BHSEC: 2009 (a) How can a galvanometer be converted into an ammeter of desired range? Explain with the help of diagram. By connecting a low resistance (shunt) in parallel to the galvanometer. As per ohm s law, V = R = total current in the circuit g =Current through galvanometer s =Current through Shunt s = ( - g) PD across galvanometer = PD across shunt S g G G g g g S (b) The diagram given below has T2>T1. Explain. We know that V R, T indicates the temperature 1 V R i T 1 (Lower temperature) V T 2 (Higher Temperature) V Smaller the slope, larger the resistance and higher the temperature. Moreover, current is more at T1 than T2. Therefore, resistance is higher at T2. Therefore, T2 is greater. (c) Define internal resistance of a cell. A battery of emf 10 V and internal resistance 3 is connected to a resistor. The current in the circuit is 0.5 A. What is the resistance of the resistor? nternal resistance is the resistance offered by the electrolyte of the cell to the flow of current through it. Given V = 10V, r = 3 and = 0.5 A V ( R r) 10 0.5( R 3) R 17 Objective Questions (i) f an electric current of 2mA flows through a wire, then the number of free electrons passing a given point in a wire per second will be A 1.25x10 13 B 1.25x10 16 C 1.25x10 18 D 1.25x10 21 Ans: (B) 1.25x10 16 s g 0 V G Shunt (Low resistance) T 1 T 2 (ii) The electrolyte in a Leclanche cell is.and its common name is.. (manganese dioxide, ammonium chloride, secondary cell, dry cell) Ans: ammonium chloride, dry cell
BHSEC: 2010 (i) Kirchhoff s first law deals with the conservation of A charge. B energy. C momentum. D angular momentum. (A) Charge (ii) Write any three differences between Joule s heating effect and Peltiers effect. Differences between Peltier Effect and Joules Effect Write any three Peltier Effect Joules Effect 1. Reversible 1. rreversible 2. Occurs at junction only 2. Occurs throughout the circuit 3. Both heating and cooling are 3. Only heating possible 4. Proportional to current 4. Proportional to square of the current 5. Depends on the direction of current 5. ndependent of the direction of current (iii) Neutral temperature changes with change in temperature of cold junction of the same thermocouple. False, neutral temperature remains same. (v) (v) (vi) The depolarizer used in a Leclanche cell is. while. is used in a Daniel cell. (NH 4Cl / MnO2 / CuSO4 / H2SO4) MnO2, CuSO4 State Kirchhoff s laws for electrical circuit. Kirchhoff s laws of electrical circuits are as follows i. First Rule: Kirchhoff's Junction Rule (Current Rule/Law) The algebraic sum of the currents at a junction in a closed circuit is zero. = ii. Second Rule: Kirchhoff's Loop Rule (Voltage Rule/Law) The algebraic sum of the potential differences in any loop including those associated with emf and those of resistive elements must be equal to zero. = () A power line of resistance 0.4 carries a constant current of 90A. How much energy is lost per day in the form of heat in the line? Soln.: Energy lost = 2 RT Total duration -Time = 1 day = 24 x 60 x 60s Energy lost = (90) 2 x 0.4 x 24 x 60 x 60 = 2.799 x 10 8 J or 2.799 x 10 5 kj
(vii) Explain with the help of a circuit diagram, how potentiometer is used to compare the emfs of the cells. Let the known emf be E1 and unknown emf E2 which are connected to the circuit via two-way switch. Let the potential gradient of the wire be K. When the switch touches at A E1 is in closed circuit while E2 is in open circuit. At null-point let the balancing length be l1. The emf of E1 is therefore 1 =.() When the switch touches at B E2 is in closed circuit while E1 is in open circuit. At null-point let the balancing length be l2. The emf of E2 is therefore 2 =.() Dividing (i) and (ii) we get = = (viii) Write down the definition of an ampere based on the force between two current carrying wires. One ampere is that current which when flowing through each of the two parallel conductors of infinite length and placed in free space at a distance of one meter from each other produces between them a force of 2 x 10-7 Newton per meter of their length. BHSEC: 2011 (i) A carbon resistance reads brown-red-yellow. ts resistance will be A 124. B C D 3 1210. 4 1210. 5 1210. (C) 4 1210. (ii) When two unequal resistances are joined in parallel to a cell, the current will be the same in both the resistances. False. Current is different in both the resistances but potential difference is same. (iii) An electron from rest is accelerated through a p.d of V volts. The final velocity acquired by it is m/sec. Obtain an expression for e of electron in terms of V and. m 1 2 m ev 2 2 e m 2V
(iv)the variation of potential difference with length in case of two potentiometers A and B is given below. Which of the two is more sensitive? State one use of a potentiometer. Smaller the potential gradient ( K V ) of potentiometer wire, greater is the sensitivity. l Therefore, potential gradient ( K V ) is smaller for A. So A is more sensitive. l Use (ANY ONE) Comparison of the emfs of two cells Comparison of resistances Determination of internal resistance of a primary cell (v) Define electric conductance of a material. Give its S unit. Electric conductance of a material is the reciprocal of its resistance. S unit: mho OR ohm -1 ( -1 ) OR Siemen (S) (vi) A 200 W 240 V bulb is connected to a 120 V d.c source. Calculate the power consumed by the bulb. 2 V 240 240 R 288 P 200 2 V 120120 P 50W R 288 (vii) Write the direction of a thermoelectric current at the i. cold junction of a Cu-Fe thermocouple and ii. hot junction of a Sb-Bi thermocouple. iii. Which of these two thermocouples produces a higher thermo-emf for the same temperature of the hot and the iv. cold junctions? Fe to Cu Bi to Sb Sb-Bi thermocouple produces a higher emf as Sb and Bi are more separated than Cu and Fe in the thermoelectric series.
