5F30.20 - R-C Time Constants

Code Number:
Demo Title:
R-C Time Constants
Circuit Characteristics
Area of Study:
Electricity & Magnetism
2200 Uf Capacitor, 100 Ohm Resistor, 6 Volt Battery, Key Switch, Digital Oscilloscope, Resistor Box, Decade Capacitor Unit, and Wave Generator (Wavetek).

Video Credit: Jonathan M. Sullivan-Wood.

Put the generator, resistor, and capacitor, in series as shown.  Set the generator to the square wave function at around 100 Hz.  Measure across the capacitor with the oscilloscope.  Set the capacitor and resistor to the proper values to obtained the desired trace.

Arrange the circuit as shown.  The battery will charge the capacitor when the switch is pressed down.  Release the key and the capacitor will discharge through the 1/2 watt, 100 ohm resistor. It is critical to set the scope as shown.  The scope setting is stored at #10 in the Recall/Save menu.

The standard oscilloscope will not allow you to observe voltage and current at the same time due to grounding parameters.  To get around this drive two identical circuits from the same wave generator with one circuit having the components in reverse order of the other.  In this way, the oscilloscope grounding remains valid for both cases and both can be shown on the same scope.

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Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist.  The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose.  The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources.  Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks.  These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition; and property loss or damage.  Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.