Physics: Discovery of Electron

Discovery of Electron

• In 1879, William Crooks conducted experiments on electricity conduction through low-pressure gases using a discharge tube.
• The discharge tube is a 60 cm long cylindrical glass tube with two metal electrodes, sealed at both ends.
• Electrical discharge is observable only at low pressures and high voltages, leading to the flow of particles known as cathode rays from the cathode to the anode.

Properties of Cathode Rays

• Cathode rays travel in straight lines and originate from the cathode moving towards the anode.
• They are invisible directly but can cause fluorescent materials to glow upon contact.
• Cathode rays consist of negatively charged particles, demonstrated by their deflection towards a positive plate in an electric field.
• Characteristics of cathode rays remain consistent regardless of the electrode material or gas type in the tube.

Discovery of Charge/Mass Ratio for Electrons

• J.J. Thomson experimentally determined the charge/mass ratio (e/m) of electrons using electric and magnetic fields.
• The degree of deflection of the particles depends on the charge magnitude, the particle's mass, and the strength of the electric/magnetic fields.
• Thomson calculated the charge/mass ratio to be e/m = 1.758820 x 10^11 C kg^-1.

Charge on the Electron

• R.A. Millikan's oil drop experiment was critical for determining the charge of an electron.

Discovery of Proton

• In 1886, Goldstein modified the discharge tube with a perforated cathode, observing luminous rays called anode rays emerging through the perforations.
• Anode rays move in the opposite direction to cathode rays and are generated between the anode and cathode.
• The charge on particles in anode rays varies based on the gas used, and the charge/mass ratio also differs.

Properties of Anode Rays

• Positive charge (e) of particles is gas-dependent, and some particles carry multiples of the fundamental electrical charge.
• The behavior of anode rays in magnetic or electric fields is contrary to that of cathode rays.

Proton Characteristics

• The smallest positive ion, the proton, originates from hydrogen.
• The mass of a proton is 1.676 x 10^-27 kg, and its charge is +1.602 x 10^-19 C.

Discovery of Neutron

• Neutrons, neutral particles, were discovered by Chadwick in 1932 through the bombarding of beryllium with alpha particles, resulting in highly penetrating neutral rays.

Thomson Model of Atom

• J.J. Thomson proposed that an atom is a positively charged sphere, with embedded negatively charged electrons, suggesting a uniform distribution of charge.

Explore the groundbreaking experiments that led to the discovery of electrons, including the work of William Crooks and J.J. Thomson. This quiz covers the properties of cathode rays and their significance in understanding electrical conduction. Test your knowledge on the historical experiments that shaped modern physics.