Electroscope
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Coulomb's Law
One of the fundamental laws of physics is Coulomb's law. It describes electrostatic interactions between electric charges.
Coulomb's law states that the electrostatic force of interaction between two point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them:
accessed: February 18, 2013
A simple device that demonstrates the consequences of Coulomb's law is an electroscope.
An electroscope is a simple apparatus used to detect electric charge and voltage (i.e., the difference in electric potential). A basic leaf electroscope consists of a vertical metal rod with two rectangular leaves made of thin, lightweight conductive foil attached at the bottom via a hinged connection.
As is well-known, like charges repel each other. If we charge the electroscope, its leaves will diverge because they acquire the same charge (positive in this case, but the effect is the same with a negative charge):
The degree of divergence of the leaves depends on the amount of charge accumulated on them. By measuring the deflection, we can estimate the magnitude of the charge.
Fun fact: Cosmic rays were discovered using an electroscope. It was observed that a charged electroscope slowly discharged on its own. The effect became more pronounced with increasing altitude. The explanation was air ionization caused by particles from space, which allowed the charge to "escape."
Nothing prevents us from building our own working electroscope.
Construction
You will need:
- A glass jar with a plastic lid
- Aluminum foil
- Some wire with a diameter of about 1.5 mm (≈ 0.06 inches)
- Wire with a diameter of about 0.5 mm (≈ 0.02 inches)
- Glue, such as cyanoacrylate (super glue)
The first step is to straighten the thicker wire. Then cut a piece of suitable length and pass it through a hole drilled in the jar’s lid. Secure the wire with glue so that its end reaches about halfway down the jar. Strip the insulation from both ends of the wire, and bend a small hook at the end inside the jar:
Next, we need some precision. Using the thinner wire, carefully bend a suspension frame for the electroscope leaves. The photos below show examples you can follow. Adjust the size to suit your needs.
As we can see, the suspension frame is bent from a single piece of wire.
Now, cut the electroscope leaves from aluminum foil. Their width should be slightly less than that of the suspension frame:
Fold the ends of the foil strips so that they can be placed onto the previously bent suspension frame.
The final step is to hang the assembled element on the hook inside the jar. Our electroscope is ready!
By bringing any charged object close to the exposed end of the thicker wire, you can observe the divergence of the foil strips. The greater the charge, the larger the deflection.
Enjoy your fun and educational experiment! :)
Further readings:
- Derry T.K., Williams T., A Short History of Technology from Earliest Times to A.D. 1900, Courier Dover, 1993, pp. 609
- Henderson T., The Physics Classroom: Static Electricity, the National Science Foundation, 2011
- Williams E.R., Faller J.E., Hill H.A., New Experimental Test of Coulomb's Law: A Laboratory Upper Limit on the Photon Rest Mass, Physical Review Letters, 1971, vol. 25, no. 12, pp. 721-724
Marek Ples