BHSEC: 2012 i. The S.. unit of specific conductance ( ) is A B C D A -1 m). mho. m.. m -1 m). ii. Explain the basis of difference between magnetic and electric lines of force. Diagram Electric lines of force start at one point (positive charge) and end at some other point (negative charge).a magnetic line of force has no beginning and end. t forms a closed loop. iii. iv. The V- graphs of two resistors and their series combination are shown in the figure given below. Which of the graphs represents the series combination of the other two? Give reasons for your answer. Graph 1 shows the series combination of 2 and 3 Rseries = sum of individual resistance 3 2 Slope of graph = inverse of resistance 1 Graph 1 has least slope and therefore the maximum resistance which can be the resistance represented by graph 2 and 3. V Derive the expression for internal resistance of a cell. n the figure, Let emf of the cell be E, the internal resistance r is connected to a resistance wire R and an ammeter A through KEY k. Let be the current in the circuit. When a steady current is established in the circuit a charge dq flows in the circuit in time. By definition of emf, the work done by the cell is ( dq = idt) The potential difference across the external resistance R is V. Hence the external work done by the cell is given by The potential drop across the internal resistance r inside the cell is
Therefore, the internal work done by the cell is given by, = = By law of conservation of energy, we have = + = + E = V + ir E-V r= i E-V = (QV=iR) V R E r=r -1 V v. Define drift velocity of free electrons in a conductor. Give its mathematical expression. When an electric field is applied across a conductor, the free electrons are accelerated opposite to the direction of the field. However, due to frequent collision with the atoms, their average velocity is very small. This average velocity with which the free electrons move in a conductor under a potential vi. difference is called drift velocity. Where e = charge on electron E = electric field = Current A = Cross-sectional Area = relaxation time M = mass of electron = Solve for the value of R using Kirchoff s laws only in the diagram given below. Total current flowing through R = 5A + 1A = 6A n the loop AFEBA, according to Kirchoff s voltage law, = -3 5 6 R + 20 = 0 = -15 6R + 20 = 0 5 R= 6
S vii. Establish the equation i g i with the help of a circuit diagram (the symbols have their S G usual meaning). Let G be the resistance of the coil of the galvanometer and ig the current. f i is the maximum current ig of i should pass through galvanometer and (i-ig) through shunt S. G and S are parallel, the potential difference across them will be same. ig G = (i ig) S igg = is igs S i g i S G BHSEC: 2013 i. A wire having a very high value of conductance is said to be a. a moderately good conductor. b. a very good conductor. c. a semi conductor. d. an insulator. (b) a very good conductor. ii. iii. iv. A galvanometer of 1 resistance gives a full scale deflection with a 5 10.-3 A current. To measure 5 V with this galvanometer, a resistance of has to be connected in to the galvanometer. ( 1000/999/parallel/series) 999, series From 25 W and 100 W bulbs, the 100 W bulb glows brighter whether the two bulbs are connected in series or in parallel. False, in series 25 W bulb glows brighter and in parallel 100 W bulb glows brighter. A 100 resistor is connected to 220V-50 Hz a.c supply. Find the rms value of current in the circuit and the net power consumed for a complete cycle. Given Vrms = 220V R = 100 Vrms 220V rms = = =2.20A R 100 Net power consumed for a complete cycle is P = (rms) 2 R = (2.20) 2 100 = 484 W
v. Study the diagram given below. Use the above figure to, a) find the unknown resistance X, b) determine the balance point if resistance X and 12.5 are interchanged. c) state why the resistors in a meter bridge are connected by thick copper strips? X 39.5cm a) = 12.5 (100-39.5)cm, 39.5cm X=12.5, 60.5cm X = 8.16 b) On interchanging X and 12.5, the balance point will be at, 100 39.5 = 60.5 cm from end A c) To minimize the resistance of connections vi. vii. What are Seebeck and Peltier effects? The phenomenon of development of e.m.f. in a thermo couple when its two junction are maintained at a difference of temperature is called Seebeck effect. The phenomenon of evolution and absorption of heat, at the two junctions of a thermo couple when a current passes through it is called Peltier s effect. Given that the p.d applied across a variable resistor is constant, draw a graph between current in the resistor and the resistance R. viii. The current flowing through a conductor is given by an equation, i = VdenA. dentify each term on the right hand side of the equation. V d = Drift velocity e = Charge of electron n = electron density A = Area of cross section of conductor ix. Obtain an expression for Vd, if the current flowing through a conductor of length ' l ' has its ends maintained at a potential difference of V volt. Force experienced by electrons. F = ma - (i) and F = ee - (ii) From (i) and (ii) we have, ma = ee - (iii) We know that, v = u + at [ since u=0] From (iii) and (iv) ev V = d ml