The electric field. Charge distribution on conductors
Lessons 5–6 · 2 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, J.E. Usarov, K.T. Suyarov, M.K. Yuldasheva. Physics, textbook for Grade 8 of general secondary schools, revised and enlarged 3rd edition. “O‘qituvchi” Publishing and Printing Creative House, Tashkent, 2019
6
Distribution of electric charges on conductors
Textbook: pp. 22–24
GoalKnow that charge sits on the surface of a conductor, the Faraday cage, and that charge is denser at sharp points.
New words
isolated conductor · yakkalangan o‘tkazgichFaraday cage · Faradey qafasisharp tip · o‘tkir uchelectric wind · elektr shamol
Explanation
Like charges placed on a metal ball repel one another and move as far apart as possible, so all of them end up on the surface of the ball and spread evenly over it. A small test ball put inside a hollow charged ball picks up no charge: there is no charge inside. Faraday built a cage covered with foil and sat inside it holding an electroscope: when the cage was charged the electroscope showed nothing, so there is no field inside a metal cage. That is why workers on high-voltage lines wear special clothing woven with metal threads. On an irregularly shaped conductor the charge gathers more densely at sharp tips; charge leaving such a tip sets the air moving as an “electric wind” that bends a candle flame.
Worked examples
A ball on an insulating handle is lowered into a charged hollow metal sphere, taken out without touching the walls and touched to an electrometer — the pointer does not move. Conclusion: there is no charge inside; the charge is only on the outer surface.
A pointed metal body is charged; a foil leaf near the tip opens wider than one on the cylindrical part. So the charge density is higher at the sharp tip, and tall structures are given pointed tops.
Class activity
Observation (safe): look at the tops of TV aerials, towers and lightning rods on the street or in pictures and write down 3 examples. Never climb up and never go near electric wires.
Practice
1
Where does the charge given to a metal ball sit?
On its outer surface, evenly spread; there is no charge inside.
2
Where is the charge denser: on a flat surface or at a sharp tip?
At the sharp tip.
3
Why does an electroscope inside a Faraday cage stay closed even when the cage is charged?
The charge is on the outside; there is no field inside the cage.
4
Why do workers on high-voltage lines need clothing with metal threads?
The clothing acts like a Faraday cage and shields the body from the electric field.