Cathode Ray Experiments

This topic is part of the HSC Physics course under the section Structure of The Atom.

HSC Physics Syllabus

  • investigate, assess and model the experimental evidence supporting the existence and properties of the electron, including:

– early experiments examining the nature of cathode rays
– Thomson’s charge-to-mass experiment
– Millikan's oil drop experiment (ACSPH026) 

Cathode Ray Experiments

The discovery of the electron started with observations of various experiments involving cathode ray tubes.

This video discusses the observations and conclusions of a few important cathode ray experiments including:

  • use of maltese cross
  • use of electric and magnetic fields
  • use of paddle wheel

 

 

What are Cathode Rays?

Cathode rays were produced in partially evacuated discharged tubes called Crookes tubes (cathode ray tubes).

 

Gas discharge tube

A simple representation of a gas discharge tube

 

Similar to a discharge tube, a cathode ray tube consists of two electrodes connected to a high potential difference. The positive electrode is the anode, and the negative electrode is the cathode. The high potential difference causes electrons to move from the anode to the cathode. In a near-vacuum setting where there are few to none air molecules, these electrons can travel unimpeded. 

 

Cathode ray tube

A simple representation of a cathode ray tube 

 

A cathode ray was the name given to the observation of these electrons when they were incident on the glass coated with fluorescent material behind the cathode. 

    It is important to keep in mind that at the time when cathode rays were observed, scientists were not aware of the existence of electrons. 

    Particle vs Wave Nature of Cathodes

    After cathode ray was demonstrated, further experiments were performed to investigate its nature and identity. These experiments produced conflicting conclusions on the nature of cathode rays: some suggested that cathode rays were waves and others suggested that they were particles. 

     

    Charged Particle Properties

    Electromagnetic Wave Properties

    Can be deflected by magnetic fields

    They are identical, regardless of material used

    Can be deflected by electric field

    It emanates from the cathode and travels in a straight line

    The rays carry energy and momentum

    Would cast a shadow of a solid object

    Are attracted to positive charges

    Could penetrate thin metal foils

    Maltese Cross Experiment

    Maltese cross experiment

     

    • What was done: An anode in the shape of a maltese cross was placed in the path of the cathode ray. 

      

       

      • Observation: a shadow of the maltese cross was formed directly behind the anode. 
      • Conclusion: Cathode rays travel in a straight line and can cast a shadow. Some scientists argued that since waves such as light can produce a similar observation, cathode rays are wave in nature. However, this observation could also be produced by particles.  

      Cathode Ray Tubes Containing Electric and Magnetic Fields

        cathode ray tube containing a plate coated with fluorescent material

       

      • What was done: a metal plate coated with fluorescent material was used to visualise the trajectory of a cathode ray. The cathode ray was passed through a uniform electric field and magnetic field (in separate experiments).

       


         

       

        • Observation: in the presence of an electric field, the path of the cathode ray was deflected towards the positively charged plate. In the presence of a magnetic field, the path of the cathode ray was deflected in a direction that was consisted with a negatively charged mass.
        • Conclusion: cathode rays are streams of negatively charged particles. 

        Paddle Wheel Experiment

         

        • What was done: a glass paddle wheel that could move and rotate freely was placed in the path of the cathode ray.

         

          

        Cathode ray tube with paddle wheel

         

        • Observation: when the glass paddle wheel was struck by the cathode ray, it rotated and moved towards the cathode.
        • Conclusion: cathode rays have momentum and kinetic energy. Therefore, they have mass and are particles in nature.

        Are Cathode Rays Waves or Particles?

        Thomson's determination of the charge to mass ratio of cathode rays settled the debate of the nature of cathode rays. Thomson demonstrated that cathode rays are particles in nature. 

         

        Previous Section: Energy Sources of Stars

        Next Section: Thomson's Discovery of The Electron

         

        BACK TO MODULE 8: FROM THE UNIVERSE TO THE ATOM