Ans : Before shaking a tree, the tree and its leaves are both at rest. When the tree is shaken, the lower portion of the tree comes in motion put its upper part and the leaves tend to remain at rest due to inert is of rest. Consequently, the leaves fall from the free.

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Ans: For an electron revolving around the positively charged nucleus, the required centripetal force is provided by the force of attraction which is along the tangent at any point of the circular path. The centripetal force, which is directed radially towards the centre of the circular path, has no effect on the tangential velocity. That's why, the electron continues to move around the circular path and it doesn't collapse into the nucleus.

2.       

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Electrons and Photons

7. Which metals are suitable for photoelectric effect? Why?

Ans: The metals (generally alkali metals) which have low work function are suitable for photoelectric ef

fect. As work function of a metal is the minimum amount of energy required to eject an electron from the surface of the metal (without imparting any kinetic energy), so electrons can be ejected from such metals even by less energetic radiation (i.e. visible light).

8. Why does electric discharge take place at low pressure and high potential difference?

Ans: At low pressure and high potential difference, mean free path for the charged particles increased and they acquire sufficient kinetic energy to produce ionization. More ions and electrons are produced in the gas due to collisions. The electrons move towards the anode and the positive ions move towards the cathode. Hence, electric discharge takes place through the gas.

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Electrons and Photons

1. What are work function and threshold frequency in photoelectric effect?

Ans: Threshold frequency: It is the minimum frequency of the incident light on the surface of a metal so that electrons are just ejected from the metal surface without acceleration. It is constant for a given metal but it differs metal to metal.

Work Function: The minimum amount of energy required to eject an electron just from the surface of a metal is called the work function of the metal. It is constant for a given metal but it differs metal to metal.

2. What are cathode ray?

Ans: Cathode rays are a beam of fast moving electrons. They are not actually rays but they are particles (electrons). They are deflected in an electric field and magnetic field. Its speed is less than that of light in air and hence cathode rays aren't electromagnetic wave.

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Electrons and Photons 

1. Discuss the physical principles Invovoled in Milikan's experiment for the determination of the charge of an electron.

Ans: The physical principles involved in Millikan's experiment for the determination of the charge of an electron are as follows:

i) Oil drops get ionized due to friction.

ii) A charged particle experienced force in an electric field.

2. Cathode rays cannot be regarded as electromagnetic waves. Why?

Ans: Cathode rays cannot be regarded as electromagnetic waves because of all the elements. Cathode rays are defected by electric and magnetic fields but electromagnetic waves do not. Also, cathode rays do not show the nature of electromagnetic waves. Hence, they cannot be regarded as electromagnetic waves.

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Electrons and Photons- Short Question-Answers

1. What is threshold frequency?

Ans. There is a certain minimum frequency of radiation below which there is no photoelectrice emission whatever be the intensity of radiation. This minimum frequency is called the should frequency. Different metals have different threshold frequencies. Illumination of a surface with light of frequency less than threshold frequency will not cause photoelectric emission.

2. Explain why photoelectric effect cannot be observed with all wavelengths of light.

Ans: Each metal surface has a work function (W-=hf⁰) which is the minimum energy required to eject an electron from an atom of its surface. So, the light photo must have at least energy which is equal to work function of the metal in order to eject an electron from its surface. If the energy of the incident photon is less than the work function of the metal then there will not be ejection of any electron. Since the energy of photo in only frequency- dependent (E=hf), only a particular wavelength of light can eject electron. So photoelectric effect is not observed with all wavelength of light.

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