CURRENT ELECTRICITY PREVIOUS EAMCET QUESTIONS ENGINEERING

Transcription

1 Current lectricity CUNT LCTICITY PVIOUS AMCT QUSTIONS NGINING. In the circuit shown below, a voltmeter of internal resistance, when connected across B and C reads 00/ volts. Neglecting the internal resistance of the battery, the value of is : ) 00kΩ ) 75kΩ ) 50kΩ 4) 5kΩ Ans : From the given circuit, 50 : 50: : [since 50Ω and Ω are in parallel and series to 50 Ω] V: V : 50 + [current is same as they are in series] V (009 ) Given 50kΩ. A cell in secondary circuit gives null deflection for.5 m length of potentiometer having 0m length of wire. If the length of the potentiometer wire is increased by m without changing the cell in the primary, the position of the null point now is : ( 009 ) ).5m ) m ).75m 4).0m Ans : From the Principle of potentiometer vα l For 0m long potentiometer wire, the balancing length is.5 m (.5) For m long potentiometer wire, the balancing length is.75m 0. A current of A flows in an electric circuit as shown in figure. The potential difference V V m in volts (V and V S are potentials at and S respectively) is (008 ) ( ) S

2 Current lectricity ) 4 () + () + 4 (4) Ans: Currrent divides equally and equal to A in each arm Potential across upper part and lower part is same and is equal to 0V Potential at is V 7V Potential at S is VS 0 7 V V V 7 4V S Hence () is the correct choice 4. When a battery connected across a resistor of 6Ω, the voltage across the resistor is V. When the same battery is connected across a resistor of 0Ω, voltage across it is V. The internal resistance of the battery in Ohms is (008 ) () 0 () 0 () 5 (4) Ans: Potential V i i + r V + r 6..() 6 + r 0..() 0 + r 0 r Ω Dividing () & () 7 5. Two unknown resistance X and Y are connected to left and right gaps of a meter bridge and the balancing point is obtained at 80 cm from left. When a 0Ω resistance is connected in parallel to X, the balancing point is 50 cm from left. The values of X and Y respectively are (007 ) () 40 Ω, 9Ω () 0 Ω, 7.5Ω () 0 Ω, 6Ω (4) 0 Ω, Ω Ans: From the principle of meter bridge P l Q 00 l X 4Y..()

3 X0 50 X 0 Y 50 ( + ) X0 4 X X ( + 0) Current lectricity 40 X + 0 X 0Ω X 0 Y 7.5Ω 4 4 Hence () is the correct choice 6. The current in a circuit containing a battery connected to Ω resistance is 0.9 A. When a resistance of 7Ω connected to the same battery, the current observed in the circuit is 0.A. Then the internal resistance of the battery is (007 ) () 0. Ω () 0.5 Ω () Ω (4) Zero Ans: ii, i + r + r MF since i + r Total resis tan ce 0.9 r 7 + r r 7 + r 7 + r 6+ r 7+ r + r r r 0.5Ω Hence () is the correct choice 7. One end each of a resistance r capacitance C and resistance r are connected together. The other ends are respectively connected to the positive terminals of the batteries P, Q, having respectively e.m.f s, and. The negative terminals of the batteries are then connected together. In this circuit, with steady current the potential drop across the capacitance is : (006 ) ) ) ) 4) Ans: As a capacitor is connected there is no current in second brach, in steady state. current through the outer loop i r+ r r

4 Current lectricity 4 potential difference across upper branch + r r This is also p.d. through middle branch 4 p.d. across capacitor 8. Twelve cells, each having e.m.f volts are connected in series and are kept in a closed box. Some of these cells are wrongly connected with positive and negative terminals reversed. This cell battery is connected in series with an ammeter, an external resistance ohms and a two-cell battery (two cells of the same type used earlier, connected perfectly in series). The current in the circuit when the -cell battery and -cell battery aid each other. Then the number of cells in -cells battery that are connected wrongly is (006 ) ) 4 ) ) 4) Ans: 4 Let no. of cells be wrongly connected is n n+ i () n i () Dividing () & () n + n 4 n 0 n 7 n 5 n 4 n 5 n n 9. A 6V cell with 0.5 Ω internal resistance, a 0V cell with Ω internal resistance and a Ω external resistance are connected in parallel. The current (in amperes) through the 0V cell is [005 ] ) 0.60 ).7 ).87 4) 5.4 Ans: Potential difference across AB, CD & F are same. Applying Kirchoff s Laws 4

5 ( ) 6 0.5i 0 i i + i From above equation i.87a 5 Current lectricity 0. In a meter bridge a resistance is connected in the left gap and a pair of resistances P and Q in the right gap. Measured from the left, the balance point is 7.5 cm when P and Q are in series and 7.4 cm when they are parallel. The values of P and Q (in ) are : [005 ] ) 40; 0 ) 5; 5 ) 0; 0 4) 5; 5 Ans: If P & Q are in series () P Q If P & Q are in parallel () P Q P Q From () & () 0Ω, 0Ω P Q. n conducting wires of same dimensions but having resistivities,,,...n are connected in series. The equivalent resistivity of the combination is (004 ) n( n+ ) n + n + n ) ) ) 4) n n + Ans : ρl Sol : We know that A When n conducting wires of same dimensions but having resistivities,, n are connected in series. + + Let ρ is the equivalent resistivity ρl L ( n) A A ρ L L n( n+ ) A A n( n+ ) equivalent resistivity ρ. Two cells A and B are connected in the secondary circuit of a potentiometer one at a time and the balancing length are respectively 400cm and 440 cm. The e.m.f. of the cell A is.08 volt. The e.m.f. of the second cell B in volts is (004 ) ).08 ).88 ).88 4).8 Ans :

