PhysicsLAB Lab
Parallel and Series Circuits
The purpose of this lab is to practice constructing electric circuits and to observe characteristics of series and parallel circuits.
 
To complete the lab you will be given the following equipment:
 
  • 1.5-3 VDC power supply
  • 3 light bulbs & socket board
  • 7 connecting wires
  • 8 alligator clips
  • ammeter/voltmeter
  • 2 diffraction gratings
  • 1 knife switch
 
 
In this lab activity you will only be using the VDC terminals on the power supply. You may not turn on the power supply without first having Mrs. Colwell certify that your connections are correct. We do not want to destroy any equipment through incorrect wiring which might be hazardous to you and your lab partner(s). If these directions are not followed, disciplinary action will be taken, plus you will be responsible for any damaged apparatus.
 
Be aware that light bulbs are non-ohmic resistors. This means that their resistance increases with their temperature. Their temperature increases with the length of time current passes through their filaments. So the purpose of the switch is to allow you to make only momentary connections.
 
In the diagram on the left, the red line would represent an ohmic resistor having constant resistance, R = V/I. The blue line shows the modified graph when the resistance increases with temperature.
 
 
 
 
 
 
 
 
 
 
 
Part I. Single bulb
 
Initially, you are to wire a simple circuit including a power supply, a switch, an ammeter, a voltmeter, and a single light bub.
 
 
After your circuit has been approved, set the DC voltage output to 3.0 volts, close the switch, and take the following readings.
 
As soon as your readings have been taken, immediately open the switch and turn off your power supply.
 
What was the exact digital readout on the power supply (while the bulb was shinning)? 

Use the diffraction grating supplied to view the light being emitted by the light bulb. Check the colors seen.
 
bulb measured
current 		measured voltage
(amps) (volts)
1
 
 
bulb calculated resistance calculated
power
(ohms) (watts)
1
Based on the emf of the power supply and the ammeter reading, what is the total resistance present in the circuit? 

Comparing your calculated resistance for the light bulb in the table with your previous answer using the emf of the power supply, does there seem to be a significant amount of resistance in our connecting wires and circuit board? Explain. 

 
Part II. Two bulbs in series
 
This time, you are to wire a new circuit including the power supply, the switch, an ammeter, a voltmeter, and two light bubs in series.
 
After your circuit has been approved, set the DC voltage output to 3.0 volts, close the switch, and take the following readings.

As soon as your readings have been taken, immediately open the switch and turn off your power supply.
 
 
What was the exact digital readout on the power supply (while the bulb was shinning)? 

Were the bulbs in this part of the experiment shinning dimmer, the same, or brighter than the single bulb in Part I?
 
Before you move to the next section, open the switch, unscrew one of the light bulbs, and then close the switch. What happens?
 

bulb measured
current 		measured voltage
(amps) (volts)
1
2

Why did you only need to take one ammeter reading? 

Did the voltages lost across the two bulbs add to equal the total voltage supplied by the power supply?
 
 
bulb calculated resistance calculated
power
(ohms) (watts)
1
2

Based on the emf of the power supply and the ammeter reading, what is the total resistance present in the circuit? 

Based on the rule for calculating total resistance for resistors in series, what was the total resistance in the circuit? 

What this value for resistance equal to, greater than, or less than the resistance calculated based on the emf of the power supply and the current through the ammeter?
 
 
Part III. Two bulbs in parallel
 
This time, you are to wire a new circuit including the power supply, the switch, an ammeter, a voltmeter, and two light bubs in parallel.
 
After your circuit has been approved, set the DC voltage output to 3.0 volts, close the switch, and take the following readings.

As soon as your readings have been taken, immediately open the switch and turn off your power supply.
 
 

What was the exact digital readout on the power supply (while the bulb was shinning)? 

What was the ammeter reading when it was placed in series with the power supply? 

Were the bulbs in this part of the experiment shinning dimmer, the same, or brighter than the single bulb in Part I?
 
Before you move to the next section, open the switch, unscrew one of the light bulbs, and then close the switch. What happens?
 

bulb measured
current 		measured voltage
(amps) (volts)
1
2

Why did you only need to take one voltmeter reading? 

Why did you only need to take multiple ammeter readings? 

Did the currents flowing through the two bulbs add to equal the ammeter reading when it was placed in series with the power supply?
 
 
bulb calculated resistance calculated
power
(ohms) (watts)
1
2

Based on the emf of the power supply and the ammeter reading when it was placed in the main line with the power supply, what is the total resistance present in the circuit? 

Based on the rule for calculating total resistance for resistors in parallel, what was the total resistance in the circuit? 

What this value for resistance equal to, greater than, or less than the resistance calculated based on the emf of the power supply and the current through the ammeter?
 
 
Part IV. Series and Parallel
 
In this final part of the experiment you are going to add a bulb in series with your previous parallel arrangement. Given below is a diagram of the desired circuit.
 
 
In the diagram, a 3rd light bulb has been added in series with the power supply. You can accomplish this by carefully opening and closing the lugs on the circuit board.
 
After your circuit has been approved, set the DC voltage output to 3.0 volts, close the switch, and take the following readings.
 
As soon as your readings have been taken, immediately open the switch and turn off your power supply.
 
What was the exact digital readout on the power supply (while the bulbs were shinning)? 

bulb measured
current 		measured voltage
(amps) (volts)
1
2
3

Was the bulb in series with the power supply shinning dimmer, the same, or brighter than the other two bulbs that were in the parallel configuration?
 
 
bulb calculated resistance calculated
power
(ohms) (watts)
1
2
3

Based on the emf of the power supply and the ammeter reading when it was placed in the main line with the power supply, what is the total resistance present in the circuit? 

Based on the rule for calculating total resistance for resistors arranged both in parallel and in series, what was the total resistance in the circuit? 

What this value for resistance equal to, greater than, or less than the resistance calculated based on the emf of the power supply and the current through the ammeter?
 
Which, if any, of the bulbs could be unscrewed and still allow the other two bulbs to remain shinning? Explain.
 




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