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