Unit 2bb tủ tài liệu training

VERY SHORT AND SHORT-ANSWERS QUESTIONS 46 Define neutral temperature and temperature of inversion 47 Thermo-emf is given by the expression Ε = αθ + βθ 2 Write the expression for thermoelectric power (Seebeck coefficient) (AISSCE Delhi 1990 C) 48 49 50 51 52 53 54 What is Peltier effect? Define Peltier coefficient What is Thomson effect ? What is a Thermopile ? Mention some applications of thermoelectric effect Which has greater resistance – a kW heater or a 100 W bulb ? Name the carriers of current in the following voltameters: (a) Copper electrodes in CuSO4 solution (b) Platinum electrodes in dilute sulphuric acid 55 Write the expression which gives the relation of the thermoelectric emf of a thermocouple with the temperature difference of its cold and hot junctions (AISSCE 1993, 95) 56 Give one practical application of thermoelectricity (AISSCE Delhi 1991) 57 Why is an electrolyte dissociated when dissolved in liquids ? S Chand & Company Limited 58 How is the electrical conductivity of an electrolyte affected by increase of temperature ? (AISSCE Delhi 1995) 59 Write one main difference between primary and secondary cells 60 What is the direction of the thermoelectric current at the hot junction of an iron-copper thermocouple ? (AISSCE Delhi 1990 C, 91) 61 A small heating element connected to a 10 V d.c supply draws a current of A How much electric power is supplied to the heater ? (AISSCE Delhi 1991 C) 62 A heating element connected to a 30 V d.c supply draws a current of 10 A How much heat is produced per second ? (AISSCE Delhi 1990 C) 63 What is the relation between temperature of cold junction, neutral temperature and inversion temperature ? 64 For a thermocouple the temperature of the cold junction is 0°C and the inversion temperature is 510°C Calculate the neutral temperature 65 A lamp of 100 W works at 250 V What are its resistance and current capacity ? 66 Calculate the current carrying capacity of a bulb rated 40 W, 120 V 67 Name the thermocouple which is used to measure a temperature of 300 K 68 How are metals purified by electrolysis process ? 69 Describe briefly a lead-acid accumulator, giving its charging and discharging chemical equations 70 Plot a graph showing the variation of thermoelectric power with temperature difference between the hot and the cold junctions 71 Which one has lower internal resistance – a secondary cell or a primary cell ? 72 Seebeck effect is reversible What does it mean ? S Chand & Company Limited 73 What are the units in which the thermoelectric coefficients α and β are generally expressed ? 74 At room temperature, what is the order of the ratio of the conductivity of an electrolyte to that of a conductor ? 75 Name a liquid which allows current through it but does not dissociate into ions 76 On what factors does the magnitude of thermo-emf depend ? 77 Derive the relation between Faraday constant and Avogadro number 78 Are all pure liquids bad conductors of electricity ? 79 Define International Ampere 80 State Faraday’s laws of electrolysis Derive the relation connecting chemical equivalent and electrochemical equivalent of an element (AISSCE 1996) 81 Define chemical equivalent and electrochemcial equivalent of a substance Establish the relation between the two (AISSCE 1997) 82 How many units of energy does a 1500 W bulb, used for two hours, consume ? 83 When does a source of emf deliver maximum power ? 84 What is the relation between Peltier coefficient and Seebeck coefficient ? 85 Why some covalent salts (which are not ionic in solid state) become conducting when dissolved in water ? 86 Two bulbs of 50 W and 30 W, both rated at 220 V, are connected in series across a 220 V mains Will they work correctly ? 87 With the help of a suitable diagram, explain the construction and working of an alkali accumulator 88 What is a button cell ? Write its main components and reactions taking place at anode and cathode ? S Chand & Company Limited 89 Which has higher internal resistance—lead accumulator or alkali accumulator ? 90 What are the advantages of alkali accumulator over lead accumulator ? 