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MERLOT II


    

Learning Exercise


Material: The Ohm Zone
Submitted by: Terry Bradfield on Jul 25, 2002
Date Last Modified: Jul 26, 2002
Title: Combinations of Resistors
Description: This is an activity on simple series and parallel combinations of resistances.
Time Required Approximately 1 hr.
Topics: Ohm's law, series and parallel resistance combinations
Prerequisites Skills: Students should be familiar with the definitions of basic electrical quantities and Ohm's law.
Learning Objectives: To understand the behavior of simple combinations of resistances.
Text of Learning Exercise:

Combinations of Resistors



Introduction


In this activity, you will study the behavior of combinations of resistances.
There are two basic ways that this can be done.  In the first, the components
are connected sequentialy, so that the same current flows through each. 
This is known as a series connection.


In the second, the components are connected so that the current may flow
through either component. This is known as a parallel connection.


The applet for this is located at:
http://www.article19.com/shockwave/oz.htm



Procedure No. 1:  Resistances in series



Measurements




When using the applet, remember that voltages are measured by connecting
the voltmeter across the component being measured, while currents are
measured by placing the ammeter at the point where the current is
desired.

Construct a simple circuit consisting of a battery and a single light
bulb. Now measure the batteryvoltage and current in the circuit. To measure
the battery voltage, connect the voltmeter across the battery by dragging
it over the battery's location. To measure the current, drag the ammeter
to a point in the circuit between the lamp and the battery. Your meter readings
will appear in the upper-right section of the applet window.



Voltage __________    Current _________   Resistance
V/I = __________.



Next, add a second light bulb in series with the first. Record your voltage
and current readings and calculate the total resistance.



Voltage __________    Current _________   Resistance
V/I = __________.



Connect the voltmeter across each lamp by dragging it into position over
the lamp. Record the voltage across each lamp and calculate the ratio V/I.


 





















Lamp No.


1


2


Voltage

   

V/I

   



Questions


1.  How does the resistance of  the series combination compare
to the resistance of a single lamp? ________________________________

2. What relationship can you see between the voltages across each lamp
and the battery voltage? ________________________________



3. How does the ratio V/I for each lamp when connected in series compare
with its resistance?   _________________________________



Procedure No. 2:  Resistances in Parallel

Modify the circuit from Procedure 2 so that the two bulbs are in parallel.
Measure the voltage across each lamp and the current in them and calculate
the ratio V/I.

 

























Lamp No.


1


2


Voltage

     

Current

   

V/I

   


Finally, measure the total current from the battery:  __________
and calculate the ratio of the battery voltage over the battery current _________. 
This value is the effective resistance of the parallel combination.



Questions


1.  How does the effective resistance of the two lamps in parallel
compare with the resistance of a single lamp? _____________________

________________________________________________________________________________________________________



2.  How does the current in each lamp in the parallel case compare
to the current in a single lamp?  _____________________________



________________________________________________________________________________________________________.



3.  What is the relationship between the the current in each lamp
and the total current from the battery?  __________________________



________________________________________________________________________________________________________


 

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