 Solution

ID: past-exam-of-the-physics-a-course-of-the-university-of-cambridge/2012/paper-1/section-a/5/solution

For independent electron arrivals, the shot-noise counting model gives mean transferred charge

Q = I Δ t = 1 0^{- 15} C

and count

\overset{ˉ}{N} = Q / e ≃ 6.24 \times 1 0^{3}

. Poisson statistics imply

σ_{Q} = e \overset{ˉ}{N} = e I Δ t ≃ 1.27 \times 1 0^{- 17} C, \frac{σ _{Q}}{Q} ≃ 1.27%.

This is about 79 electron charges rms. It assumes Poisson shot noise; charge discreteness by itself does not rule out correlations, thermal noise or other apparatus noise.

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