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Al Tobias (wat4y) - Office: Gibson S123 & Physics 218, (434) 924-0538

Physics Demo Manual

Demonstrations are cataloged according to PIRA Bibliography


Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.

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 Electric Fields And Potential
 Current and Resistance
 Electromotive Force And Current
 DC Circuits
 Ohm's Law
 Resistivity of a Pencil
 Resistive Heating of Nichrome Wire
 Blowing a Fuse
 Wheatstone Bridge
  video  - Wheatstone Bridge II
  video  - Circuit Analysis
  video  - RC Circuit w/ Function Generator
 RC Circuit w/ Battery
 RC Time Constant (DECAY)
 Telephone model as simple DC circuit
 Magnetic Materials
 Complex Circuits
 Semiconductors And Tubes
 Transmission Lines and Antennas

RC Time Constant (DECAY)


To show the relationship between R, C, and discharge time.


Connect battery (6V) and charge RC circuit. A schematic of the set up is shown below. Electrometer hooked across capacitor should read 6 V.D.C. Place gates 1m apart. Load gun and aim so bullet will go through both gates and sever them. This is a one shot affair. The first coil will disable battery circuit when cut. A short time later the RC circuit is disabled and the remaining voltage on the capacitor is read from the electrometer. One could consider placing the demo inside the Plexiglas® box that is used for the mousetrap "atomic bomb".

Note: The speed of sound in air at sea level and T=20°C is ν=344m/s=768mph. Therefore, the speed of the rifle bullet can be supersonic.
  • 1 m/s = 2.24 mph (conversion factor)
  • 500 m/s = 1118 mph (rated maximum bullet speed)
Some past data:

Initial Voltage Final Voltage Bullet Speed Instructor Date
6.4V 3.8V 519m/s Craig G. 31 Jan 2013
6.4V 2.5V 300m/s Nilanga L. 29 Jan 2015
6.9V 3.9V 469m/s Cass S. 30 Sep 2015
6.5V 3.0V 346m/s Bob H. 09 Feb 2016
6.4V 3.4V 423m/s Nilanga L. 09 Feb 2016
6.4V 1.1V 152m/s Cass S.. 30 Sep 2016
6.6V 3.0V 350m/s Nilanga L.. 04 Oct 2018


  • A fully "singly" wrapped gate with our thin brown wire measures a resistance of about 10 Ω.
  • Make sure both gates are "closed circuit" (check continuity of wires). If, for example, the 2nd gate is open, the RC circuit will not discharge.
  • Make sure both gates are not accidentally shorted at the terminal lugs ie. bare wire touching bare wire. If the 2nd gate is shorted, then the circuit will discharge completely.
  • RC time constant is 0.00374s. Maximum speed for the bullet is 1640 ft/s (500m/s). Distance between the front faces of the gates is 1m. Thickness of wood that holds gates is 1.5-inch (3.81cm). This factor is only important if one gate is taped to front face and the other is taped to back face.


  • Power Supply
  • Electrometer
  • .22 Caliber Rifle
  • Alligator Clips
  • Connecting Wires
  • 2 RC Time Constant Apparatus
  • Capacitor
  • Resistor