Teaching Physics with the Physics Suite Edward F. Redish

The Electroscope

In the figure below are shown schematics of two versions of an electroscope: a historic version on the left with gold foil leaves, and a more modern one with a pivoting metal arm on the right.

(a) The charged metal disk of an electrophorus is touched to the metal knob of the electroscope and removed. The movable leaves of the electroscope, which hung straight down when the electroscope was uncharged, now stand apart as shown in the two figures above. Explain why this happens, tell what sign of charge you think the electroscope has, and why you think so.

(b) The electroscope is neutralized by touching the knob. Now the charged metal disk of the electrophorus is brought near to the knob but doesn’t touch it. The leaves separate. Explain why.

(c) While the disk of the electrophorus is near to the electroscope’s knob (but not touching) the knob of the electroscope is touched with a finger and then the electrophorus disk is taken away. The leaves stand out again. Explain why this happens, tell what sign of charge you think the electroscope has, and why you think so.

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