Describe the discovery of the electron using a cathode ray tube. Draw a labelled diagram and give three properties of cathode rays.

1. 1 · Set up the tube. Two metal electrodes are sealed in a glass tube from which most of the air is removed, and joined to a high voltage.
2. 2 · Observe. Rays leave the cathode (−) and travel in straight lines to the anode (+): these are cathode rays.
3. 3 · Test with a field. Between charged plates, the rays bend towards the positive plate, so they carry negative charge.
4. 4 · Draw the tube. Glass tube, cathode (−), anode (+), high voltage source, straight beam, and field plates with the beam bending towards +.
5. 5 · Properties.
   • Travel in straight lines
   • Have mass and kinetic energy
   • Carry negative charge
   • Are the same for every gas and metal (same e/m)
6. 6 · Conclusion. J. J. Thomson (1897) named the particles electrons and concluded that they are present in all atoms.
7. 7 · In real life. Old television picture tubes worked on cathode rays.

Scoring tip: Label both electrodes with their signs; an unlabelled diagram loses most of its marks.