Chemistry · Virtual lab
Thomson's Cathode Ray Tube Experiment
Deflect a cathode ray with electric and magnetic fields, balance the two forces, and measure the charge-to-mass ratio of the electron.
Background
In 1897 J. J. Thomson used a cathode ray tube to demonstrate that cathode rays are streams of negatively charged particles called electrons. By applying perpendicular electric and magnetic fields to the beam and adjusting them until the beam travelled in a straight line, he measured the charge-to-mass ratio (e/m) of the electron.
Thomson's experiment was one of the most important in the history of physics: it proved that atoms are not indivisible and led to the discovery of the first subatomic particle. The measured e/m ratio of approximately 1.76 × 10¹¹ C/kg showed that the electron is far lighter than any atom.
In this virtual lab you control the electric and magnetic field strengths, observe the deflection of the cathode ray, balance the two forces, and calculate e/m from your measurements.
What you will learn
- Understand the setup and operation of a cathode ray tube
- Apply electric and magnetic fields to deflect the beam
- Balance the two fields to achieve zero net deflection
- Calculate the charge-to-mass ratio of the electron
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- Millikan's Oil Drop: Catch a charged oil drop, weigh it by timing its fall, and find the charge of the electron as the smallest step every droplet is a multiple of.
- Rutherford's α-Particle Scattering: Fire α-particles at a thin gold foil, count where they scatter, and discover the tiny dense nucleus at the centre of the atom.