Teaching Physics with the Physics Suite

Edward F. Redish

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Counting charges - 1

A. In chemistry, one often uses a unit of charge known as the Faraday, which has the magnitude of the charge of one (gram) mole of electrons. Since the Coulomb has come to be the international standard of unit of charge, the Faraday is taken to have the unit "C/mole" or Coulombs/mole. How many Coulombs/mole is there in a Faraday?

B. In lab, suppose that we take two small aluminum spheres of 1 gram each and hang them from a common support point by 23 cm long non-conducting threads. While they are touching, we put some negative charge on the two spheres. Suppose that the two spheres share the charge equally. When things settle down, the spheres repel and settle down to hang as shown in the figure at the right.

1. Estimate the amount of charge (in Coulombs) on each sphere.

2. Estimate what fraction of the aluminum atoms in a sphere have an extra electron.

Page last modified February 16, 2012: E65