Resistances In Series And Parallel Circuits

Austin E. Barnett              Bogan High School
                               3939 West 79th Street
                               Chicago IL 60652
                               (312) 536-2180 

Objectives:
  
1. To observe the difference in illumination for a given set of bulbs when 
   placed in series and in parallel circuits.       
2. To calculate the IR drop across a network of resistances in series and 
   parallel.

Materials: 

Six flashlight bulbs and sockets, one switch for each of the two circuits, four 
"D" cells, masking tape, twenty 10-cm pieces of bell wire, two DC ammeters of 
range zero amp. to one amp., two DC voltmeters of range zero volts to five 
volts. 

Strategies:

Activity 1.  Series Circuit

1.  Connect three bulbs in sockets in series with a switch, an ammeter, and
    two "D" cells that are firmly taped together top to bottom.  Place the
    ammeter between the switch and the two-cell battery.  Connect the negative
    terminal of the ammeter to the negative terminal of the battery.

2.  Close the switch.  Read and record the amount of current.  Open the switch. 

3.  Reconnect the ammeter between two of the bulbs.  Close the switch.  Read and 
    record the amount of current.  Open the switch. 

4.  Reconnect the ammeter as in step one.

5.  Connect a voltmeter across one of the bulbs.  Be sure that the negative 
    terminal of the voltmeter is connected toward the negative terminal of the 
    battery. 

6.  Close the switch.  Read and record the voltage drop.  Open the switch.
 
7.  Repeat steps 5 and 6 with the voltmeter connected across two bulbs.

8.  Predict what will happen to the voltage drop if the voltmeter is connected 
    across all three bulbs.  Repeat steps 5 and 6 with the voltmeter connected 
    across all three bulbs. 

9.  Remove one of the bulbs from its socket without disconnecting the socket.
    Remove the voltmeter.  Close the switch.  Record the ammeter reading.  Open 
    the switch. 
 
10. Reconnect the circuit, so that, there are only two bulbs, and sockets in 
    series with the ammeter, switch, and battery.  Record the current value and 
    voltage (IR) drop across the circuit.  Open the switch. 

Students, this concludes our first activity.  Now, let's discuss what took 
place. 

Activity 2.  Parallel Circuits

Students, the purpose of this activity is to study the properties of a parallel 
circuit. 

1.  Connect three flashlight bulbs in parallel sockets. 

2.  Connect 2 "D" cell batteries, a switch, and an ammeter in series.  Place 
    this in parallel with the 3 sockets.  Be sure, that the negative terminal of 
    the ammeter is toward the negative terminal of the battery. 

3.  Close the switch.  Read and record the current value.  Notice how bright
    the bulbs are.  Open the switch.

4.  Without disconnecting the socket, remove one bulb.  Close the switch.  
    Record the current value.  Compare the brightness of the remaining bulbs to 
    their brightness before.  Open the switch.  Replace the bulb. 

5.  Reconnect the ammeter in series with one of the bulbs.  Predict how the 
    current will compare to the previous reading. 

6.  Test your prediction by closing the switch.  Record the current and open
    the switch.

7.  Remove a bulb in a different branch from the ammeter.  Close the switch.
    Read and record the current value.  Open the switch.

8.  Connect a voltmeter across one socket.  Close the switch.  Read the meter,  
    and record the voltage.  Open the switch.  Repeat for each of the other 
    sockets. 

References:  
     
    Modern Physics  Holt: Rinehart and Winston (1990)
    Physics: Principles and Problems (1982)