Unit 2ac kho tài liệu bách khoa

VERY SHORT AND SHORT-ANSWERS QUESTIONS 91 Give an example of a non-ohmic device 92 A current flowing in a copper wire is passed through another copper wire of double the radius What is the effect on the drift velocity of electrons? 93 Why is the terminal voltage of a cell less than its emf? 94 When is a Wheatstone bridge most sensitive? 95 The current flowing through a conductor is ampere Calculate the number of electrons flowing through any cross section per second? 96 A current of A is flowing in a conductor Calculate the charge flowing through the conductor in 10 minutes 97 A current of 4.5 amperes is flowing in a copper wire of cross-sectional area 3×10–4 m2 Calculate the current density in the wire 98 109 electrons pass from a point A to a point B in a conductor in 10–3 s Calculate the strength of the current and give its direction 99 A copper wire has a resistance of 20 Ω at 0° C and 40 Ω at 273° C Find the temperature coefficient of resistance of copper 100 Arrange the following substances in order of decreasing value of temperature coefficient of resistance: (a) copper (b) manganin (c) carbon 101 The side of a cube is 50 cm What is the resistance of this cube if its resistivity is 50 × 10–8 Ω m and the source is connected between any two opposite faces 102 What is the emf of the following battery if the emf of each cell is V: A B Fig A.5 S Chand & Company Limited 103 Three resistances are connected as shown in Fig A.6 Find the equivalent resistance between the points A and B C A R D R B R Fig A.6 104 Compute the value of i, in the following electrical network: i 3A 4A 5A 1.5A 3.5A Fig A.7 105 Given three resistors, each of Ω, how will you combine them to get an equivalent resistance of Ω 106 Two resistors of 20 Ω and 60 Ω are joined in parallel Find the equivalent resistance S Chand & Company Limited 107 Calculate the equivalent resistance between the points X and Y: 4Ω 4Ω Y X 4Ω Fig A.8 108 In the given network what is the current through the Ω resistance and the potential drop across the Ω resistance? 1Ω 5Ω 2A 3Ω 9Ω Fig A.9 109 A wire of resistance Ω is bent at its middle by 180° and both the halves are twisted with each other What is the new resistance? 110 A high voltage supply must have very large internal resistance Why? 111 Calculate the resistance of a conductor whose voltage-current characteristic is shown in Fig A.10 S Chand & Company Limited V (volt) 1 I (mA) Fig A.10 112 A carbon resistance of 47 k Ω is to be marked with rings of different colours for its identification Write the sequence of colours 113 Three identical cells, each of emf V and internal resistance 0.2 Ω, are connected in series to an external resistor of 7.4 ohm Calculate the current in the circuit (AISSCE 1993) 114 It is easier to start a car engine on a warm day than on a chilly day Why? 115 Given the resistances of 4Ω, 2Ω, 6Ω, how will you combine them to get equivalent resis12 Ω? tances of 12Ω and 11 116 Why is a potentiometer so named? 117 Name the type of cell generally used in the main circuit of the potentiometer 118 A cell can supply A current for 40 hr What is its capacity ? 119 What is the conductance of a wire of resistance mΩ ? 120 Name the materials used for making standard resistances Give reasons S Chand & Company Limited 121 122 123 124 125 126 127 Thicker a wire of given length and material, lesser is the resistance Why? A carbon resistor has coloured strips in the sequence yellow, violet, brown and gold What is its resistance? Give reason why the electrical conductance of electrolytes is less than that of metals (AISSCE 1996) Name the device used for measuring the internal resistance of a secondary cell (AISSCE Delhi 1996) Name any one material having a small value of temperature coefficient of resistance Write one use of this material (AISSCE 1997) A student obtains resistances 3, 4, 12 and 16 ohm using two metallic resistance wires either separately or joined together What is the value of resistance of each of these wires? (AISSCE 1997) V-I graphs for a metallic wire at two different temperatures T1 and T2 are as shown in the following figure Which of the two temperatures is higher and why? (AISSCE 1998) T1 I T2 V Fig A.11 S Chand & Company Limited 128 129 130 131 132 133 134 Two wires A and B are of the same metal, have the same area of cross section and have their lengths in the ratio 2:1 What will be the ratio of the currents flowing through them, respectively, when the same potential difference is applied across the length of each of them? (AISSCE Delhi 1998) If a wire is stretched to double its original length without loss of mass, how will the resistivity of the wire be influenced ? (AISSCE Delhi 1999) A carbon resistor is marked in green, red and orange bands What is the approximate resistance of the resistor? (AISSCE Delhi 1999) If the temperature of a good conductor increases, how does the relaxation time of electrons in the conductor