A decade before Einstein published his theory of relativity, J.J. Thomson propos

Question

A decade before Einstein published his theory of relativity, J.J. Thomson proposed that the

electron might consist of small parts and attributed its mass to the electrical interaction of the

parts. Furthermore, he suggested that the energy equals mc2. Make a rough estimate of the electron mass in the following way: assume that the electron is composed of three identical parts

that are brought in from infinity and placed at the vertices of an equilateral triangle having sides

equal to the classical radius of the electron, 2.82 x 10-15 m. (a) Find the total electric potential

energy of this arrangement. (b) Divide by c2 and compare your result to the accepted electron

mass (9.11 x 10-31 kg). (The result improves if more parts are assumed)

Explanation / Answer

E = 3*k (e/3)^2/r (the 3 comes from three pairs)

E = 3*9E9*(1.6E-19/3)^2/2.82E-15=2.72E-14

m = E/c^2 = 2.72E-14/(3E8)^2=3.026E-31 kg

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