Write the expression which gives the relation of the thermoelectric emf of a thermocouple with the temperature difference of its cold and hot junctions.. What is the relation between tem
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VERY SHORT AND SHORT-ANSWERS QUESTIONS
46 Define neutral temperature and temperature of inversion.
47. Thermo-emf is given by the expression
2
1
= αθ + βθ
2
Ε
Write the expression for thermoelectric power (Seebeck coefficient)
(AISSCE Delhi 1990 C)
48. What is Peltier effect?
49. Define Peltier coefficient
50. What is Thomson effect ?
51. What is a Thermopile ?
52. Mention some applications of thermoelectric effect
53. Which has greater resistance – a 1 kW heater or a 100 W bulb ?
54. 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 ?
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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
61. A small heating element connected to a 10 V d.c supply draws a current of 5 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
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 equa-tions
70. Plot a graph showing the variation of thermoelectric power with temperature difference be-tween 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 ?
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73. What are the units in which the thermoelectric coefficients ααα and βββββ 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
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 do 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 accumu-lator
88. What is a button cell ? Write its main components and reactions taking place at anode and cathode ?
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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
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
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
tem-perature of inversion (θθθθθi).
= αθ + βθ
2
E
Thermo electric power d
d
E
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
re-versed This phenomenon is called Peltier effect.
49 Peltier coefficient is defined as the amount of heat absorbed or evolved per second at a
junc-tion when a current of 1 A is passed through it
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50. The production of an electric potential gradient along a conductor as a result of a
tempera-ture gradient along it is called Thomson effect Thus points at different temperatempera-tures 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.
2
V
54. (a) Cu++ and SO4– – ions
(b) H+ and OH–ions
= αθ + βθ
2
E
where α and β are constants
56. Measurement of temperature
57. 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
58. The electrical conductivity of an electrolytes increases with the increase in temperature
59. The primary cells can not be recharged while in secondary cells reversible reactions take place
so that they can be recharged
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60. From copper to iron
61 Electric Power P = VI = 10 × 5 = 50 W
62 Heat produced per second = VI
= 30 × 10
= 300 W
2
n
where θn is neutral temperature,
θc is temperature of cold junction, and
θi is inversion temperature
64 θi =510°C, θc=0° C
θ + θ 0 + 510
i c
65. P = 100 W
V = 250 V
2 250 × 250
100
V
R
100 Current capacity = = =
250
P I
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120
P
I =
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 re-versed 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 electro-lyte of dilute sulphuric acid
Reactions: H2SO4 dissociates into H+ and 4––
During discharging:
At Cathode: Pb + SO4– –→ PbSO + 2e4 –
At anode: PbO + 2H + 22 + e–→PbO + H O2
PbO + H SO → PbSO + H O
During charging:
At cathode: PbSO + 2H + 24 + e–→Pb + H SO2 4
At anode PbSO + SO4 4– – + 2H O2 →PbO + 2H SO + 2e2 2 4 –
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70. Thermo electric power S is given
by
S = α + βθ
The graph is shown in fig 2 B.5
71. Secondary cell
72. It means that if the hot and the cold junctions are
interchanged, the emf changes sign and the
circulat-ing current reverses direction
73. α: µ V/°C
β : µ V/°C2
74 10– 5 to 10– 6
75. Mercury
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 1 equivalent weight of a
substance by electrolysis So the amount of charge required to liberate 1 mole of the substance
is Fp where p is valency of the substance.
The charge required to liberate one atom of substance is, therefore Fp.
N Now, the charge on each ion is pe, where e is electronic charge Thus
or
Fp
= pe N
F = Ne
78. No For example, mercury is a good conductor
A
B S
Temperature Difference (θ)
Fig 2 B.5
α
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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 m2respectively
Then, from Faraday’s second law 1 1
=
But m1=z1 it and m2= z2 it where z1 and z2are the ece’s of the two substances So,
1 1 1
2 2 2
1 1
2 2 or
=
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
depos-ited 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
va-lency
For relation – see question 80
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1500W = = 1.5 kW
100
Energy consumed in two hours = 2 × 1.5
= 3.0 kWh
83. Power delivered is maximum when the external resistance is equal to the internal resistance of the source
84. π = TS whereπis the peltier coefficient, S is the Seebeck coefficient and T is the temperature
of the cold junction
85 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
86 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
87 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 ter-minal 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:
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Steel vessel
Ni (OH)2
KOH + LiOH Perforated
steel container
Fe (OH) 2
+ –
Fig 2 B.6
(b) Ni(OH) + 2OH2 →Ni(OH)4
The positive ions move towards cathode and then react with Fe(OH)2 to
form iron:
(c) Fe(OH) + 2K2 →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)
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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
Main parts of a button cell are
(a) Anode can
(b) Cathode can
(c) Separator
(d) Gasket.
Silver Oxide zinc cell is shown in fig 2 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:
At Cathode : Ag O + H O + 22 2 e– →2Ag + 2OH–
The emf of the cell is 1.60 V It has a high energy output per unit weight and a constant voltage level
89. Alkali accumulater has higher internal resistance
90. (i) Alkali accumulator is not damaged if it is not charged for a long time.
(ii) Excess charging or discharging do not damage it.
91. See Q 80
92. In open circuit, i.e., when no current flows through the cell
93. Graph - See Q 44
Neutral temperature is independent of the temperature of the cold junction
Gasket SeparatorCathodeAnode Cell can
Anode cap