Lightning storms are dangerous for several reasons: (1) If lightning strikes you or something you are touching, it will produce a fatal charge flow in your body. (2) If it strikes an object near you, a portion of the charge flow can jump through the air to you (an effect known as side flash). (3) If lightning strikes the ground near you, part of the charge flow it produces along the ground can be diverted through your body. Recently a fourth danger of a lightning storm was recognized. The photograph here, in which lightning strikes a tree, contains a hint.
What is this additional danger of a lightning storm?
The answer is in this chapter.
23-1 What Is Physics?
One of the primary goals of physics is to find simple ways of solving seemingly complex problems. One of the main tools of physics in attaining this goal is the use of symmetry. For example, in finding the electric field 
of the charged ring of Fig. 22-10 and the charged rod of Fig. 22-9, we considered the fields 
(= k dq/r2) of charge elements in the ring and rod. Then we simplified the calculation of by using symmetry to ...
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