We know that the negative charge on the electron and the positive charge on the

Question

We know that the negative charge on the electron and the positive charge on the proton are equal. Suppose, however, that these magnitudes differ from each other by 0.00008%. With what force would two copper pennies, placed 0.9 m apart, repel each other? Assume that each coin contains 3*1022 copper atoms. (Hint: A neutral copper atom contains 29 protons and 29 electrons.)

(From your answer, it should be obvious that the magnitude of the charge on a proton and the charge on an electron cannot possibly differ by as much as the given percentage.)

Explanation / Answer

Suppose the charge of a proton is more positive than the negative charge on an electron. Each coin would carry an excess charge Q = 29* 12*10^(-7) * 3*10^(22) *1.6*10(-19) Coulomb = 0.167 Coulomb.
For a rough estimate, we assume the excess charge is centered in the coin so we can use Coulombs law for calculating the repulsive force with one distance d:
F = Ke * Q^2 / d^2
= 8.988 * 10^9 N m^2/C^2 * (0.167 C)^2 /(0.9 m)^2 = 3.16 * 10^8 N ....
The spirit of the exercise is that electrostatic forces can be HUGE. If they did not balance on a macroscopic scale, matter as we know it would not exist. Since gravity is the force that we see dominating on a larger scale, this actually tells us that the stuff in the universe is electrically neutral on the large scale. Compare the constant Ke in Coulomb's law with G in Newton's law of universal gravitation.

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