SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
PHYSICS II
LAB 5
SP212
Kirchhoff’sRulesandRCCircuits
I. Introduction
Today’s Lab will investigate Kirchhoff’s junction law and Kirchhoff’s voltage law, as well as
experiment with parallel and series circuits. The final part of the lab is to study a capacitor and
resistor together in a circuit (RC circuits).
II. Procedure
A. Preliminary Measurements
1. Measure and record the resistance of each of the resistors on your lab bench.
(Remember you can look at the resistor color code to determine the expected value).
Now measure the equivalent resistance of two of the resistors in series. Does it
match what you expected? Now measure the equivalent resistance of two of the
resistors in parallel. Does it match what you expected?
2. Setup (if directed by your instructor)To test our three ammeters and our voltage
probe to ensure that they work properly before we start the experiment. Your
instructor will show you how to do this using the procedure in the Sp212 Lab
Manual. If something isn’t working correctly, tell your instructor and get a new
sensor.
B. Two-Loop Circuit with an EMF in each loop:
1. Build the circuit shown. Set the voltage of the power supply to be about 3 V.
Page 1 of 6 SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
2. Measure the current passing through the three ammeters.
3. After you build the circuit, take data, and then gently disconnect the D‐cell and
power supplies. The idea is to avoid running the D‐cell down, but to leave the circuit
almost intact, so you can take another data set if you need to.
4. Calculate what current should pass through each of the three ammeters. (This circuit
is very close to the one you considered with your pre-lab assignment).
5. Discussion: Do your measurements support or refute Kirchhoff's Rules for analyzing
circuits? Please write a comprehensive and comprehensible paragraph. What is your
percent error between the calculated and measured currents?
C. An RC Circuit
1. Build this circuit and collect data on V and i as a function of time for both
charging and discharging. Use the 25,000 F (0.025 F) capacitor similar to the
one shown below and the Tektronix power supply with 5 V.
i.
Starting with the capacitor fully discharged, start collecting data and then
throw the switch to the charging position. After the capacitor is completely
charged, flip the switch to the discharge position, and complete the data
run.
ii.
Fit the appropriate exponentials to the charging and discharging curves
from both the ammeter and voltmeter. Determine the time constant from
each of these, and average them together.
2. Calculate the theoretical time constant value for this circuit and calculate the
uncertainty in your predicted value. τ = RC
Page 2 of 6 SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
3. Discussion: Compare your measured value for the time constant with the
theoretically-predicted value. What was the percent error? Write in complete
sentences. In addition to comparing predicted and measured values for the time
constant, explain the signs and shapes of each of the four curves in your graph.
Use complete, grammatically correct sentences structured into a coherent
paragraph.
D. Switches
1. Connect two dry-cell batteries in series to a flashlight bulb through a LoggerPro
ammeter and a switch. Add a LoggerPro voltmeter in parallel across the switch.
2. Collect data with the switch both open and closed. Calculate the resistance in this
circuit using Ohm’s law.
3. What do the ammeter and voltmeter read when the switch is open? What do they read
when the switch is closed? Do these results make sense?
E. Bulbs and Batteries in Series and Parallel (If time permits):
1. You will have two boards with small light bulbs: one board will have two light bulbs
wired in parallel, while the other will feature three light bulbs wired in parallel. Set
up a circuit with two 1.5 V D-cell batteries connected to the board with two bulbs.
Observe how bright the bulbs are. Now gently unscrew one of the bulbs. What
happened to the brightness of the other bulb? Remove one banana lead while
answering these questions to avoid draining the battery needlessly.
2. Now set up a circuit with two 1.5 V D-cell batteries connected to the board with three
bulbs. Observe how bright the bulbs are. Now gently unscrew one of the bulbs.
Then, gently unscrew a second bulb. What happened to the brightness of the other
bulb?
Page 3 of 6 SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
3. Now connect the two batteries to both of the light bulb boards, with the two boards
connected in series. Observe the relative brightness of the bulbs. Now unscrew on of
the bulbs on the board with three bulbs. What happens to the brightness of the other
bulbs? If your answer is different from your answers to parts 1 or 2, explain why the
answers are different. Remove one banana lead while answering these questions to
avoid draining the battery needlessly.
4. Discussion: What would happen if you connected your two bulbs in parallel to two
batteries in parallel as shown in the next diagram?
III.LabReporttohandin:
1.
Annotated Graph of currents, calculated (predicted currents), and discussion from
Part B.
2.
Annotated Graphs of voltage and current, calculated (predicted time constant), and
discussion from Part C.
3.
Annotated Graphs of voltage and current, calculated resistance of bulb, and discussion
from Part D.
4. Answers to discussion from Parts E (if directed by your instructor).
Don’tforgetuncertainties,units,annotationsanddiscussions.
Page 4 of 6 SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
IV.
Clean‐Up(ensureallequipmentisoffandallwiresare
disconnected!)
A. GoldenRule:“Dountoothersasyoudesirethemtodountoyou.”
ThisappliesasmuchhereinthelabasitdoesintheFleet.AsfutureNavalOfficers,
howcanyouexpectyourenlistedsailorstomaintainacleanworkareaifyour
stateroom,workareas,messarea,etcisa“pigsty?”Soasofficersitisimperativethat
wecleanupafterourselvesnotonlytofollowtheGoldenRule,butalsotoleadby
examplefortheenlistedpersonnelunderourcharge.
1. EndofLabCheckout:Beforeleavingthelaboratory,pleasetidyuptheequipmentat
theworkstationandquitallrunningsoftware.
2. Thelabstationshouldbeinbetterconditionthanwhenyouarrivedandmore
importantly,shouldbeofanappearancethatyouwouldbePROUDtoshowtoyour
legalguardiansduringa“ParentsWeekend.”
3. Haveyourinstructorinspectyourlabstationandreceivetheirpermissiontoleavethe
LabRoom.
4. YouSHALLfollowthisprocedureduringeverylabforSP212!
Many thanks to Dr. Huddle, Dr. Katz, Dr. Helton, and Dr. Fontanella for their assistance in producing this Laboratory procedure; specific references can be supplied on request. LCDR Timothy Shivok Page 5 of 6 SP212 Lab: Five Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
Appendix A
List of Objectives for this lab.
At the end of this activity, you should:
1. Be able to determine the equivalent resistance of resistors in series. Then observe that the
experimental results match theory.
2. Be able to determine the equivalent resistance of resistors in parallel. Then observe that the
experimental results match theory.
3. Develop an ease with Kirchhoff’s junction rule, aka Kirchhoff’s Current Law (KCL), including
being able to solve for various currents.
4. Observe that the algebraic sum of all the currents entering a node must be equal to the algebraic
sum of all the currents exiting a node.
5. Develop an ease with Kirchhoff’s loop rule, aka Kirchhoff’s Voltage Law (KVL), including
being able to solve for various voltages and/or currents.
6. Observe again, that the algebraic sum of all the voltage drops and voltage sources in a closed loop
must add up to zero.
7. Develop an ease with simultaneous equations (in your calculator or preferably by hand) and solve
a complex circuit involving two emf sources and parallel resistors. Then observe that the
experimental results match theory.
8. Observe the effects batteries and light bulbs in Series or Parallel arrangements.
9. Observe the voltage across the capacitor and current through the resistor as functions of time in a
simple RC series circuit when the capacitor is charging.
10. Observe the voltage across the capacitor and current through the resistor as functions of time in a
simple RC series circuit when the capacitor is discharging.
Page 6 of 6