Principle of Millikan’s determination of the electronic charge, e (A-level only) Flashcards – AQA A-level Physics (7408)

What is the electric field strength between Millikan’s plates in terms of their potential difference $V$ and separation $d$?

$E=\frac{V}{d}$

What force-balance equation applies to a stationary oil droplet when the electric force acts upward?

$\frac{QV}{d}=mg$, where $Q$ is the magnitude of the droplet’s charge.

What is Stokes’ law for the viscous drag on an oil droplet moving through air?

$F=6\pi\eta rv$, where $\eta$ is the air viscosity, $r$ is the droplet radius and $v$ is its speed.

With the electric field off, what force-balance equation applies to an oil droplet falling at terminal speed $v_0$? Neglect air buoyancy.

$mg=6\pi\eta r v_0$

How is an oil droplet’s mass related to its radius $r$ and oil density $\rho$?

$m=\frac{4}{3}\pi r^3\rho$

An oil droplet falls at terminal speed $v_E$ while the electric force acts upward. What is the force-balance equation?

$mg=\frac{QV}{d}+6\pi\eta r v_E$

How does the terminal-speed force balance change if the electric force on a falling oil droplet acts downward rather than upward?

The electric force is added to the weight: $mg+\frac{QV}{d}=6\pi\eta r v_E$.

What does it mean for an oil droplet’s net charge to be quantised?

$Q=ne$, where $n$ is an integer and $e$ is the elementary charge.

What approximate value of the elementary charge was determined from Millikan’s oil-droplet measurements?

$e\approx 1.60\times 10^{-19}\,\mathrm{C}$

What is the charge of an electron in terms of the elementary charge?

$-e$

What pattern in Millikan’s measured droplet charges provided evidence for charge quantisation?

The charges were integer multiples of a common smallest charge unit.