Physics Department Homepage
Table of ContentsRC Charging Curve on Scope -- an oscilloscope is hooked across a capacitor which is in series with a battery and resistor. When the switch is closed the capacitor begins to charge and the assymptotically rising voltage is seen on the scope. Releasing the first switch and pressing another discharges the capacitor, with the resulting discharge curve shown on the scope. Circuit elements are laid out on a vertical board for easy viewing and analysis.
RC Circuit Analog -- a flat disc tied to the end of a spring is submerged in a large glass jar of water. Pull up suddenly on the end of the spring and the disc will rise in the water, quickly at first then more slowly as it approaches equilibrium, just as does the RC circuit described above. The spring is analogous to the capacitor in the RC circuit - pulling on it suddenly is the same as applying a sudden voltage - and the resistance of the water to the motion of the disc is the analogue of the electrical resistance in the circuit. The distance that the disc moves is the analogue of capacitor charge.
Relaxation Oscillator -- capacitor, resistor, and 90V battery are hooked in series, with a neon bulb in parallel with the capacitor. Capacitor charges to about 80V (the breakdown voltage of the neon bulb), then discharges through the bulb and begins the cycle again. The capacitor voltage curve can be displayed on an oscilloscope.
Emergency Flasher -- commercial emergency flasher uses a 9V battery to flash a neon lamp at approximately 2 Hz. (Battery voltage is stepped up electronically to provide the 600 volts needed to flash the tube).
Lamp in Parallel with Solenoid -- large DC current introduced suddenly to this large inductor cannot pass through the coil at first, so an incandescent lamp in parallel with the coil lights brightly. After the current becomes steady, the coil draws more current and bulb dims. When the current is switched off suddenly, the induced voltage in the coil (back EMF) again lights the lamp. A separate neon lamp in parallel with the coil shows that the direction of the second voltage surge is the opposite of the first.
LR Circuit -- a large inductor in series with a resistor, a battery and a switch, When the switch is closed the the current rises slowly from zero to a steady-state value as shown by the voltage across the resistor.
Driven LRC Circuit -- an LRC circuit is driven by a 60 Hz input, and the resulting voltage and current across any component can be displayed on an oscilloscope. I and V are tested by means of a probe that plugs into a highly-visible schematic board which shows the electrical location of the probe. All components can be varied to determine the effect on resonance as shown by the voltage on the screen. Phase shifts between I and V may be shown for each component.
Swept LRC Circuit --an LRC circuit is hooked in series with an audio oscillator, which provides the driving voltage. The frequency of the audio oscillator is swept through the resonant frequency of the LRC circuit and the voltage across the capacitor is displayed on an oscilloscope. The voltage hits its peak at the resonant frequency, but the resonance is rather broad.
Damped LRC Oscillation -- the capacitor in an LRC circuit is fully charged with a battery, then the battery is removed, a switch is closed and the circuit is allowed to oscillate down to equilibrium. Voltage across the capacitor is displayed on the scope as the oscillations die away.
Damped LRC Circuit Analog -- a flat disc with a weight on it is tied to the end of a spring and submerged in a large jar of water. Pull up suddenly on the free end of the spring and the weight will shoot up, pass its new equilibrium point, then oscillate about the equilibrium point, damping out within 4 or 5 oscillations, just as does the damped LRC described above. The spring is analogous to the capacitor in the LRC circuit; pulling on it suddenly is the same as applying a sudden voltage. The resistance of the water to the motion of the disc is the analogue of the electrical resistance in the circuit. The weight on the disc is analogous to an inductor. The position of the disc is the analog of capacitor charge.
University of Washington Homepage
Physics Department Homepage
Table of Contents