6 l l l l Current lectricity. In a potentiometer experiment, the balancing length with a cell is 560 cm. When an external resistance of 0 ohm is connected in parallel to the cell, the balancing length changes by 60cm. The internal resistance of the cell in ohms is (00 ) ).6 ).4 ). 4) 0.6 Ans : ( l l) 0 60 r. Ω l 500 Since l l 60 and l 560 cm l l 500 cm 4. Two resistances of 400Ω and 800Ω are connected in series with 6 volt battery of negligible internal resistance. A voltmeter of resistance 0,000 Ω is used to measure the potential difference across 400Ω. The error in the measurement of potential difference in volt approximately is(00 ) ) 0.0 ) 0.0 ) 0.0 4) 0.05 Ans: 4 The expected voltage drop across 400Ω resistor is V volt When the volt meter is included in parallel, the combined resistance of 400Ω and 0000Ω is Ω The voltage drop is V V V V V 5. The balancing length for a cell is 560 cm in a potentiometer experiment. When an external resistance of 0 is connected in parallel to the cell, the balancing length changes by 60cm. The internal resistance of the cell in ohms is [00 ] ).6 ).4 ). 4) 0. Ans : 0r When 0Ω resistance is connected in parallel, the effective resistance to be balanced is 0 + r l l Since the resistance is connected in parallel, < and l < l. Hence l l 50

7 Current lectricity cm 560 ( 0 + r) 500 r 0 r r.ω 6. A conductor of resistance ohm is stretched uniformly till its length is doubled. The wire now is bent in the form of an equilateral triangle. The effective resistance between the ends of any side of the triangle in ohms is [00 ] ) 9 ) 8 ) 4) Ans : ρl ρl ρl, where the volume does not change on stretching. A Al volume The new resistance ( l) ρ 4 4 Ω volume ach side will have resistance 4Ω Two sides in series have resistance 8Ω and the third side of resistance 4Ω is in parallel. The effective resistance Ω A uniform conductor of resistance is cut into 0 equal pieces. Half of them are joined in series and the remaining half of them are connected in parallel. If the two combinations are joined in series, the effective resistance of all the pieces is [00 ] ) ) ) 0 4) Ans: ach part has resistance 0 0 parts in series have total resistance 0. The remaining 0 parts in parallel have 0 resistance The resistance of these two combination in series is Two wires of equal diameters, of resistivities ρ, ρ and lengths x and x respectively are joined in series. The equivalent resistivity of the combination is [00 ] ρx+ ρx ρx ρx ρx + ρx ρx ρx ) ) ) 4) x+ x x x x+ x x x Ans: quivalent resistance of the combination 7

8 Current lectricity + [As they are in series] ρ ( x+ x) ρx ρx + π r πr πr ρx+ ρx ρeq x + x MDICAL 9. In the circuit shown below, a voltmeter of internal resistance, when connected across B and C reads 00/ volts. Neglecting the internal resistance of the cell, the value of is : Ans :. ) 00kΩ ) 75kΩ ) 50kΩ 4) 5kΩ (009 M) pd across B, C effective 50 + V i V kΩ 0. A flash light lamp is marked.5v and 0.8A. The filament temperature is 45 0 C. The filament resistance of 0 0 C is 4Ω. Then, the temperature coefficient of resistance of the material of the filament is: (009M) ) / K ).5 0 / K ) / K 4) 5 0 / K Ans: 4 Given resistance at 45 0 C V.5.5Ω i 0.8 8

9 esistance at 0 0 C 4Ω α t ( ) / K 445 ( ) Current lectricity. I and V are respectively the current and voltage in a metal wire of resistance ''. Two I-V graphs at two different temperatures T and T are given in the graph. Then (008 M) I T T () T T () T > T () T < T (4) T T Ans: V i For given voltage i esistance increase with increase of temperature and temperature are inversely proportional to voltages. T < T. A projector lamp can be used at a maximum voltage of 60 V, its resistance is 0 Ω, the series resistance (in ohms) required to operate the lamp from a 75 V supply is (008 M) () () () 4 (4) 5 Ans: 4 V i For 60V, 0Ω resistance is required For 75V i.e., extra 5V is required V V The required additional resistance is Ω. A teacher asked a student to connect N cells each of e.m.f. e in series to get a total e.m.f. of Ne. While connecting, the student, by mistake, reversed the polarity of n cells. The total e.m.f. of the resulting series combination is : [006 M] n ) e N ) e( N n) ) e( N n) 4) en 9

