Electric Current
Electricity and Magnetism

En-lightning!

Stories from Physics for 11-14 14-16 IOP RESOURCES

  • The most common kind of lightning, around 75% of activity, is cloud-to-cloud lightning which is seen as a diffuse light within clouds. Of lightning that travels from sky to ground, 90% is negatively-charged downward lightning. Upward travelling lightning, distinguishable by rising branching lines, is rarer and is thought to arise from objects taller than 100 m. Positively-charged lightning accounts for some of the largest recorded strikes, including some in the range 200-300 kA. Discharges involving both positive and negative charge may occur.
  • Around 60 volcanic eruptions are reported to have caused lightning-like phenomena. The motion of dust and ash particles can lead to the build
  • of electrostatic charge.
  • Flashes of lightning have been reported after the detonation of thermonuclear bombs.
  • Various forms of discharge in the sky were reported before the Kobe earthquake in Japan in 1995, including flashes, domes of light and luminous funnels. An orange arc-like phenomenon was described as 100 times brighter than moonlight.
  • Researchers have triggered lightning by firing rockets with trailing ground wires into clouds.
  • Lightning is not only found on the Earth – the phenomenon has been observed on Jupiter. The Voyager spacecraft recorded 20 lightning events in 192 seconds. It is thought Jovian lightning originates from a band of water and ice cloud 30 km thick around the planet.
  • Though the typical energy of a cloud to ground lightning strike is 10 9 −  10 10 J, most of the energy is transferred to heat in the air, sound, light and radio waves. Nevertheless, a strike can heat sand to 30,000 K, much higher than the melting temperature of silica. When lightning strikes rocks or sand, the passage of large currents can heat the sand so that it forms hollow or branching mineral structures called fulgurites, or petrified lightning. The longest recorded fulgurite is over 5 m long.

References

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