Charged Particles

Discovery of ElectronElectron beams in Fields

Electron TubesMeasurement of the Charge of the

Electron

Cathode Rays (Discharge Tubes)

Crooke’s Tubes (1870s)

Neon for bright red-orange

Oxygen and Nitrogen for purple

Thomson’s Experiments:

Do atoms have parts?

The Electron gun

Thermionic emission The work done on each electron The electron-volt

Electron beams in fields

Deflection in a uniform electric field Force on each electron Acceleration of each electron Time taken to travel whole plate width Vertical displacement, y Vertical speed, Vy

Direction of the beam Speed of the beam at any point

Example An electron gun operating at 3000 V is used to

shoot electrons into the space between two oppositely charged parallel plates. The plate spacing is 50 mm and its length is 100 mm. Calculate the deflection of the electrons at the point where they emerge from the field when the plate p.d. is 1000 V. +

-

d

Electron beams in fields

Deflection in a uniform magnetic field The force on the electron The acceleration Radius of the electron path The specific charge, e/m

The velocity selector× × × × × × × ×× × × × × × × ×× × × × × × × ×× × × × × × × ×× × × × × × × ×

vF=Bev

Electron gun

r

Example

An electron gun operating at 2000 V is used to shoot electrons into a magnetic field strength of 1.9 mT. The initial angle of the beams is at right angles to the field direction. Calculate the specific charge on an electron if the

beam forms a circle of diameter 160 mm. Describe the beam path if the initial direction of

the beam is changed to 30 to the field direction.

Electron tubes

The TV tube

Electron tubes Oscilloscope tube

Measurement of the charge of an electron (1909)

The End