A student sets up a fine-beam tube containing helium gas at low pressure to demonstrate the properties of electrons. The electron source is a heated filament inside an electron gun. Electrons are accelerated from rest through a potential difference VVV towards an anode. A uniform magnetic field of flux density BBB, acting perpendicular to the electron path, deflects the beam into a circular trajectory of radius rrr.
Explain, with reference to the work function, the mechanism of thermionic emission by which electrons are liberated from the heated filament.
By considering both the work done on an electron by the accelerating potential difference VVV and the magnetic force acting as a centripetal force on the electron beam in the magnetic field BBB, derive the expression for the specific charge of the electron:
em=2VB2r2 \frac{e}{m} = \frac{2V}{B^2 r^2} me=B2r22Vwhere eee is the electron charge and mmm is the electron mass.
In the student's demonstration, the following experimental measurements are recorded:
Calculate the specific charge of the electron from these data. Express your final answer to an appropriate number of significant figures.
When J.J. Thomson first measured the specific charge of cathode ray particles, he compared his result to the known specific charge of a hydrogen ion. State how these two values compared, and explain the conclusions Thomson drew regarding the identity and mass of the particles making up cathode rays.
