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