Science Buddies: "Ask an Expert"

Hi,

I am performing the "How Far Can Sparks Jump?" experiment located at https://www.sciencebuddies.org/science- ... c&from=TSW. As it recommends, I am going to calculate the voltage produced using the formula "dielectric breakdown voltage for air is approximately 3000 V/mm (= 3 kV/mm)". The maximum distance I moved the metal spheres while still seeing a spark was 5mm. If I use this equation, then I would say the voltage produced was 15,000 V (3000 V/mm *5mm). This seems like a lot! Is this correct? If not, what formula do I use to calculate the voltage?

Trey, Indianapolis

tahubbard - All your observations and calculations seem correct. The dielectric breakdown of dry air is approximately 3000V/mm. If you can initiate a spark over 5 mm distance between the spheres the voltage is certainly in the 15,000 volt area. Congratulations.

Rick Marz

Yes, quite correct. Scary isn't it?!

However, there are a few qualifiers.
1. Spheres require a slightly higher voltage than needle points.
2. This applies to clean and dry air.
3. This applies to a static charge that will not continue the arc once the static charge is gone.

A few additional items. Damp or dirty air requires only a little lower voltage since it is easier to ionize. Higher elevations will also reduce the voltages until you get to about 50,000 feet, then it goes back up due to a lack of air.

If the power source can sustain a current flow once the arc is started you get a lot of power being dissapated in the arc, as in a lightning bolt or a power line flashover. This can cause considerable damage and frequently starts fires.

Thanks for all your help!