Science Buddies: "Ask an Expert"

Hello,
My partner and I have decided to do our Science Fair Project on Electricity. Our idea was to use a home-made Leyden Jar to store static electricity and use that energy to power a small device like a screenless mp3 player. I was wondering if this will be hazardous to us. I know that the jar needs to be small in size so as to not recieve a hazardous static shock. However, I cannot find any information in the dangers of using the Leyden Jar to power a device.
Will the device explode? Will the electricity 'go wild'? I am sorry if these questions seem absurd or mundane.
Thank you,
The Third One

Hi,

a Leyden jar essentially acts as a capacitor. How much static electricity you can store in it depends on size, materials, and your ability to charge it. So in theory you can come up with a jar that has a hazardous charge. Therefore you should always adhere to basic electric safety rules. In particular, never touch a possibly charged object with your skin, and when handling such objects, use only one hand while leaving the other in your pocket (to make sure it is not grounded and current will not flow through your heart). Avoid experimenting among flammable materials (e.g. in a garage next to cans of paint thinner or gasoline). Also, if you do experience a sudden complete discharge of your Leyden jar, the first thing to go may be your MP3 player; those run on 5V or less and you could get 10,000V out of a jar. You will have to design a circuit which adequately protects the equipment you intend to power from the Leyden jar.

So much for safety which always has to come first. In practice, you have leakage working against building up enough of a charge to sustain a harmful current. The wikipedia article on the Leyden jar has a list of references which I recommend reading.

Thank you very much for your help ivogougheschrich.
Sincerely,
The Third One

TheThirdOne - Ivogougheschrich was correct. A classic Leyden jar uses a glass jar, similar to a canning jar, with an internal and external foil or conductive coating to form the capacitive 'plates'. Glass is a low K dielectric, but capable of extremely high voltage operation. This type of Leyden jar would have a very low capacitance, and would never be able to provide power even for a small MP-3 player if you attempted to use a low voltage charge. It would be capable of storing a 10 or 20kV charge, but that is of no use in this experiment. Consider using another power source, like a battery made of a citrus fruit or acid vegetable. The voltage and current would probably easily power your MP-3 device. Solar power? Wind power? There are many alternatives you might use. Good luck.

Rick Marz

The Leyden jar is a capacitor with a very small capacitance but able to withstand a very high voltage charge. The result is its ability to store a relatively large quantity of energy. The major problem with using one is getting the energy out in a slow and controlled manner.

To run a low-voltage device like an MP3 player, you need a current source of tens to hundreds of milliamps at a voltage less than five volts. The Leyden jar could only deliver that amount of current for a fraction of a second before it was discharged, and, in the process, you would need to regulate the voltage to a useful level with a voltage regulator, and the voltage regulator would consume almost all the energy and dissipate it as heat.

Second problem, how to charge the jar? There is the small matter of energy efficiency and a battery is a much better device for making that conversion, chemical to electrical. A battery will also hold a lot more energy than the jar.

Bob