## PHYS208 Fundamentals of Physics II

### Construction of a Parallel-Plate Capacitance

 MacGyver is in another tight spot. He has been locked in the kitchen supply area of a military camp. Unbeknownst to his captors he has concealed an electronic homing beacon in his shoe -- he will simply wait for Pete to arrive! Unfortunately his foot was run over by a humvee in the excitement of the chase and he suspects that the capacitance unit of the tuning circuit is now malfunctioning. What to do? Mac recalls that the circuit specs called for a 0.02 microfarad capacitor. Looking through the supply cabinets he finds a roll of wax paper and a roll of aluminum foil. He will make a new capacitor! Recalling a formula from the most useful and interesting class from his college days, he begins some quick calculations: Image borrowed fromThe MacGyver Homepage

• Desired capacitance is 20 nF.
• Epsilon0 is 8.85 pF/m -- call it 10 pF/m.
• So the desired A/d is about 2000 m.
Mac recalls that the thickness of wax paper is around a mil (a thousandth of an inch).
• A thousandth of an inch is 25 microns (0.025 mm).
• So the desired area for the capacitor is (2000 m)(0.025 x 10-3 m).
• That is, 0.05 m2 or 500 cm2.
Mac also recalls that there are 2.54 cm per inch. He is no stickler for SI! Besides he knows that U.S. currency is 6 inches long and he may have to improvise for a ruler...
• Squaring to convert to area yields about 6 cm2 per sq.in.
Remember! time is of the essence and there is no calculator handy...
• So an area of about 80 sq.in. will suffice.
• 6" x 12" pieces of foil will be close enough!

 Finally Mac places the two sections of foil between alternating layers of wax paper. Rolling the sandwiched layers up tightly with a rolling pin, to get a uniform separation of the two conductors by the amount of one layer of wax paper, the capacitor is formed! Pulling out a few paper clips, appropriate connections to the homing beacon are made. Mac is rescued soon thereafter!

THE END

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"http://www.physics.udel.edu/~watson/phys208/waxpaper.html"
Last updated March 10, 1998.
Copyright George Watson, Univ. of Delaware, 1998.