Important Questions and Answers: The Electrons

1. What is the use of X-rays and atomizer in Millikan’s oil drop experiment?

Ans: The use of X-rays in Millikan’s oil drop experiment is to charge the oil drop and the use of atomizer is to create smaller oil drop due to which the oil drop can attain its terminal velocity at smaller space.

2. Can we use water instead of oil in Millikan’s oil drop experiment?

Ans: No, we cannot use water instead of oil in Millikan’s oil drop experiment because water is a volatile liquid and can evaporate before it gains its terminal velocity and its terminal velocity cannot be measured.

3. Mention the significance of Millikan’s oil drop experiement?

From Milikan’s oil drop experiment:

• Quantum Nature of charge (Q =ne) was established.
• It helps to find the mass of an electron.
• It help to find the radius of electron.
• It showed that the electronic charge was the smallest possible charge in charged particles.

4. What property of cathode rays indicate that they consist of electrons?

Ans: The values of charge (e) and specific charge (e/m) for cathode rays is same as that of electrons. Hence the cathode rays must consist of electrons.

5. Cathode rays cannot be regarded as electromagnetic wave. Why?

Ans: Electromagnetic waves are charge less waves which cannot be deflected by electric or magnetic field. But cathode rays are negatively charged waves which are deflected by both electric and magnetic field. Hence cathode rays cannot be regarded as electromagnetic waves.

6. What is specific charge of an electron?

Ans: The ratio of charge and mass of electron is called specific charge of an electron.

We  know, mass of electron = 9.1 × 10-31 kg

Charge of electron = 1.6 × 10-19 C

$$\text{Specific charge} = \frac{\text{Charge of electron(e)}}{\text{Mass of electron(m)}}$$

$$\text{or, Specific charge} =\frac{1.6 \times 10^{-19}}{9.1\times 110^{-31}} = 1.76 \times 10^{11} \text{C Kg}^{-1}$$

So, the value of specific charge  of an electron is 1.76 × 1011 C Kg-1.

7. The value of e/m is constant for cathode rays but not for positive rays. Why??

Ans: Cathode rays are streams of electrons. The value of e/m  is always constant for electrons. But positive rays consists of different rays like gamma rays, beta rays, alpha rays etc. So, the value of e/m varies for positive rays.

8. Why is a metal piece heated up when cathode rays strike it?

Ans: Cathode rays are streams of electrons which possess kinetic energy due to their mass and motion. When this ray strikes with the metal piece the kinetic energy gets transferred to the molecules of the metal and increases their internal energy. As a result of this the temperature of the metal piece increasers.

9. Why is a magnetic field preferred over electric field to deflect electron beam in T.V.?

Ans: If we use electric field than we require a very high voltage and a very long tube to deflect he electrons. But if we use magnetic field than even a small field can produce large deflection and the required size of T.V is also reduced. Due to this reason a magnetic field is preferred over electric field to deflect electron beam in T.V.

10. Why lightning discharge takes place at high altitude but not at low altitude?

Ans: The lightning discharge is an example of electric discharge through gas at low pressure. The atmospheric pressure is low at high altitude and the electric discharge takes place there but it does not take place at low altitude where there is high pressure. Hence lightning discharge takes place at high altitude but not at low altitude.

11. Write down some uses of discharge tube:

Ans: Some uses of discharge tube are:

1. It is used in fluorescent tubes, flood lights, sodium lamps etc.
2. It is used in television, video cameras, etc
3. It is used in cathode ray oscilloscope.

12. The electric discharge stops at very low pressure. Why?

Ans: At very low pressure the concentration of gas molecules is very low such that there is no sufficient gas molecule to eject electrons from the cathode. Hence electric discharge cannot take place and stops at very low pressure.

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