10 Current lectricity Ans: Total emf Ne emf N n e Present ( ) When n cells reversely connected the emf of n cells get cancelled. 4. Two wires A and B, made of same material and having their lengths in the ratio 6 : are connected in series. The potential differences across the wires are V and V respectively. rb If r A and r B radii of A and B respectively, then is : (005 M) r A ) 4 ) ) 4) Ans: V lr atio of potentials V l r B A [As they are connected in series current is same] Sl l r ia ib A l r 6 rb ra rb ra 5. For a chosen non-zero value of voltage, there can be more than one value of current in : (005 M) ) Copper wire ) Thermistor ) Zener diode 4) Manganine wire Ans: () All semi conductors devices give more than one current value for a given voltage. 6. The temperature coefficient resistivity of a material is /K. When the temperature of the material is increased by 50º C, its resistivity increases by 0-8 ohm-meter. The initial resistivity of the material in ohm-meter is (004 M) ) ) ) ) Ans : ρ ρ + α θ where is the coefficient of resistivity ( ) ρ ρ 8 0 α θ ρ Ω m 7. Two cells with the same MF and different internal resistances r and r are connected in series to an external resistance. The value of so that the potential difference across the first cell be zero is (00 M) 0

11 Ans: ) rr ) r +r ) r - r ) Total. M. F. i + r+ r Total resis tan ce r V ir + r + r r r 0 [since p.d. across first cell 0] ( + ) ( + r + r ) 0 r + r Current lectricity + r r r r 8. Three unequal resistors in parallel are equivalent to a resistance ohm. If two of them are in the ratio : and if no resistance value is fractional, the largest of the three resistances in ohms is (00 ) ) 4 ) 6 ) 8 4) Ans : If then Ω Ω 6Ω Ω Largest resistance 6Ω 9. In potentiometer experiment a cell of emf..5 V connected in the secondary circuit gives a balancing length of 65cm of the wire. If a resistance of 5 is connected parallel to the cell, the balancing length of the wire is 50cm. The internal resistance of the cell is [00 M] ) 5 ).5 ) 4) 0.5 An s: 4 l l If r is the internal resistance of the cell, 5r 65 50( 5 + r) 5+ r r 5r r 0.5Ω 0. The sides of a rectangular block are cm, cm and 4cm. The ratio of the maximum to minimum resistance between its parallel faces is [00 M]

12 Current lectricity ) 4 ) ) 4) Ans: ρl The dimensions of the block are 4 A ρlmax ρ 4 ρ max Amin ρlmin ρ ρ min Amax 4 6 max 4 min. Three equal resistances each of are in series and connected to a cell of internal resistance one ohm. If these resistances are in parallel and connected to the same cell, then the ratio of the respective currents through the electric circuits in the two cases is [00 M] ) 8 ) 7 ) 5 4) Ans: + 0Ω In the series arrangement, ( ) V V i 0 () In the parallel arrangement, Ω, + Ω V V i () i Dividing () & () i 5. An ideal battery of emf V and a series resistance are connected in the primary circuit of a potentiometer of length m and resistance 5 Ω. The value of to give a potential difference of 5mV across the 0cm of potentiometer wire is [00 M] ) 80 Ω ) 90 Ω ) 95 Ω 4) 00 Ω Ans: Voltage drop across the potentiometer wire is 00 V V 0 V Current, i Where Series esistance V i, where resistance of potentiometer wire Ω

13 Current lectricity. A nichrome wire 50cm long and one square millimeter cross-section carries a current of 4A when connected to a V battery. The resistivity of nichrome wire in ohm-meter is[00 M ] ) x 0-6 ) 4 x 0-7 ) x 0-7 4) x 0-7 Ans: 6 ρl A V A 0 ρ. A l l l Ω m 4. When a resistor of Ω is connected in series with an electric cell, the current flowing in it is 0.5A. Instead when a resistor of 5Ω is connected to the same electric cell in series, the current increases by 0.4A. The internal resistance of the cell is [00 M] ).5 ) ).5 4).5 Ans: Since the same cell is used, V i( + r) i( + r) 0.5( + ) ( )( 5 + r) r.5ω 5. Four resistances 0, 5,7 and are connected so that they form the sides of a rectangle AB, BC, CD and DA respectively. Another resistance of 0 is connected across the diagonal AC. The equivalent resistance between A and B is [000 M] ) ) 5 ) 7 4) 0 Ans: (i) From the circuit, Ω&7Ω resistors are in series. Their resultant resistance is 0Ω (ii) 0Ω and are in parallel and their resultant is 5Ω (iii) 5Ω and 5Ω are in series and are parallel to0ω 6. In a meter bridge experiment, the ratio of the left gap resistance to right gap resistance is :, the balance point from left is (007 ) ) 60 cm ) 50 cm ) 40 cm 4) 0 cm Ans. X l l 00 l 00 l 00 l l 00 5l

14 l 40cm 7. An aluminium ρ Current lectricity 8. 0 Ωm wire of a diameter.4 mm is used to make a 4Ω resistor. The length of the wire is (007) ) 0m ) 000m ) 80m 4) m l A r ρ π l A ρ ρ (007 ) ( ) 4 80m