Producing and Using Stream of Electrons 1. (a) What do you understand by the following terms : (i) Free electrons, (ii) Thermions? (b) State one way by which thermions differ from free electrons. Ans.(a) (i) Free electrons : The electrons, revolving in the outermost orbit of an atom, which are weakly held by the nucleus are called free electrons. (ii) Thermions : The electrons emitted from the surface of a metal, when heat energy is supplied to the metal are called thermions. (b) Whereas free electrons are free in a sense that they can move in the metals in any direction, the thermions completely break loose from the metal surface due to the absorption of heat energy. 2. What do you understand by the following terms? (i) Space charge (iii) Electron volt (ii) Threshold energy or work function (iv) Threshold temperature. Ans. (i) Space charge : The space around cathode which is filled with free thermions is called space charge. (ii) Threshold energy or work function : The minimum heat energy required to make a metal emit an electron (thermion), from its surface is called threshold energy or work function. 1
(iii) Electron volt (ev) : It is the smallest unit of work and is generally used to measure threshold energy. It is defined as the amount of work done, when a charge of 1.6 10 19 coulombs (charge of one electron), moves through two points in an electric field, at a potential difference of one volt. 1 ev = 1.6 10 19 C V = 1.6 l0 19 J. (iv) Threshold temperature : The minimum temperature at which a particular material emits thermions from its surface on heating is called threshold temperature. 3. (a) Can a metal emit thermions at all temperatures? Explain your answer. (b) Calculate the value of 500 electron volts in joules. Ans. (a) No. A metal cannot emit thermions at all temperatures. The thermions are emitted by a metal only when its temperature is higher than the threshold temperature. (b) 1eV = 1.6 l0 19 C 1 V = 1.6 l0 19 J 500 ev= 500 1.6 10 19 J = 8.0 10 17J. 4. State three factors which determine the rate of emission of thermions. Ans. Factors determining the rate of emission of thermions : 1. It is inversely proportional to the work function of a material. 2. It is directly proportional to the temperature of the surface emitting thermions. 3. It is directly proportional to the surface area of the surface emitting thermions. 2
5.(a) State two characteristics of a thermion emitter. (b) Name three commonly used thermion emitters and state their work function in electron volts. (c) Why are materials of low work function preferred as electron emitter. Ans. (a) Characteristics of material acting as thermion emitter. 1. It must have low work function. 2. It must have high melting point. (b) (i) Tungsten (work function is 4.52 ev). (ii) Thoriated tungsten (work function is 2.6 ev). (iii) Tungsten coated with oxides of alkali metals, (work function is less than 1 ev.) (c) The materials with low work function emit thermions at lower temperature. If the temperature of these materials is increased then large number of thermions are evolved. 6. (a)what do you understand by the following terms : (i) Directly heated thermion emitter? (ii) Indirectly heated thermion emitter? (b) Which of the above thermion emitter is superior and why? Ans.(a) (i) Directly heated thermion emitter : A thermion emitter, which gets heated on the passage of electric current and when its temperature becomes more than the threshold temperature, starts emitting electrons, is called directly heated thermion emitter. 3
(ii) Indirectly heated thermion emitter : A material, which itself does not get heated upon the passage of electric current, but when heated by some other source emits thermions is called indirectly heated thermion emitter. (b) The indirectly heated thermion emitter is better because: (i) It emits thermions at a far lower temperature on account of its lesser work function. (ii) It emits large number of thermions when heated beyond the threshold temperature. 7. Why is the rate of thermionic emission less in an evacuated bulb, as compared to one, which is filled with inert gas at low pressure? Or Why is the pressure inside the cathode ray tube is kept very low? Ans.Vacuum is the poorest conductor of electricity. Thus, few electrons can migrate from cathode to anode. However, if the bulb is filled with inert gas at low pressure, the rate of thermionic emission increases because the inert gas ionises. 8. (a) What do you understand by the term valve, as applied to electronic circuits? (b) Name the material of cathode and anode of : (i) Directly heated diode, (ii) Indirectly heated diode. (c)why is the indirectly heated diode valve considered superior to directly heated diode valve? Ans. (a) An electronic device, which allows the electrons to move in one direction only, and stops them, if made to flow from other direction is called a valve. 4
