PHYS 1402 – General Physics II

Kirchhoff’s Laws

 

Materials

Pasco circuit Board set

DMM

2 pieces alligator clip cable

 

Introduction

In this activity we will examine and develop the rules that will allow us to calculate the currents flowing in each part of a resistive circuit.

 

Part 1 Models of Current Flow in a Series Circuit

In this part, we will explore some models of the way current flows in a circuit that are commonly held by people and determine which one correctly describes the behavior of a circuit.

 

Procedure

Use the Pasco circuit board to connect the circuit shown below.  Instead of using alligator clips, pinch hook up wire between the coils to make the circuit.  To preserve battery life, only have the circuit closed when you are taking data.  Configure the DMM as an ammeter with the red lead in the 200 mA socket.

 

 

Model A

A common thought people have about current flowing through a resistor is that it is consumed by a resistor as it flows through the circuit.

 

Q1)  Use model A to predict how the relative amount of current will compare at points A, B, and C.  (i.e. greater, less, equal)

 

Model B

Another common thought people have about current flow in a circuit is that it will be less flowing into a larger resistor and greater flowing into a smaller one.

 

Q2)  Use model B to predict how the relative amount of current will compare at points A, B, and C.  (i.e. greater, less, equal)

 

 

Model C

A third idea frequently held by people is that current flow is constant everywhere in a series circuit.

 

Q3)  Use model C to predict how the relative amount of current will compare at points A, B, and C.  (i.e. greater, less, equal)

 

 

Q4)  With which, if any, of the models posed do you agree?  Explain.  If you don’t agree with any of the models posed, what is your model for current flow in the series circuit?

 

 

Q5)  In the space below, list a procedure you can use to test which model correctly describes current flow in the series circuit.  You are limited to using one DMM for this procedure.

 

 

 

 

Q 6)  Carry out your procedure and list your data below.

 

 

 

Q7)  Which of the models did your data agree with?

 

 

Q8)  Make an argument using conservation of charge why model C must be the correct model.

 

 

 

Part 2  Kirchhoff’s Current Law

In this part we will examine the behavior of current at a junction of several wires.  Such a point is referred to as a node.

 

Procedure

Add a 560 Ω resistor in parallel with the circuit you have already made as shown.  Remember to preserve battery life by only closing the circuit when taking data.  Points B and C on the figure are two nodes on the circuit.

 

Q9)  Connect the DMM as an ammeter between the 330 Ω resistor and point B.  Close the circuit and measure and record the current.

 

 

Q10)  Connect the DMM as an ammeter between the 100 Ω resistor and point B.  Close the circuit and measure and record the current.

 

 

Q11) How does the current you measured in Q10) compare to that you measured in Q9)?

 

 

Q12)  You probably noticed that is slightly less.  Where did the remaining current flow?

 

 

Q13)  Connect the DMM as an ammeter between the 560 Ω resistor and point B.  Close the circuit and measure and record the current

 

 

Q14)  How does the sum of the currents in Q13) and Q10) compare to the current measured in Q9)?

 

 

Q15)  Explain using conservation of charge why this should be the case.

 

 

Q16)  Write a brief sentence describing the behavior of currents at a node.  This result is known as Kirchhoff’s current law.

 

 

 

 

Part 3  Kirchhoff’s Voltage Law

 

Procedure

Build the circuit shown in the figure.  Configure the DMM as a voltmeter. 

 

Q17)  Place the COM lead at point D and the VΩ lead at point A and record the potential difference.

 

 

Q18)  Why is the potential difference +?

 

 

Q19)  Place the COM lead at point A and the VΩ lead at point B and record the potential difference.

 

 

Q20)  Why is the potential difference -?

 

 

Q21)  Place the COM lead at point B and the VΩ lead at point C and record the potential difference.

 

 

Q22)  Why is the potential difference -?

 

 

Q23)  Place the COM lead at point C and the VΩ lead at point D and record the potential difference.

 

 

Q24)  Why is the potential difference -?

 

 

Q25)  Add your results for Q17), Q19), Q21), and Q23).  What do you obtain?

 

 

Q26)  Use conservation of energy to explain why you obtained your answer in Q25).

 

 

The answer in Q25 is referred to as Kirchhoff’s Voltage Law.  Notice that we measured potential differences around a loop starting and ending at point D.

 

Q27)  State Kirchhoff’s Voltage law for the sum of the potential differences around a loop in your own words.