91 State Faraday’s laws of electrolysis Write down the relation connecting chemical equivalent and electrochemical equivalent (AISSCE Delhi 1999) 92 State the condition in which terminal voltage across a secondary cell is equal to its e.m.f (AISSCE Delhi 2000) 93 Draw the graph showing the variation of thermo - emf of a thermocouple with the temperature difference of its junctions How does its neutral temperature vary with the temperature of the cold junction ? (AISSCE Delhi - 2000) ANSWERS 46 If the temperature of the hot junction of a thermocouple is gradually increased, the thermo-emf first increases and attains a maximum value This temperature is called neutral temperature (θn) If the temperature is further increased, the emf decreases to become zero again and then it changes direction The temperature at which the thermo-emf changes direction is called temθi) perature of inversion (θ 47 E = αθ + βθ dE Thermo electric power S = = α + βθ dθ 48 If a current is passed through a junction of two dissimilar metals, heat is either absorbed or evolved at the junction On reversing the direction of current, the heating effect is also reversed This phenomenon is called Peltier effect 49 Peltier coefficient is defined as the amount of heat absorbed or evolved per second at a junction when a current of A is passed through it S Chand & Company Limited 50 The production of an electric potential gradient along a conductor as a result of a temperature gradient along it is called Thomson effect Thus points at different temperatures in a conductor are at different potentials 51 Thermopile is a series combination of thermocouples It is used to detect and measure the intensity of heat radiation 52 Some of the important applications of thermoelectric effect are: (a) Power generation (b) Measurement of temperature (c) Refrigeration 53 100 W bulb R= V P ⇒ R∝ P –– 54 (a) Cu and SO4 ions ++ (b) H 55 56 57 58 59 + and OH – ions E = αθ + βθ , where α and β are constants Measurement of temperature The ionic bonds between the ions of the solute are made weak by polar molecules of liquids Therefore the ions of electrolyte (solute) get dissociated The electrical conductivity of an electrolytes increases with the increase in temperature The primary cells can not be recharged while in secondary cells reversible reactions take place so that they can be recharged S Chand & Company Limited 60 From copper to iron 61 Electric Power P = VI = 10 × = 50 W 62 Heat produced per second = VI = 30 × 10 = 300 W 63 θn = θi + θ c , θn is neutral temperature, where θc is temperature of cold junction, and θi is inversion temperature 64 θi =510 C, θc=0° C ° θn = θi + θ c 65 P = 100 W V = 250 V R= V p = = + 510 250 × 250 100 Current capacity I = = 255°C = 625 ohm P V = 100 250 = 0.4A S Chand & Company Limited 66 I= P V = 40 120 = (1/3) A 67 Copper-Constatantan thermocouple 68 For purification of metals by electrolysis, anode is made of the impure metal and cathode of pure metal The electrolyte used is any soluble salt of pure metal When current is passed through the electrolyte, pure metal gets deposited on the cathode 69 Lead-acid accumulator is a secondary cell which can be recharged by passing a current through it in the reverse direction The chemical process that occurred at the electrodes are then reversed and the cell recovers its original state The electrodes consist of alternating parallel plates of lead dioxide (positive electrode) and spongy lead (negative electrode) insulated from each other They are immersed in an electrolyte of dilute sulphuric acid Reactions: H2SO4 dissociates into H + –– ions and SO4 During discharging: –– → P bS O + 2e – At Cathode: Pb + SO At anode: PbO + 2H + 2e + – → PbO + H 2O PbO + H SO → PbSO + H O During charging: At cathode: + – PbSO + 2H + 2e → Pb + H SO At anode PbSO + SO –– + 2H O → PbO + 2H SO + 2e S Chand & Company Limited – 70 Thermo electric power S is given by S = α + βθ The graph is shown in fig B.5 71 Secondary cell 72 It means that if the hot and the cold junctions are interchanged, the emf changes sign and the circulating current reverses direction 73 α: µ V/°C β : µ V/°C2 B S A α 74 10– to 10– Temperature Difference (θ) 75 Mercury Fig B.5 76 The magnitude of thermo-emf depends on two factors: (a) Nature of the metals forming the thermo couple (b) Temperature difference between the two junctions 77 Faraday constant (F) is the amount of charge required to liberate equivalent weight of a substance by electrolysis So the amount of charge required to liberate mole of the substance is Fp where p is valency of the substance Fp The charge required to liberate one atom of substance is, therefore N Now, the charge on each ion is pe, where e is electronic charge Thus Fp = pe N or F = Ne 78 No For example, mercury is a good conductor S Chand & Company Limited 79 International ampere is defined as the steady current, which when passed through a silver voltameter, deposits 0.001118 g of silver in one second on the cathode 80 For Faraday’s laws see Q No 12 From Faraday’s first law of electrolysis, we have m = zit, where z is the electrochemical equivalent (ece) Now we consider two