In this experiment, students use the GLX to determine the relationship between the time constant for certain RC circuits and the value of the resistance and capacitance for those circuits.

• Microsoft Word version of this lab:
  
  • rcConstantGLX_docs.zip (172 KB)  or
  • rcConstantGLX_docs.sit (188 KB)

• Xplorer GLX (PS-2002)
• Voltage-Current Sensor (PS-2115)
• Short Patch Cords (SE-7123)
• Charge/Discharge Circuit (EM-8678)
• 2 AA or AAA Batteries

1. Arrange the batteries, patch cords, Charge/Discharge Circuit, and Voltage-Current Sensor as in the diagram above.


2. Connect the Voltage-Current Sensor to the GLX.


3. Place the switch in the "DISCHARGE" position.



4. Press the button and select "Two Graphs" as in the picture above.



5. If necessary, change one of the graphs so that one is a Current graph and the other is a Voltage graph by pressing the button and navigating to the correct axis label as in the picture above.

1. Press the Start Button. Wait about 5 seconds.


2. Move the switch from the "DISCHARGE" position to the "CHARGE" position.


3. Press to autoscale the data.


4. When the values remain constant, move the switch to the "DISCHARGE" position.


5. When the values remain constant, press the Stop button.


6. Sketch or print the graphs. To print the graphs, press the button and select "Print."


7. Record the value of the resistance and the capacitance in the Data Table.

Data Table

Resistance ()	Capacitance (F)	Voltage at Full Charge (V)	Time to Charge to 63.2% of Full Charge(s)	Time to Charge to 36.8% of Full Charge(s)

1. Press to deselect the "Two Graphs" setting.



2. If necessary, change the graph to the Voltage graph by pressing the Button twice and selecting "Voltage."


3. Press and select the Smart Tool. Using the or arrow button, determine the value of the voltage at full charge. Record this value in the Data/Analysis Table.


4. The time constant for a Resistor-Capacitor (RC) circuit is the amount of time to charge a capacitor to 63.2% of full charge; or the amount of time to discharge it from full charge to 36.8% of full charge.



5. Press and select the Delta Tool. Using the or arrow button, accurately drag one of the Delta Tool's cursors to the point in the graph as close as possible to the time the switch was placed in the "CHARGE" position.


6. Press and select Swap Cursors. Using the left or right arrow button, accurately drag the other cursor to 63.2% of full charge.


7. Record the amount of time required to charge to 63.2% of full charge in the Data/Analysis Table.


8. Drag one of the cursors to the point in the graph as close as possible to the time the switch was placed in the "DISCHARGE" position.


9. Press and select Swap Cursors. Using the left of right arrow button accurately drag the other cursor to 36.8% of full charge.


10. Record the amount of time required to discharge to 36.8% of full charge in the Data/Analysis Table.


11. Repeat the Set Up and Procedure steps above for the 33 Ω and 100 Ω resistors.

1. Using standard symbols for electronic components, draw a diagram of the circuit when it is charging and when it is discharging.


2. Is the Voltage Sensor placed in parallel or in series in this circuit? Explain.


3. Is the Current Sensor placed in parallel or in series in this circuit? Explain.


4. Which component of the circuit is the voltage sensor measuring? Would the result of the time constant be different if it was placed across a different component? Explain.


5. At what location is the current sensor in the circuit? Would the result of the time constant be different if it was placed elsewhere in the circuit? Explain.


6. Observe your sketch/print-out of the two simultaneous graphs. Describe and explain the changes in the current and voltage during charging and during discharging.


7. The time constant can be found using the following equation: = RC, where is the time constant, R is the resistance, and C is the capacitance. Determine the percent difference between the theoretical and actual values of the time constant for each circuit.


8. Account for any discrepancies between the theoretical and actual values of the time constant. Hint: look closely at each resistor.


9. Are those discrepancies different for each resistor? Explain.

Repeat the lab. Determine how the time constant relates/applies to the current during charging and discharging.