(b) Kind of diode Material of cathode Material of anode (i) Directly heated diode Tungsten Copper or aluminium (ii) Indirectly heated diode Tungsten coated with oxides of alkali metals Copper or aluminium. (c) (i) It supplies electrons at a far lower temperature than directly heated diode. (ii) It supplies more number of electrons when its cathode is heated beyond the threshold temperature. 9. (a) (i) What is thermionic emission? (ii) Name two factors on which the rate of emission depends. (b) Why is diode called valve? Ans. (a)(i) Thermionic emission : The phenomenon due to which free surface of the metal emits electrons, on being heated is called thermionic emission. (ii) Factors determining the rate of emission of thermions : 1. It is inversely proportional to the work function of a material. 2. It is directly proportional to the temperature of the surface emitting thermions. 3. It is directly proportional to the surface area of the surface emitting thermions. (b) As diode allows the thermions to flow from cathode to plate (anode) in external circuit but not from plate to cathode, therefore it acts as an electronic valve. 5
10. Diagram below, shows a cathode ray tube circuit. Answer the following questions : (a) (i) What is F? (ii) Why must F be heated? (b) (i) What is A1 & A2? (ii) Why must A1 & A2 be maintained at high potential relative to F? (iii) Why is there a hole in A1 & A2? (c) (i) What is C? (ii) What is the material coated on C? (d) (i) Name P. Name two materials, which can be coated on P. (ii) How do above materials help in the formation of image? (e)what is R? What is the function of R? (f) Name Q. What is the function of Q? (g)what is G? What is the function of G? (h) (i) How can the production of thermions be increased? (ii) In what way will this increase affect P? (iii) What is the effect on P if the e.m.f. of H.T. is increased? (i) How does the tiny spot produced on screen move when : 6
(i) only R is connected to alternating current? (ii) only Q is connected to alternating current? (iii) Q and R simultaneously are connected to alternating current? (j) State two uses of cathode ray tube. Ans.(a)(i) F is filament of heater. (ii) It is heated to generate thermions in cathode. (b)(i) A1 and A2 are anodes. (ii) When A1 & A2 are at higher potential relative to anode, it attracts thermions from cathode. (iii) The hole concentrates the beam of electrons drifting from cathode to anode. (c) (i) C is cathode plate. (ii) Oxides of alkali metals are coated on the cathode plate. (d) (i) P is the screen. Materials coated on screen are: (i) Zinc sulphide, (ii) Barium platinocyanide. (ii) When the electrons strike on the materials named above, they show fluorescence. The amount of brightness produced will depend upon the intensity of electrons. (e) R is a pair of deflecting plates commonly called vertical plates. They help in deflecting electron beam in horizontal plane when connected to high voltage alternating source. (f) Q are horizontal plates. They deflect the electron beam in vertical plane when connected to high voltage alternating source. 7
(g) G is grid. The electrical signals are fed into the G, which are then converted into visual signals as electron beam strikes the screen. (h) (i) The production of thermions can be increased by passing more current through F. (ii) It will increase the brightness on screen. (iii) It will increase the contrast on the screen. (i) (i) The tiny luminous spot moves in the horizontal direction. (ii) The tiny luminous spot moves in the vertical direction. (iii) The tiny spot moves diagonally. (j) (i) It is used as an oscilloscope. (ii) It is used as a television screen. 11.(a) Why is graphite coating made on the inner side of the cathode ray tube? (b) How can the brightness of the pattern on the screen of cathode ray tube be changed? (c) How is the cathode ray tube used to convert an electrical signal into a visual signal. Ans. (a) The graphite coating on the inner side of cathode ray tube is earthed. This shields the electron beam from any external electric field. It also increases the life of cathode ray tube. (b) The brightness of screen depends upon the number of electrons striking the screen per unit time. The brightness of screen can be increased by increasing the current in L.T. circuit as it generates more electrons. Conversely, by decreasing current in L.T. circuit the brightness decreases. 8
(c) When a varying electric signal is fed in the grid of cathode ray tube, it either increases or decreases the thermions travelling towards screen. At some points of the screen becomes bright and at some points dark, depending upon the intensity of electrical signal. This in turn appears as black and white visual signal. 9