Science Buddies: "Ask an Expert"

I did a project on whether larger jars can hold more static electricity and I found that different sized jars can hold the same amount of static electricity, but i need to know why. Would someone please help me?

sciencejeff - The confusion seems to be "what is more static electricity"? A Leyden jar is fundamentally a capacitor. Think of a simple capacitor as two parallel metal conductors separated by an insulating spacer, or dielectric. The capacitor is capable of storing a quantity of energy. One of the conductors having a positive charge, the other, a negative charge. There is an attraction between the two that creates an electric field, but because the insulator efficiently prevents the flow of electrons, the charge can remain stored on the capacitor for a period of time. The Leyden jar was charged by some source that provided a high voltage charge. There are some definitions that have to be considered. The measurement shown in the Science Buddies experiment measures the potential, or voltage, stored on the capacitor (Leyden jar). This is determined by measuring the distance that the spark initially jumps as the two electrodes are brought into close proximity. In dry air, this voltage can be estimated as about 3,000 volts/mm. Voltage is only one attribute of the energy stored. The other is the amount of energy stored which is a function of the number of electrons stored and the voltage potential that is observed.

So the problem is, what do you mean by your question "which (larger or smaller Leyden jars) can hold more static electricity"? If you have two capacitors, same insulator thickness, but different surface areas of the electrodes, the larger capacitor can store more energy at the same voltage. It may be at exactly the same voltage, but many more electrons will be involved in the larger capacitor. Because your charging source for this experiment may limit the number of electrons flowing into the Leyden jar, the smaller capacitor may actually charge to a higher voltage, while the larger capacitor will store more energy (probably or possibly at a lower voltage).

I know this is may sound very confusing, but do a little investigation of capacitors and capacitance and it will help you to understand this phenomena.

Rick Marz

I was measuring the voltage and I understand a little better now, but I need to know why different sized jars can store the same voltage. Thanks!

sciencejeff - Different size Leyden jars will store whatever level of voltage charges it. If your source produces 20KV, a Leyden jar will store a charge with a potential of 20KV, independent of size. This is assuming that the source is capable of producing a sufficient flow of electrons to fully charge the capacitance. Remember, this is a 'static charge' not a connection to a power supply. The larger Leyden charge will store more electrons (think energy), but the stored voltage will be the same.

Rick Marz

Thank you very much for helping!

Measuring how much energy is stored in a Leyden Jar (capacitor) requires knowing both the initial voltage and something that is related to the total amount of stored charge. Assuming you can charge different Leyden Jars to the same initial voltage, you can control the discharge current by using a resistor of say 10 to 100 Meg Ohms and determining how long current flows. If I remember correctly, the amount of time it takes to discharge to to 1/3 the original voltage is the time constant for a given RC value (resistance in Meg Ohms times capacitance in micro Farads).

Research RC circuits and time constants to understand how this works.