1. Discovery of the Electron
Atomic Physics 1

2. 1. Discovery of the Electron
Scientists wanted to know if atoms were made up of even smaller particles
When a voltage source was supplied to a evacuated tube, electrons moved from the cathode to anode and gave off a distinctive green glow which were called cathode rays
The green glow in the cathode tube (CRT) , similar to the vacuum tube of a television, was a flow of electrons The electrons could not be seen themselves but their interaction with the gas in the tube caused the electrons to emit light – we will learn more with the Bohr model of the atom

3. 2. Properties of Cathode Rays
Beams of electrons (have (-) charge)
Produced by the negative cathode in an evacuated tube and travel toward the anode
Travel in straight lines (cast sharp shadows) and are emitted perpendicular to the cathode surface
Have energy and can do work
Are deflected by electric and magnetic fields

4. Produce similar chemical reactions to that of light
Eg) exposes photographic film, creates similar reactions in plants( photosynthesis)

5. The Guy Behind It All The mass of the electron was measured indirectly
J.J. Thompson (1897) determined the charge to mass ratio of (q/m) of the electron
Thompson passed cathode rays into an electric field which was perpendicular to a magnetic field to determine the speed of the electrons

6. Cathode Ray Tube

7. When Fields Were Applied
Case 1 No deflection
Forces must be balanced.
Enabled scientists to find the speed of the cathode rays.

8. Case 2 Magnetic Field only
An unbalanced force produces circular motion.

9. Charge to Mass Ratio The ratio of q/m was always the same value q/m =
q was the charge on a single electron determined by Robert Millikan’s oil drop experiment

10. Examples
1. In a Cathode ray tube an electron travels through an electrical field of 5.80 x 103 N/C and a magnetic field of 6.60 x 10-3 T undeflected.
a. what is the velocity of the electron?

11. b. If the electric field is turned off the magnetic field causes the electron to travel in a circular path, what is the radius of the circular path of the electron?

12. 2. An electron travels through a CRT and a magnetic field of 4
2. An electron travels through a CRT and a magnetic field of 4.20 x 10-3 T, the electron travels a circular path with a radius of 4.90 x 10-3 m.
a. What is the velocity of the electron? 

13. b. What was the potential difference in the CRT?

14. 3. An unknown particle travels in a CRT tube through a magnetic field of 4.60 x 10-2 T at a velocity of 5.20 x 105 m/s, the magnetic field causes the particle to travel in a circular path with a radius of m.
What is the charge to mass ratio of this particle?