Beginner Topic 30 of 1102 Atomic Structure & Periodicity
Cathode Rays
Streams of electrons observed in discharge tubes, deflected by electric and magnetic fields.
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In-depth explanation of Cathode Rays
Cathode rays are streams of electrons emitted from the negative electrode, or cathode, in an evacuated discharge tube. They travel in straight lines, cast shadows and can turn a small paddle wheel placed in their path. They are deflected by electric and magnetic fields, and the electric deflection is toward the positive plate, which proves that the particles are negatively charged. The study of cathode rays by Crookes, Thomson and others led directly to the discovery of the electron. Cathode ray tubes were used for decades in televisions and oscilloscopes.
Going deeper: Cathode Rays
Key ideas: Rays in a discharge tube at low pressure (about 0.01 mmHg) at high voltage travel in straight lines from cathode to anode, are deflected by electric and magnetic fields toward the positive side, and are the same for any cathode material. Thomson balanced the two fields (v = E / B) to find e/m = 1.76 x 10^11 C per kg.
Common mistakes and connections: Students think the rays come from the anode or are electromagnetic waves like light; they are negatively charged particles. The identical behavior for all metals showed electrons are universal constituents of matter. They lead to the electron, television tubes and the anode (canal) rays that revealed protons.
Visualization of Cathode Rays
Thomson found cathode ray particles have a charge-to-mass ratio about 1800 times that of a hydrogen ion, so they are very light electrons. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Old television tubes steer a cathode ray beam across the screen to paint the picture.
Frequently asked questions about Cathode Rays
What is Cathode Rays?
Streams of electrons observed in discharge tubes, deflected by electric and magnetic fields.
Is Cathode Rays a beginner, intermediate or advanced topic?
Cathode Rays is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Cathode Rays?
Study these earlier topics in Atomic Structure & Periodicity first: Avogadro's Number, Molar Mass, Mass Spectrometry.
What comes after Cathode Rays?
The next topics in Atomic Structure & Periodicity are Millikan's Oil Drop Experiment and Electromagnetic Radiation.
Can I practise Cathode Rays with quizzes?
Yes. The interactive lesson includes a self-check question on Cathode Rays, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Cathode Rays fits in Atomic Structure & Periodicity
Cathode Rays is topic 30 of 115 in Atomic Structure & Periodicity.