Science Buddies: "Ask an Expert"

I did my science project on " Where there is charge, there can be sparks." I was wondering where the 30,000 volts came from. I know 30,000 volts equals one centimeter,and that 3,000 votls equals one milimeter but where does the 3,000 come from?

Sophie99 - I'm not sure I understand your question exactly, but as you know, air is an excellent insulator and will not readily conduct electrons. However, every insulator has a limit at which time the molecules, under high voltage stresses, will break down and another phenomena takes over. In air, the molecules ionize and the insulating properties disappear and you see the breakdown and conduction as a spark discharge. In dry air, at atmospheric pressure the electrical potential for this to occur is approximately 3,000 volts per millimeter (30,000 volts per centimeter). In this experiment you are using this known insulating strength to measure the electrical charge voltage by bringing the electrodes closer together until the discharge occurs. Then, by simply measuring the gap distance, you can calculate the voltage with the numbers above. For example, if the distance that the discharge occurs was 3.5 millimeters, just multiply 3.5 times 3,000 and you calculate the stored voltage on your device to be 10,500 volts. Hope this helps.

Rick Marz

So your saying, for this experiment it is exactly 3,000 volts per millimeter is when the molecules start to break down.

Hi:

Rick said:

In dry air, at atmospheric pressure the electrical potential for this to occur is approximately 3,000 volts per millimeter

This number will be approximate, as it's for dry air (and your conditions may vary )

For a short discussion on the dielectric breakdown of air, see this: http://hypertextbook.com/facts/2000/AliceHong.shtml

Keith