How to turn your physics book into a capacitor!
A page by Devin Boyer

The Theory - The Finished Product - The Tools, The Plan - Parallel Circuits - Series Circuits - Bibliography

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Parallel Circuits

No experiment would make sense without a control, so the first circuit built and tested did not include the capacitor. This circuit is a very simple voltage divider. Because both ohm-rangers are set for 1k ohm, the first oscilloscope channel should about half the voltage of the second. Because there are no significant sources of inductance or capacitance in the circuit, the two waveforms are nearly identical in timing and shape. When the source goes high, the output follows nearly instantaneously.


Simple resistive circuit diagram


Simple resistive circuit


Oscilloscope output from resistive circuit

With an idea of what a circuit looks like without capacitance, it is possible to now observe the changes a capacitor will have. For this circuit, we put the capacitor in parallel with the second resistor. Our measurements are the voltage across the capacitor and, because they are parallel, the second resistor. It can be seen from the output that the waveform is drastically modified by the addition of the capacitance. The output from this experiment shows that capacitance simply makes the change in states slower. Although it takes nearly a quarter of a period for the voltage to settle, it does eventually, until the next switching event. This transient response is typical of a capacitor. If the frequency of the signal was increased, it would eventually look like a constant zero volts because there would be no time to charge the capacitor between events. If the frequency was decreased, it would eventually look a lot like a simple resistive circuit, because the charging time will be an insignificant part of the period.


Simple parallel RC circuit diagram


Simple parallel RC circuit


Oscilloscope output from parallel RC circuit