Fred's World of Science:Fast Discharges, Exploding Apples

Potential energy stored in a capacitor bank is given by the following equation: E = 1/2 C V², where E is measured in Joules, C in Farads, and V in Volts. So, in our experiment, we have the following parameters: 17microfarads, 20kV. Plugging in, we get: E = 0.5 (17 x 10^-6) (2 x 10⁴)² = 3400 J. This is a large amount of energy. A defibrillator pack used to "jump start" hearts has a maximum energy of 300 J, typically.

Using an oscilloscope and a HV probe, we were able to measure the time to discharge the capacitors, and therefore calculate the total energy dissapated into the load. For a typical apple, the time to discharge the entire energy of the capacitor bank was about 1.75 microseconds. Plugging that in, we get the total power dissapated into the apple as:

Several people research exactly what happens to this massive amount of power in discharges. Typically, a large amount of energy is dumped into a small amount of water, and calorimetric measurments are made. Using the simple thermodynamic calculation dQ = m c dT, we can calculate what the change in temperature for a given amount of water. If dQ is 3400 Joules, as in our experiments, the dT should be great. The most careful experiments show a temperature rise in agreement with the preceeding equation. However, in even the most careful experiments, there arises a strange phenomenon. When the electrode spacing is close enough in the water "load" and the energy of the capacitor bank is high enough, there is a large explosion.

If thermodynamic measurments are made, the rise in pressure and temperature that one might expect do not add up to meet the total energy dissipated in the system. Several theories are being investigated, from anomalous "free" energy production, to sonoluminescence-like pressure wave generation, to explain the phenomenon. My guess is one of two possibilities. 1) A thin channel of water (like the thin channel of ionized gas in a spark discharge) is flash-boiled and expands the expected 1000x just like in a steam engine. Since the expansion occurs within an incompressible medium, a large explosion follows. 2) Similarly a thin channel of water is ionized, and dissociated. The dissociated gasses recombine exothermically in an explosive manner (just like rocket engines). Whatever the real process here, it is fun to create explosions in a semi-controlled manner. We simply used this technique for making fun pictures and creating stunning effects.

In our experiments, electrodes were spaced roughly 1cm apart, 2.5 cm under water. The beaker was a pyrex 200ml graduated model. The caps were charged to 3.4kJ, and discharged remotely. The glass and water both vaporized, leaving a fine mist in the room, and pieces of glass no larger than grains of sand. Pieces of glass were found 25 feet away. The top 0.75" of the beaker was left intact, with a smooth crack winding its way around the beaker. Interestingly, this was approximately the water-line in the beaker. Below, the glass was gone, above the water level, the glass was virtually intact. The light produced in the blast was roughly blue, mostly white. The sound from the blast was very loud, and not unlike a mid-gauge shotgun blast.

The following images are prints of some photos we made during our experiments. Each photo was made in complete darkness. The lighting in the picture is due to the light produced in the explosion. The camera's shutter was left open while the capacitor bank was discharged. After the voltage had fallen to zero, we closed the shutter and turned on the lights. At that point, we hand-discharged the caps and shorted them for safety.

You can clearly see in the background of both photos a 6' relay rack full of the charging equipment and the trigger electronics. The spark gap tube required at least 300A in its trigger pulse to prevent damage to the tube. They suggested that 1000A would be optimal for preserving the lifetime of the tube. That is why the electronics were so demanding, and space-consuming.

Notice in the background water droplets frozen in air by the flash of light produced by the exploding water.

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You can clearly make out the apple's stem in the photo.