The chocolate crumb mystery. Explosions ignited by electrostatic discharges (spa

Question

The chocolate crumb mystery. Explosions ignited by electrostatic discharges (sparks) constitute a serious danger in facilities handling grain or powder. Such an explosion occurred in chocolate crumb powder at a biscuit factory in the 1970s. Workers usually emptied newly delivered sacks of the powder into a loading bin, from which it was blown through electrically grounded plastic pipes to a silo for storage. As part of the investigation of the biscuit factory explosion, the electric potentials of the workers were measured as they emptied sacks of chocolate crumb powder into the loading bin, stirring up a cloud of the powder around themselves. Each worker had an electric potential of about 7.0 kV relative to the ground, which was taken as zero potential, (a) Assuming that each worker was effectively a capacitor with a typical capacitance of 213 pF, find the energy stored in that effective capacitor. If a single spark between the worker and any conducting object connected to the ground neutralized the worker, that energy would be transferred to the spark. According to measurements, a spark that could ignite a cloud of chocolate crumb powder, and thus set off an explosion, had to have an energy of at least 163 mJ. (b) Could a spark from a worker have set off an explosion in the cloud of powder in the loading bin (type 1 if yes, 0 if no)?

Explanation / Answer

Part A)

Energy of a capacitor can be found from

E = .5CV2

E = (.5)(213 X 10-12)(7000)2

E = 5.22 X 10-3 J, which is 5.22 mJ

Part B)

Since you need 163 mJ to set of the explosion, 5.22 is not enough.

So NO the spark could not set off the explosion.

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