battery?

++

-- Round

BulbRoundBulb

A B


Q3. If one bulb is disconnected, what is the current through the remaining bulb? How does itcompare with the case when both bulbs were in parallel?

Q4. If one bulb is disconnected, what is the current delivered by the battery? How does itcompare with the case when both bulbs were in parallel?


Q5. Given your answers to Q3-Q4, can you explain your experimental observations regardingthe change in bulb brightness and the change in total current when one of the bulbs isunscrewed? Are any of your experimental observations INCONSISTENT with your theoreticalanalysis? Which one(s)?


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Lab Activity 1: A Single Loop Circuit and Resistors in Series

The circuit shown above uses components from your kit.

Q1. Draw a clear circuit diagram that represents the circuit. Each bulb can be simply viewed as aresistor. Your circuit should show an EMF direction for each battery as well as a direction for thecurrent in your circuit. In your diagram, remember to take into account that the batteries of EMFE are not ideal and have some unknown internal resistance.


Q2. Set up the circuit but leave one of the leads to a battery disconnected.Use the digital multimeter (DMM) to measure the following quantities:

• The voltage differences VB-VC and VH -VA between the + and - terminals of each battery.Note: to correctly measure the sign of VB - VC, for example, connect the "COM" or"ground" terminal of the meter to C and the positive (red) terminal of the meter to B.

• The resistance of each bulb and each of the connecting wires.

VB-VC = VH -VA =

Resistance of round bulb = Resistance of long bulb =

Q3. Now complete the circuit. What do you immediately notice?

Q4. Measure the potential differences listed in the table below; if any quantities are obviouslyzero or negligible, indicate this clearly. Discuss whether your measured data are consistent withthe "Loop Rule."

Pair of terminals Potential Difference(V)

VA-VB

VB-VC

VC-VD

VD-VE

VE-VF

VF-VG

VG-VH

VH-VA


Discuss whether the data follow the "loop rule":

Q5. Why is there a discrepancy between the EMF of each battery measured before the circuit iscompleted and after the circuit is completed? How could you use such an observation to estimatethe combined internal resistance of the two batteries? What additional information would youneed to know? (Remember that you measured the bulb resistance when the bulb was NOT lit.)


Q6. Explain why the bulbs behave as observed when the circuit is completed. Note again thatyou do not know the resistance of the bulbs when they light up, but it is safe to assume that theresistances you measured with the DMM correctly tell you which bulb has a smaller resistanceeven when they light up.


Lab Activity 2: Resistors in Parallel

Q7. Draw a circuit diagram representing the circuit shown above. Note that one of the bulbs isround and the other one is long (i.e. they have different resistances). As in activity 1, assume thatthe batteries are NOT ideal.


Q8. Connect the above circuit and use the DMM to record the potential differences listed in thetable below. Unscrew the ROUND bulb from its socket, leaving the rest of the circuit intact.Carefully observe the brightness of the LONG bulb while you do this. Again, record the potentialdifferences listed below.

Pair of terminals Potential Differencewith both bulbs (V)

Potential Differencewith long bulb only (V)

VB-VE

VA-VF

VC-VD

Q9. Carry out an analysis of the circuit in the following way:• Assign 3 unknown currents in the circuit (I1, I2 , I3 that go through sections BE, AF and

CD, respectively);• Write down 3 independent equations that relate these currents to the following quantities

that we treat as "known" quantities: the combined EMF of the two batteries (E) , theresistances of the bulbs (Rlong and Rshort) and the combined internal resistance of the twobatteries Rint.

• Solve these equations for each of the 3 currents.

Eq. 1:

Eq. 2:

Eq. 3:


Solution to the 3 simultaneous equations:


Q10. Using your analysis in Q9, discuss what you expect to happen to the brightness of the longbulb when the round bulb is removed from its socket and compare this expectation with yourobservations. Compare also the results of this analysis to the analysis in your prelab, where youassumed that the batteries were IDEAL.

dc circuits - penn...b. dc circuits: • loop rule: sum up the changes in potential that you...

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