substances having chemical equivalents E1 and E2 When the same quantity of charge is passed through the electrolytes containing them, let the masses of the two substances liberated be m1 and m2 respectively Then, from Faraday’s second law E1 E2 = m1 m2 But m1=z1 it and m2 = z2 it where z1 and z2 are the ece’s of the two substances So, E1 E2 or E1 E2 = = m1 m2 = z1 it z it z1 z2 or E/z= Constant The constant is denoted by F and is called Faraday constant: E/z = F 81 The electrochemical equivalent of a substance is defined as the mass of the substance deposited on any one of the electrodes when one coulomb of charge passes through the electrolyte The chemical equivalent of a substance is defined as the ratio of atomic weight to the valency For relation – see question 80 S Chand & Company Limited 82 1500W = 83 84 85 86 87 1500 = 1.5 kW 100 Energy consumed in two hours = × 1.5 = 3.0 kWh Power delivered is maximum when the external resistance is equal to the internal resistance of the source π = TS where π is the peltier coefficient, S is the Seebeck coefficient and T is the temperature of the cold junction The dielectric constant of water is large (81) It weakens the attraction between the atoms of covalent salts In some cases, the salts ionise and conduct electricity Since the bulbs are connected in series, the applied voltage of 220 V gets divided in the ratio of their resistances Thus each bulb gets less than 220 V, (voltage for which bulbs are rated) So the bulbs will not be as bright as they should be Further, the 30 W bulb will be brighter than the 50 W bulb as the former has higher resistance Alkali Accumulator: This accumulator consists of a steel vessel containing a 20% solution of KOH with 1% LiOH LiOH makes it conducting Perforated steel grid is used as anode The anode is stuffed with nickel hydroxide Another perforated steel grid stuffed with finely divided iron hydroxide is used as cathode To lower the internal resistance traces of mercury oxide are used in it Working:– Potassium hydroxide dissociates as (a) 2KOH → 2K + + 2OH – During charging, anode is connected to the positive terminal and cathode to the negative terminal of a d.c source, Inside the cell the current flows from anode to cathode Hydroxyl ions are attracted towards anode where they lose their charge and form nickel peroxide: S Chand & Company Limited – + Steel vessel Ni (OH)2 KOH + LiOH Fe (OH)2 Perforated steel container Fig B.6 (b) Ni(OH) + 2OH → Ni (OH) The positive ions move towards cathode and then react with Fe(OH)2 to form iron: (c) Fe(OH) + 2K → Fe + 2KOH The complete reaction during charging is given by adding (a) (b) & (c): Ni (OH) + Fe(OH) → Ni(OH) + Fe During discharging, the current flows from cathode to anode inside the cell and reaction is given by Ni (OH) + Fe → Ni (OH) + Fe (OH) S Chand & Company Limited 88 Button Cell:The button cell is a solid state miniature dry cell, which is widely used in electronic watches, cameras etc It is also known as silver oxide zinc cell Some other button cells are mercury cell, Lithium cell, Alkaline cell Anode cap Main parts of a button cell are (a) Anode can Gasket Cathode Anode Cell can Separator (b) Cathode can (c) Separator (d) Gasket Silver Oxide zinc cell is shown in fig B.7 The anode is amalgamated zinc powder with gelatinised KOH electrolyte Cathode is of silver oxide and it is separated by an absorbent cellulosic material Reactions taking place inside the cell are: 89 90 91 92 93 At Cathode : Ag O + H O + 2e At anode : Zn + 2OH – – → 2Ag + 2OH – → ZnO + H O + 2e – The emf of the cell is 1.60 V It has a high energy output per unit weight and a constant voltage level Alkali accumulater has higher internal resistance (i) Alkali accumulator is not damaged if it is not charged for a long time (ii) Excess charging or discharging not damage it See Q 80 In open circuit, i.e., when no current flows through the cell Graph - See Q 44 Neutral temperature is independent of the temperature of the cold junction S Chand & Company Limited ... ? 72 Seebeck effect is reversible What does it mean ? S Chand & Company Limited 73 What are the units in which the thermoelectric coefficients α and β are generally expressed ? 74 At room temperature,... electrochemcial equivalent of a substance Establish the relation between the two (AISSCE 1997) 82 How many units of energy does a 1500 W bulb, used for two hours, consume ? 83 When does a source of emf deliver... → 2Ag + 2OH – → ZnO + H O + 2e – The emf of the cell is 1.60 V It has a high energy output per unit weight and a constant voltage level Alkali accumulater has higher internal resistance (i) Alkali