1. Discovery of the Electron
Unit D: Atomic Physics
Discovery of the Electron

2. Outcomes
You will explain how the discovery of cathode rays contributed to the development of atomic models
You will explain J. J. Thomson’s experiment and the significance of the results for both science and technology

3. *Background (Not Testable) Early Models of the Atom
Dalton’s Model: Billiard Ball Model
All matter consists of small, indivisible, indestructible particles called atoms.
All atoms of a given element are the same.
Each elements atomic mass is characteristic for that element. That is, it is different from the mass of any other element.
Atoms combine together to form compounds.
Atoms are neither created nor destroyed during a chemical reaction. They are only rearranged.

4. *Background (Not Testable) The Trouble with Dalton’s Model
The Dalton model has trouble explaining several questions:
How do atoms combine to form compounds - what holds molecules together?
How can the periodic table be explained – what causes the repetition of chemical properties?

5. Cathode ray Tubes
Several scientists noted the following characteristics of cathode rays:
Cathode rays travel in straight lines without interference of an electric or magnetic field
Rays travel from cathode to anode
Cathode rays can be considered to be moving particles
They can be deflected by magnetic fields
They can be deflected by electric fields
**The material the cathode is made of does not affect the properties of the cathode rays**

6. Thomson’s Cathode ray Experiments
Thomson showed that the particles that made up the cathode ray could be deflected by magnetic and electric fields
He used these experiments to determine the speed of the cathode ray

7. Thomson’s Cathode Experiment
Where the magnetic and electric forces were equal, the ray would travel in a straight line:

8. Sample Problem
A beam of electrons passes undeflected through a 0.50-T magnetic field combined with a 50-kN/C electric field. The electric field, the magnetic field, and the velocity of the electrons are all perpendicular to each other. How fast are the electrons travelling?

9. Thomson’s Charge to mass Ratio
When Thomson switched off the electric field so only the magnetic field was acting on the cathode rays, the rays moved in a circular path (centripetal force!)
Thomson equated the magnetic force to the centripetal force and solved for the charge to mass ratio of an electron

10. Charge-to-Mass Ratio
When a beam of electrons, accelerated to a speed of x 105 m/s, is directed perpendicular to a uniform µT magnetic field, they travel in a circular path with a radius of 3.37 cm (Figure 15.6). Determine the charge-to- mass ratio for an electron.

11. Why was Charge-to-Mass Ratio important?
**The charge to mass ratio of the electron demonstrated that atoms were divisible**
Since, the smaller the charge-to-mass ratio means a larger particle
From this, Thomson’s raisin bun or plum pudding model was created – he knew that atoms had a neutral charge and therefore hypothesized that electrons were embedded in a positively charged fluid.

12. Homework
Conceptual Q’s: p. 760 #3 Calculation Q’s: p. 760 #1, 4-7, 9, 10 *for 10b) choose arbitrary mass