Physics 109 - Resistors and Capacitors

This experiment begins with an invention activity which will help with the data analysis in the rest of the experiment. Data for this invention activity is available in a spreadheet by clicking here.

In this experiment you will study a simple circuit containing a resistor and a capacitor. When the switch S is closed in the circuit shown below, the capacitor 'charges up' so that the voltage across the capacitor matches the DC Power Supply voltage. When the switch is opened again, the capacitor discharges via current flowing through the resistor.

To perform the experiment, assemble the circuit as indicated. Try to arrange the wiring so that it is easy to connect and disconnect the wire leading to the 'top' of the capacitior as indicated by the switch in the diagram. Set the DC power supply to provide no more than 2 Volts to the circuit. The resistor box should be set to 90,000 Ohms and the capacitance set to 0.1 microfarads. Make sure one of your Teaching Assistants checks the circuit with you.

The oscilloscope displays voltage versus time and the technical challenge of this experiment is to arrange the horizontal time scale, the vertical voltage scale, and the triggering. You need to enter the Ch1 menu and set the coupling to 'DC'. The vertical scale should be 500 mV/division and the horizontal scale should be 10 ms/division. The triggering is set so that the oscilloscope starts to measure the decay at the moment that you disconnect the voltage to the capacitor. This will take some experimenting, but two things that you should definitely do are to set the trigger source to Channel 1, the trigger slope to 'falling', and the trigger mode to 'normal'. (If you are having a hard time finding the decay, you can temporarily set the trigger mode to 'Auto' until you find the signal.)

Once you have captured the voltage drop that occurs when the capacitor discharges, set the 'Run' button to 'Stop', so that you do not accidentally erase your data. Then use the cursors to measure voltage versus time. Be sure to collect data out far enough in time that you have data where the decay is mostly completed. Use your skills with spreadsheets to linearize the data and fit it to a straight line.

Marking Scheme

invention activity: 2 marks

8 marks for high quality measurements and high quality linear least squares fit