A First Course in Physics · Ch. XIII, §§ 334–339 · Chapter 13: Static Electricity
38. Charges on conductors, points, and lightning
Goal: Your student can explain why charge sits on the outside of a conductor and crowds at points, how a lightning rod works, and why a metal cage shields what is inside.
You'll need
- The book, open to these pages
- The homemade electroscope from last lesson and a balloon
- A metal colander or a cage of aluminum foil or wire mesh
- A radio or phone (for the shielding test)
- A notebook and pencil
What's in the book
Charge lives on the outside surface of a conductor; charge is densest where the surface is most curved; points let charge escape (the electric whirl and electric wind); Franklin's kite and the lightning rod; electric screens (Faraday's box). A portrait and short life of Benjamin Franklin.
The lesson, step by step
Hook question
SayWhy is a car a fairly safe place to be in a lightning storm?
Read together
Read §§ 334–339 and the note on Franklin. The book's conclusion:
These experiments show that an electric charge resides entirely on the outside surface of a conductor.
Explain the idea
All the bits of charge repel each other, so they spread out to the outside surface and crowd onto sharp curves and points. At a sharp point the charge is so dense it leaks into the air (§ 336). A lightning rod's point quietly leaks charge from the building, which helps prevent a strike (§ 338).
Try it: a shield
Charge the electroscope by induction so its leaves stand apart. Set the metal colander over it, then bring a charged balloon close outside: the leaves don't move (§ 339). Then put a phone inside a closed metal pot or wrapped in several layers of foil and call it: often the call won't connect. The metal screens out electrical disturbances.
Safety: lightning is not an experiment
Franklin's kite (1752) was extremely dangerous, and at least one scientist (Georg Richmann, in 1753) was killed trying a similar experiment. Never fly a kite in a storm or near power lines. When thunder roars, go indoors. Sparks from electrical machines, Leyden jars and the electric whirl are also for a teacher's demonstration only: look up a video of a Van de Graaff generator instead.
What we know now
We now know a lightning rod mainly works by giving a strike a safe path to the ground, rather than mostly by preventing strikes; the book itself says there are discharges they do not stop. A car protects you because the metal body carries the current around the outside, just as in Faraday's box.
Say it back
Your student explains in their own words why the charge collects at the point of a pear-shaped conductor.
Afterwards
Look at buildings and church steeples for lightning rods, and at the metal cage inside a microwave oven door that keeps the waves in.