change? (AISSCE 2000) If the temperature of a good conductor decreases, how does the relaxation time of electrons in the conductor change? (AISSCE 2000) What is superconductivity? Mention two application of superconductivity ANSWERS 91 Junction diode 92 We know that vd = I I = neA neπr So the drift velocity of electrons in the second wire will be one-fourth of that in the first wire 93 The terminal voltage of a cell is less than its emf as a part of the energy is dissipated by the internal resistance 94 Wheatstone bridge is most sensitive when all the four resistances P, Q, R and S are of the S Chand & Company Limited same order 95 We have q = it Also q = ne ⇒ ne=it ⇒ n= Here it e i = A; t = 1s 1×1 = 6.25 ×1018 electron/s 1.6×10 –19 96 We have q = it i = A; t = 10 = 10 × 60 s ∴ q = × 10 × 60 ∴ n= = 3600 = 3.6 × 103 C i 97 We have current density j = A Here i = 4.5 A, A = × 10–4 m2 4.5 –2 ∴ j= = 1.5 × 10 Am –4 × 10 q 98 We have i = where q = ne t ne ∴ i= t Here n = 109; e = 1.6 × 10– 19 C; t = 10– s S Chand & Company Limited ∴ i= 10 × 1.6 × 10 –19 –3 –7 = 1.6 × 10 A 10 Direction of current is from B to A 99 We have Rt = R0 (1+ α t) or α= Rt – R0 = R0 t 40 – 20 20 × 273 = 273 °C –1 100 (a) Copper (b) Manganin (c) Carbon 101 We have R = ρ Here L A ρ = 50 × 10–8 Ω m L = 50 cm = 0.5 m A = 50 × 50 cm2 = 2500 × 10–4 m2 R= 50 ×10 –8 × 0.5 –4 = 10 2500 ×10 102 Effective emf = (4 × 2) – (2 × 2) = – = 4V –6 = 10 – Ω 103 All the three resistance are in parallel, so the equivalent resistance is S Chand & Company Limited R 104 Applying Kirchhoff’s junction rule, we get i = 2A i = 2A 3A 7A 4A 5A 1.5A 3.5A Fig A.32 105 Two resistors are connected in parallel and the third resistance is connected in series with the combination 106 For parallel combination, we have Req = = 20 × 60 R1R2 R1 + R2 = 15 Ω 20 + 60 107 Two resistances of Ω each are in parallel, so their equivalent resistance is Ω This combination is in series with the third Ω resistance So the equivalent resistance of the network is Req = + = Ω 108 The circuit can be redrawn as shown in Fig A.33 Total resistance between the points A and B is R1 = + + = Ω S Chand & Company Limited A 1Ω 5Ω 3Ω B Y 2A D 9Ω C Fig A.33 Resistance between points D and C = Ω Branches AB and CD are in parallel So the current in each branch is 1A (Since the resistance of each branch is the same) Potential drop across the Ω resistance is V = IR = × = 5V 109 Since the wire is bent at its middle, the resistance of each half is Ω Now the two halves are in parallel, so the new resistance is R= = 1.5 Ω 110 The external resistance may not be large So to keep the current within safe limits, the supply must have its own resistance high ∆V 111 R = ∆I ∆V = – = V From graph, ∆ I = – = mA = × 10– A S Chand & Company Limited ∴ R= 2 ×10 –3 = 10 ohm 112 Yellow, Violet, Orange 113 Current I = E R+r Total emf in the circuit E = × = V Total internal resistance r = 0.2 × = 0.6 Ω R = 7.4 Ω ∴ 114 115 116 117 118 119 120 121 I = = 7.4 + 0.6 = 0.75 A On a warm day the internal resistance of a car battery is less than that on a chilly day because the internal resistance decrease with the rise in temperature For equivalent resistance of 12 Ω , they should be connected in series and for equivalent resistance of 12/11 Ω, they should be connected in parallel Potentiometer is so named because it is used to measure potential differences Leclanche cell Capacity of cell = × 40 = 120 Ampere hour –3 = 1/ (5 × 10 ) = 200 siemens Conductance G = R Constantan and manganin The temperature coefficient of resistance for these is very low R = ρ l/A ⇒ R ∝ 1/A S Chand & Company Limited 122 123 124 125 126 127 128 Thus thicker the wire, smaller is its resistance, provided length and material are same 470 Ω ± 5% Conduction in metals is due to free electrons whereas that in electrolytes is due to positive and negative ions The number density and mobility of ions are smaller than those of free electrons in metals Therefore the conductance of electrolytes is less than that of metals Potentiometer Manganin It is used to make standard resistances 12 Ω and Ω For the same potential the current at temperature T2 is less than that at temperature T1 Therefore the resistance at T2 is greater than that at T1 But resistance increases with temperature Therefore T2 > T1 l We know R = ρ A for same values of ρ and A, R ∝ l So, R1 l = = R2 l2 Now I = V/R Therefore, I1 = I2 ⇒ I1 I2 R2 for same value of V R1 = 129 The resistivity remains the same because it is a property of the material S Chand & Company Limited 130 R = 5300 Ω = 53 ×102 Ω 131 Relaxation time decreases with increase in temperature 132 Relaxation time increases 133 The property by virtue of which certain materials lose their resistance when cooled below a certain temperature (called critical temperature) is called superconductivity 134 Applications of Superconductivity: (i) Loss-less power transmission (ii) Production of very strong magnetic fields S Chand & Company Limited