Sunday, 24 June 2018

CBSE Class 12 - Physics - CH: Atoms and Nuclei - Cathode Rays and Its Properties (#cbsenotes)(#class12Physics)(#eduvictors)

Cathode Rays and Its Properties

CBSE Class 12 - Physics - CH: Atoms and Nuclei - Cathode Rays and Its Properties (#cbsenotes)(#class12Physics)(#eduvictors)
Image Credits: Wikipedia, Author: Theresa Knott

Q1: What are Cathode Rays?

Answer: are streams of electrons observed in vacuum tubes. These rays were discoevered by a German physicist Johann Wilhelm Hittorf, and were named in 1876 by E. Goldstein, or cathode rays. In 1897, British physicist J. J. Thomson experimented and showed that cathode rays were composed of negatively charged particles, called electrons. Cathode rays are also called as electrons beam.

When a potential difference of 10 to 15 kV is applied across the two electrodes of a discharge tube and pressure is reduced to 0.01mm of mercury, the rays known as cathode rays are emitted from the cathode. These rays are independent of the nature of the gas in the discharge tube and their direction of propagation is not affected by the position of the anode.

Q2: What are the properties of Cathode Rays?

Cathode rays have the following properties:

1. Cathode rays travel along straight lines and cast sharp shadows of the objects placed in their path.

2. Cathode rays are shot out normally from the surface of the cathode.

3. The direction of the cathode rays is not affected by the position of the anode.

4. The cathode rays exert mechanical pressure.

5. The cathode rays produce heat, when they fall upon matter.

6. The cathode rays are deflected by electric and magnetic fields.

7. When cathode rays strike a solid target of high atomic weight such as tungsten, they produce a highly penetrating radiation called the X-rays.

8. Cathode rays ionise the gas through which they pass.

9. Cathode rays can excite fluorescence.

10. Cathode rays can produce chemical changes.

11. Cathode rays can penetrate through thin sheets of matter without puncturing them.

12. Cathode rays are found to have velocity upto one tenth of the velocity of light.

Q3: In his experiments on “cathode rays” during which he discovered the electron, J. J. Thomson showed that the same beam deflections resulted with tubes having cathodes made of different materials and containing various gases before evacuation. 

(a) Are these observations important? Explain your answer. 

(b) When he applied various potential differences to the deflection plates and turned on the magnetic coils, alone or in combination with the deflection plates, Thomson observed that the fluorescent screen continued to show a single small glowing patch. Argue whether his observation is important. 

(a) Yes the observations are important, it shows that electron beam is present in all kinds of atoms.

(b) Yes everything in the electrons beam has single charge-to-mass ratio.

Q4: What is a mass spectrometer?

Answer: A mass spectrometer separates ions according to their mass-to-charge ratio.