Lightning…

1. The electrical discharge that occurs when there is an increase in the electric field of the surrounding environment (i.e. atmo-sphere) through surface polarization of electric charge of its surrounding molecules (i.e. water molecules in clouds). Lightning… It’s Electrifying!

2. Historical Beliefs • Most European cultures in the ancient world had a form of deity that was related to lightning • The Native Americans believed that a thunderbird was the cause of the lightning and thunder • Priests and seers saw lightning as a way of explaining the gods’ will pertaining to political and battle tactics • In the Middle Ages it was thought that bell ringing would deter lightning from the churches, when in all reality it caused the deaths of the bell ringers

3. Dangers of Lightning • Lightning has been known to cause power outages, damage buildings, start forest fires, cause damage to aircraft, etc. • In the eighteenth century (before Franklin’s grounding rods were used), gun powder was stored in the church volts during war • St. Nazaire in Italy contained about 100 tons and was struck by lightning which was ensued by an explosion and death of over 3,000 people and destruction of about a sixth of the city of Brescia • Recently a woman was killed by lightning while hiking during a thunderstorm with her boyfriend in Asheville, N.C. • Lightning on average tends to release about 30 kA of current correlated to an electric field of approximately a few 100’s Volts per meter

4. Fulminology in the Past • Benjamin Franklin conducted the first scientific experiments involving lightning (kite and string [1749], grounded metal rods [1752], etc.) The lightning rod has provided safety for many buildings throughout history. It was usually a grounded metal rod inside the building walls, which acted as a pathway for the lightning to the ground in order to protect the building from lightning damage

5. Types of Lightning • Lightning can generally be placed in two categories: • Ground strikes: electrical discharge between a grounded object and clouds; these themselves may be ascending or descending sparks. • Intercloud: electrical discharge between clouds or inside clouds; usually evidenced by shock waves (i.e. thunder); these occur more than six times as much as ground strikes • We know more about ground strikes than intercloud

6. Lights… Camera… Action! • Most of what we know about lightning comes from photographs taken by both scientists and civilians • The most basic process of lightning sparks involves several steps, beginning with the formation of thunderstorms • The thundercloud acts as a giant capacitor, its base negatively charged and top positively, due to the polarization of water particles inside • The polarization is theoretically due to the collision of molecules and the knocking off of electrons • This creates a large enough electric field at the base of the cloud to break down gas molecules and form a plasma channel (a leader) • As the cloud moves it creates an equal but opposite charge (+) on the ground, grounding the plasma channel

7. The plasma channel acts like a wire that current passes through and forms a path of least resistance for the lightning to propagate through in order to ground the charge on the cloud • Meanwhile a streamer from the ground flies up to meet the downward heading stepped leader • As soon as the leader and streamer are joined there is a lightning discharge followed by a progression of smaller discharges between branches of the main channel and the streamer leading back up to the cloud • This is known as the return stroke • This whole process occurs within microseconds

8. http://wsx.lanl.gov/images/lightning.mpg • Lightning on average releases 30 kA of current, 500 MJ of energy and carry ~5 C of charge and the average peak power output is ~1 terawatt • Larger ones can have up to 250 kA and 350 C • The electric field necessary for the electrical breakdown of air is ~3 MV/m and that necessary for propagation is in the order of ~10 V/m • The air around the lightning reaches temperatures ~3 X that of the surface of the sun (~36,000° F)

9. Physics: Lightning, and Current, and Electric Fields… Oh, My!

10. In The Beginning There Was…The Method of Images • The simplest way to show that there is an induced charge on the ground due to the cloud is this method • Consider a cloud of negative charge as a single point of charge above the conductive plane of the Earth • Place a charge q above a conducting plane at (0, 0, d) • The potential at z=0 is Φ=0 because the plane is • grounded • As d²>>x²+y²+z², Φ→0 • Now consider the problem of placing a charge q at (0, 0, d) and a charge –q at (0, 0, -d) • The potential is then, • In this case as well, the potential at z=0 is Φ=0 and as d²>>x²+y²+z², Φ→0 Both of these cases reveal the same potential in the end when you look at the math.

11. Induced Surface Charge • The surface charge on σ on the conductor is given by: where is the normal derivative of the potential (in this case the z-axis), so • From the potential given in the previous slide: and so, • Now solving for the induced charge on the surface using polar coordinates: • Hence there is an equal but opposite charge induced in the conductive surface (in this case, Earth)

12. Lightning Sparks and Their Leaders • The leader is a plasma channel that is formed to make a path of least resistance for the lightning to propagate through with speeds comparable to that of the speed of light • Plasma is a gas made of mostly ionized particles, which, in this case, are due to the high electric field caused by the charge separation in the clouds • The clouds themselves act as capacitors and their bases and tops act as conducting planes • The induced electric field can be calculated using the force on each electrical charge:

13. And Then There Was Light • The energy stored in these electric fields can be calculated by: • Lightning bolts (in terms of simple electrostatics) can be viewed as current passing through a wire • It is known that the potential across a wire, Φ, is directly proportional to the length, L, of it: • where E is the electric field. • ljlkjio

14. References • Bazelyan, Eduard M., and Yuri P. Raizer. Lightning Physics and Lightning Protection. 1 ed. Washington, DC: Taylor & Francis, 2001. Print. • "Cumulonimbus cloud - Wikipedia, the free encyclopedia." Wikipedia, the free encyclopedia. N.p., n.d. Web. 3 June 2010. http://en.wikipedia.org/wiki/Cumulonimbus_cloud. • Griffiths, David J.. Introduction to Electrodynamics (3rd Edition). 3rd ed. San Fransisco: Benjamin Cummings, 1998. Print. • Lightning - Wikipedia, the free encyclopedia." Wikipedia, the free encyclopedia. N.p., n.d. Web. 3 June 2010. http://en.wikipedia.org/wiki/Lightning. • Malan, D J. Physics of Lightning.. Edinburgh: London: English Universities Press 1974., 1974. Print. • "NWS Lightning Safety Overview." NWS Lightning Safety. N.p., n.d. Web. 4 June 2010. http://www.lightningsafety.noaa.gov/overview.htm. • Uman, Martin A.. The Lightning Discharge. Unabridged ed. New York: Dover Publications, 2001. Print. • Zavisa, John. "HowStuffWorks "How Lightning Works"." Howstuffworks "Science". N.p., n.d. Web. 3 June 2010. http://science.howstuffworks.com/lightning.htm.