It’s Electrifying! 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).

 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

 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

 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

 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

 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

 lightning.mpg 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)

 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.

 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)

 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:

 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

 Bazelyan, Eduard M., and Yuri P. Raizer. Lightning Physics and Lightning Protection. 1 ed. Washington, DC: Taylor & Francis, Print.  "Cumulonimbus cloud - Wikipedia, the free encyclopedia." Wikipedia, the free encyclopedia. N.p., n.d. Web. 3 June  Griffiths, David J.. Introduction to Electrodynamics (3rd Edition). 3rd ed. San Fransisco: Benjamin Cummings, Print.  Lightning - Wikipedia, the free encyclopedia." Wikipedia, the free encyclopedia. N.p., n.d. Web. 3 June  Malan, D J. Physics of Lightning.. Edinburgh: London: English Universities Press 1974., Print.  "NWS Lightning Safety Overview." NWS Lightning Safety. N.p., n.d. Web. 4 June  Uman, Martin A.. The Lightning Discharge. Unabridged ed. New York: Dover Publications, Print.  Zavisa, John. "HowStuffWorks "How Lightning Works"." Howstuffworks "Science". N.p., n.d. Web. 3 June