12.1 - The discovery of the electron (A-Level Physics AQA 7408)

#12.1.1

Cathode rays

Production of cathode rays in a discharge tube.

#12.1.2

Thermionic emission of electrons

The principle of thermionic emission.

Work done on an electron accelerated through a pd V; $\dfrac{1}{2}mv^2 = eV$

#12.1.3

Specific charge of the electron

Determination of the specific charge of an electron, $\dfrac{e}{m_e}$ , by any one method.

Significance of Thomson’s determination of $\dfrac{e}{m_e}$

Comparison with the specific charge of the hydrogen ion.

#12.1.4

Principle of Millikan’s determination of the electronic charge, e

Condition for holding a charged oil droplet, of charge Q, stationary between oppositely charged parallel plates.

$\dfrac{QV}{d} = mg$

Motion of a falling oil droplet with and without an electric field; terminal speed to determine the mass and the charge of the droplet.

Stokes’ Law for the viscous force on an oil droplet used to calculate the droplet radius.

$F = 6πηrv$

Significance of Millikan’s results.

Quantisation of electric charge.

A-Level Physics AQA 7408