Friday, February 27, 2009

Static Electricity - 3


Lightning




Natural static discharge

Lightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. In general, significant charge accumulations can only persist in regions of low electrical conductivity (very few charges free to move in the surroundings), hence the flow of neutralizing charges often results from neutral atoms and molecules in the air being torn apart to form separate positive and negative charges which then travel in opposite directions as an electric current, neutralizing the original accumulation of charge. The static charge in air typically breaks down in this way at around 30,000 volts-per-centimeter (30 kV/cm) depending on humidity. The discharge superheats the surrounding air causing the bright flash, and produces a shockwave causing the clicking sound. The lightning bolt is simply a scaled up version of the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the shock wave created as the superheated air expands explosively.

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