Discovery of Electron Cathode Rays - UNSOLVED PRACTICE SET

Q7. What are cathode rays? Name two properties of cathode rays that prove they are streams of fast-moving negatively charged particles.

Q8. Describe the experimental setup used to produce cathode rays. What conditions are necessary inside the discharge tube for cathode rays to be produced?

Q9. Your physics teacher demonstrates cathode rays using a discharge tube in a darkened classroom. The rays cause a fluorescent screen to glow green. She places a magnet near the tube and the rays bend. Explain why this happens and what it tells us about cathode rays.

Q10. How did J.J. Thomson determine the charge-to-mass ratio (e/m) of cathode rays? What was the significance of this discovery?

Q11. Why was the discovery of the electron considered revolutionary? How did it change our understanding of the atom?

Q12. Cathode rays can rotate a small paddle wheel placed in their path. What does this observation tell us about the nature of cathode rays?

Q13. With the help of a labelled diagram, describe the experiment that led to the discovery of cathode rays. Explain the observations made and the conclusions drawn. How did these experiments eventually lead to the discovery of the electron?

Q14. A student is confused about why cathode rays are called "cathode rays" and not "anode rays." Help the student by explaining:

(a) The direction of movement of cathode rays in a discharge tube.

(b) Why they originate from the cathode.

(c) What would happen if the electrodes were reversed.

(d) The difference between cathode rays and canal rays (anode rays).

Q15. "The discovery of the electron through cathode ray experiments was the first crack in the idea that atoms are indivisible." Discuss this statement by explaining: (i) what people believed about atoms before this discovery, (ii) how cathode ray experiments challenged this belief, (iii) what Thomson concluded from his e/m measurements, and (iv) how this discovery paved the way for modern atomic theory.

Q16. In a cathode ray experiment, the following data was recorded:

Voltage applied (V) = 2000 V

Distance between plates (d) = 2 cm

Deflection observed = 1.5 cm

Length of plates = 5 cm

(a) Calculate the electric field strength between the deflecting plates.

(b) If the charge of an electron is 1.6 × 10⁻¹⁹ C, calculate the force experienced by a single electron in this field.

(c) Explain how this deflection experiment helped Thomson determine that cathode rays are negatively charged.

(d) If the magnetic field required to nullify the electric deflection is 2 × 10⁻⁴ T, calculate the velocity of the cathode rays.

Q17. J.J. Thomson found that the charge-to-mass ratio (e/m) of cathode rays is approximately 1.76 × 10¹¹ C/kg.

(a) If the charge of an electron is 1.6 × 10⁻¹⁹ C, calculate the mass of an electron.

(b) Compare the mass of an electron with the mass of a hydrogen atom (1.67 × 10⁻²⁷ kg). How many times lighter is an electron than a hydrogen atom?

(c) Why was this comparison important in understanding the structure of the atom?

(d) If a beam of cathode rays contains 10¹⁵ electrons, calculate the total charge and total mass of the beam.

Q18. A television picture tube (CRT) used in older TV sets works on the principle of cathode rays.

(a) Explain how cathode rays are produced and directed in a CRT.

(b) If the accelerating voltage in a CRT is 10,000 V, calculate the kinetic energy gained by each electron in joules. (Charge of electron = 1.6 × 10⁻¹⁹ C)

(c) The screen of the CRT is coated with a fluorescent material. Why does it glow when struck by cathode rays?

(d) Modern LED TVs have replaced CRTs. Why do you think this happened, and what advantages do newer technologies have?