Hands-on
Lab 6

Fun with RC Circuits

To gain hands on knowledge of RC circuits, to measure the time constant of a circuit using an oscilloscope and to find the relationship between the time constant and the resistance.

Read 29-8 and do problems 29.70,71

Stand-up computer station equipped with MPLI Box and voltage probes, a 1.0 F capacitor, a several resistors (less than 5.0 ohms), a 1.5 V battery, several wire leads, a breadboard, and a multi-meter.

Part A

1. Connect the capacitor, resistors and MPLI oscilloscope in a RC circuit, which has an equivalent resistance of less than 1.0 ohms. Draw the circuit and show the teacher.
2. What is the formula for the voltage across a charging capacitor as a function of time? What is the formula for the voltage across a discharging capacitor? (use t instead of RC for the time constant) Sketch a (1/2-page) graph of each function. Include any necessary constants including the time constant.
3. Remove the capacitor from your circuit and charge it using a 1.5 V battery.
4. Setup the MPLI Oscilloscope program: open "PHYSICS", then "MULTI PURPOSE LAB INTERFACE", go to "Oscilloscope". Under the "TRIGGER" menu set "NO TRIGGER". Press "B" and "C’ to turn off the ports not in use, use the horizontal direction keys to set the time interval to 1000 ms, and the vertical direction keys to set the "VOLTS/DIV" to 1 V.
5. Reconnect one of the terminals of the capacitor (do not connect the other or the capacitor will discharge and you’ll have to charge it again). Make sure the circuit is set up so when you connect the other terminal of the capacitor the oscilloscope will collect voltage data.
6. When the horizontal line on the oscilloscope reaches the far left connect the capacitor and watch the line on the oscilloscope curve down towards the time axis.
7. Press "O" and select "EXIT – SAVE DATA". Go to "PLOT DATA", observe and record what you see. Then use the data to calculate the time constant. Show using a diagram, how you did this.

Part B

1. Repeat the process in Part A with at least 3 different resistances and make a table to record values of R and t .
2. Plot t vs. R and fit a curve to the plot to find a function t (R).
3. Does this function agree with the physics in Chapter 29? Explain thoroughly, giving mathematical details. Your answer must include an interpretation of the physical meaning of the coefficients of your fit

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