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The Sponge Resistor Model — A Hydrodynamic Analog to Illustrate Ohm's Law, the Resistor Equation , and Resistors in Series and Parallel

### Abstract

Physics students encountering electric circuits for the first time often ask why adding more resistors to a circuit sometimes increases and sometimes decreases the resulting total resistance. It appears that these students have an inadequate understanding of current flow and resistance. Students who do not adopt a model of current, voltage, and resistance necessarily resort to memorizing formulas for calculating, e.g., the resistance of a resistor network. For these students, certain properties of electric circuits may remain mysterious or puzzling.

© 2014 American Association of Physics Teachers

Acknowledgments:
I would like to thank Rick Lindsey for assembling the prototype and for his suggestion to use the 3M Scotch-Brite™ hand finishing pad as possible suitable filler material for the resistance tubes. I am also grateful to Jonathan Barrick for creating the sharpened steel punches that were used to cut out the sponge sections. Special thanks also to Sahil Nayyar and Raheem Chouwdry for performing many measurements with the apparatus to obtain the data presented in this paper.

Article outline:

Illustrating Ohm's law
The length of a resistance element
The cross-sectional area of a resistance element
The resistivity ρ of a resistance element
Resistances in series
Resistances in parallel
Darcy's law — Or “Why does the sponge resistance model work so well?”

/content/aapt/journal/tpt/52/5/10.1119/1.4872404

http://aip.metastore.ingenta.com/content/aapt/journal/tpt/52/5/10.1119/1.4872404

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