generator, resistor, and capacitor
Wave Generator (Wavetek)
Set up for R-C Time Constants Demo
Wave Generator (Wavetek).
Simultaneous Voltage and Current Charging Curves Demo
R-C Time Constants demo

 

 

Code Number: 5F30.20

Demo Title: R-C Time Constants or Simultaneous Voltage and Current Charging Curves

Condition: Good

Principle: Circuit Characteristics

Area of Study: Electricity & Magnetism

Equipment: 

2200 Uf Capacitor, 100 Ohm Resistor, 6 Volt Battery, Key Switch, Digital Oscilloscope, Resistor Box, Decade Capacitor Unit, and Wave Generator (Wavetek).

Procedure:

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.

This circuit is already assembled and labeled "RC Charging - Discharging Circuit Only" in the 5F30.20 box.  Note that we will only be monitoring the voltage charging and discharging curves with this circuit.  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 on the digital scopes is stored at #2 in the Recall/Save menu.

This circuit is already assembled and labeled "Simultaneous Voltage and Current Curves" in the 5F30.20 box.  Note that you may have to change some of the oscilloscope settings to add the second trace when you have obtained the "Saved" setting on the oscilloscope.  You must add a resistor box to this circuit.  Two of the leads for this circuit are labeled for that.  The battery will charge the capacitor when the switch is pressed down.  Release the key and the capacitor will discharge through the resistors.  The scope setting on the digital scopes is stored at #2 in the Recall/Save menu.

The standard analog 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.

References:

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  • Norris W. Preyer, "Transient Behavior of Simple RC Circuits", AJP, Vol. 70, #12, Dec. 2002, p. 1187.
  • David E. Wilson, "A Direct Laboratory Approach to the Study of Capacitors", AJP, Vol. 57, #7, July 1989, p. 630.
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  • "E-405. Chg. and Dischg. - Square Wave", DICK and RAE Physics Demo Notebook.
  • G. D. Freier and F. J. Anderson, "Eo-12", A Demonstration Handbook for Physics.
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  • G. D. Freier and F. J. Anderson, "En-10", A Demonstration Handbook for Physics.
  • G. D. Freier and F. J. Anderson, "En-8", A Demonstration Handbook for Physics.
  • Yaakov Kraftmakher, "1.3. Charging and Discharging a Capacitor", Experiments and Demonstrations in Physics, ISBN 981-256-602-3, p. 15.
  • Robert Ehrlich, "L.9. Discharging a Capacitor Through a Voltmeter", Turning the World Inside Out and 174 Other Simple Physics Demonstrations, p. 151 - 152.

Video Credit: Jonathan M. Sullivan-Wood.

5F30.20 - R-C Time Constants
5F30.20 - R-C Time Constants - Wave Gen.
5F30.20 - Simultaneous Voltage & Current Charging Curves