# Chapter Unit - 6 : Modern Physics - Thermionic Emission and Cathode Ray Tube - Frank Solutions for Class 10 Physics ICSE

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## Chapter 6 - Modern Physics - Thermionic Emission and Cathode Ray Tube Excercise 271

Question 1
What do you mean by work function of a metal?
Solution 1
The minimum amount of energy required to emit electrons from a metal surface is called work function of that material.
Question 2
What are conduction electrons?
Solution 2
The electrons in the outer orbits are weakly attracted by the nucleus and are loosely bound. When a solid is formed, the loosely bound electrons leave their individual atom and become a part of the solid as a whole. They are called conduction electrons.
Question 3
Explain the term thermionic emission. Mention its one use.
Solution 3
when certain metals are heated to a high temperature, they emit thermions (electrons) and the phenomenon is called thermionic emission. Thermionic emission can be used to produce cathode rays oscilloscope.
Question 4
State two factors on which the rate of emission of electrons from a heated surface depends.
Solution 4
Rate of emission of electrons from a heated surface depends upon:
(i)  Work function of the material.
(ii)Melting point of the material.
Question 5
Why are materials of low work function preferred as electron emitters?
Solution 5
Materials of low work function are preferred as electron emitters because they emit electrons even at a low temperature.
Question 6
Why is the pressure inside the cathode ray tube kept very low?
Solution 6
Pressure inside a cathode ray tube should be kept very low because cathode rays are made up of electrons and mass of electrons are very low. If air exists inside the cathode ray tube then electrons would be deflected by air molecules and cathode would not be able to strike on screen properly.
So pressure inside cathode ray tube should be kept very low.
Question 7
Why is graphite coating made on the inner side of the cathode ray tube?
Solution 7
A layer of graphite is made on the inner side of the cathode ray tube close to the fluorescent screen. As a result of impacting electrons, the screen acquires a negative charge. To reduce this effect, the graphite is electrically connected to 'earth' (zero volts).This allows excess charge to drain away. Otherwise the accumulated charge would reduce the numbers of electrons arriving at the screen, reducing brightness.
Question 8
How can the brightness of the pattern on the screen of cathode ray tube be altered?
Solution 8
Brightness of the pattern on the screen of CRT can be controlled by varying the potential applied to the cathode.
Question 9
What are the three main components of a cathode ray tube? Explain, briefly the function of each part.
Solution 9
Cathode ray tube has three main sections

(i)  Electron gun : arrangement of electrodes where a stream of electrons is produced is known as electron gun
(ii)Deflection system: Stream of emerging electrons is deflected in its passage between the gun and the screen. This deflection is produced by two pairs of parallel plates arranged at right angles.
(iii)Fluorescent screen: At the end of the tube screen is coated with a mixture of fluorescent material and phosphorescent material. The phosphorescent material is responsible for the persistence of the image on the screen.
Question 10
In a hot cathode ray tube, what is the effect on the beam of particles if
(i) A hotter filament is used,
(ii) The anode voltage is increased?
Solution 10
(i)If hotter filament is used then no of electron would increase and a brightness of the spot on the screen would increase.
(ii)If anode voltage is increased then this would collimate the beam into a narrower, faster stream of electrons, producing a smaller, sharper dot on the screen.
Question 11
State important uses of a cathode ray tube.
Solution 11
(i) Cathode ray tubes are used in science research laboratories by scientist for converting electrical signals into visual signals and television tubes.
(ii) Cathode ray tubes are used by doctors for converting electrical impulses corresponding to heart beats into visual signals in ECG, EEG etc.
Question 12
What is the function of fluorescent screen in the cathode ray tube?
Solution 12
The end of the tube screen is coated with a mixture of fluorescent material and phosphorescent material. The phosphorescent material is responsible for the persistence of the image on the screen. While fluorescence material coated inside screen is responsible for production of bright spot on the screen when electron beam strikes on the screen.
Question 13
How does the deflection of a beam of electrons by an electric field differ from the deflection by a magnetic field? Explain with proper diagrams.

Solution 13

In presence of electric field, beam of electrons experience a force and deflected towards the positive plate. The path is parabolic within the electric field and after electric field the beam of electron travels straight.

In presence of magnetic field, the electrons experience a force, the direction of which is given by Fleming left hand rule. The beam of electrons gets deflected and follows a circular path. The magnetic field is perpendicular to the plane of paper inwards and is represented X. On emergence from the magnetic field, they travel along a straight path in opposite direction to their velocity.

Solution 1
Solution 2

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