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NEET : Physics [ 244 ] www.allendigital.in  Digital Exercise - I ELECTRIC CURRENT & DRIFT VELOCITY 1. If 106 electrons/s are flowing through an area of cross section of 10–4 m2 then the current will be :– (1) 1.6 × 10–7 A (2) 1.6 × 10–13 A (3) 1 × 10–6 A (4) 1 × 102 A 2. The current in a conductor varies with time t as I = 2t + 3t2 A where I is amperes and t in seconds. Electric charge flowing through a section of the conductor during t = 2 s to t = 3 s is :- (1) 10 C (2) 24 C (3) 33 C (4) 44 C 3. 10,000 electrons are passing per minute through a tube of radius 1cm. The resulting current is : (1) 10000 A (2) 0.25 × 10–16 A (3) 10–9 A (4) 0.5 × 10–19 A 4. There are 8.4 × 1022 free electrons per cm3 in copper. The current in the wire is 0.21 A (e = 1.6 × 10–19 C). Then the drifts velocity of electrons in a copper wire of 1 mm2 cross section, will be :– (1) 2.12 × 10–5 m/s (2) 0.78 × 10–5 m/s (3) 1.56 × 10–5 m/s (4) none of these 5. There is a current of 40 amperes in a wire of 10–6 m2 area of cross-section. If the number of free electrons per m3 is 1029, then the drift velocity will be (1) 1.25 × 103 m/s (2) 2.50 × 10–3 m/s (3) 25.0 × 10–3 m/s (4) 250 × 10–3 m/s 6. S.I. unit of current is :- (1) C (2) A (3) A/s (4) N/s 7. When no current flows through a conductor :- (1) the free electrons do not move (2) the average speed of a free electron over a large period of time is zero (3) the average velocity of a free electron over a large period of time is zero (4) the average of square of velocities of all the free electrons at an instant is zero 8. The number of free electrons per 10 mm of an ordinary copper wire is about 2 × 1021. The average drift speed of the electrons is 0.25 mm/s The current flowing is :- (1) 0.8 A (2) 8 A (3) 80 A (4) 5 A 9. In a Neon discharge tube 2.9 × 1018 Ne+ ions move to the right each second, while 1.2 × 1018 electrons move to the left per second; electron charge is 1.6 × 10–19 C. The current in the discharge tube is :- (1) 1 A towards right (2) 0.66 A towards right (3) 0.66 A towards left (4) zero 10. Two wires each of radius of cross section r but of different materials are connected together end to end (in series). If the densities of charge carriers in the two wires are in the ratio 1:4, the drift velocity of electrons in the two wires will be in the ratio : (1) 1 : 2 (2) 2 : 1 (3) 4 : 1 (4) 1 : 4 11. A current I flows through a uniform wire of diameter d when the electron drift velocity is v. The same current will flow through a wire of diameter d/2 made of the same material if the drift velocity of the electrons is (1) v/4 (2) v/2 (3) 2v (4) 4v 12. A wire has a non–uniform cross–section as shown in figure. A steady current flows through it. The drift speed of electrons at points P and Q is vP and vQ, then :- (1) vP = vQ (2) vP < vQ (3) vP > vQ (4) data is insufficient 13. The plot represents the flow of current through a wire for different time intervals. The ratio of charges flowing through the wire corresponding to these time intervals is (see figure) :- P Q
Current Electricity  Digital www.allendigital.in [ 245 ] (1) 2 : 1 : 2 (2) 1 : 3 : 3 (3) 1 : 1 : 1 (4) 2 : 3 : 4 OHM'S LAW & ELECTRICAL RESISTANCE 14. Three copper wires are there with lengths and cross-sectional areas as (, A); A 2 , 2       and ,2A 2       . Resistance :- (1) minimum for the wire of cross-sectional are A 2 (2) minimum for the wire of cross-sectional are A (3) minimum for the wire of cross-sectional area 2A (4) same for all the three cases. 15. A wire of uniform cross-section A, length  and resistance R is bent into a complete circle; the resistance between any two of diametrically opposite points will be :– (1) R 2 (2) R 4 (3) R 8 (4) 4R 16. The electric resistance of a certain wire of iron is R. If its length and radius both are doubled, then :– (1) the resistance will be halved and the specific resistance will remain unchanged (2) the resistance will be halved and the specific resistance will be doubled (3) the resistance and the specific resistance, will both remain unchanged (4) the resistance will be doubled and the specific resistance will be halved. 17. When a piece of aluminium wire of finite length is drawn to reduce its diameter to half its original value, its resistance will become :- (1) two times (2) four times (3) eight times (4) sixteen times 18. As the temperature of a metallic resistor is increased, the product of resistivity and conductivity :- (1) increases (2) decreases (3) may increase or decrease (4) remains constant. 19. If a wire is stretched, so that its length is 20% more than its initial length, the percentage increase in the resistance of the wire is :- (1) 40% (2) 10% (3) 44% (4) 25% 20. The length of a given cylindrical wire is increased by 100%. Due to the consequent decrease in diameter the change in the resistance of the wire will be :– (1) 300% (2) 200% (3) 100% (4) 50% 21. On increasing the temperature, the specific resistance of a conductor and a semiconductor- (1) both increase (2) both decrease (3) increases and decreases respectively (4) decreases and increases respectively 22. A conductor with rectangular cross section has dimensions (a × 2a × 4a) as shown in figure. Resistance across AB is x, across CD is y and across EF is z. Then (1) x = y = z (2) x > y > z (3) y > x > z (4) x > z > y 23. Specific resistance of a conductor increases with:– (1) increase in temperature. (2) increase in cross–sectional area (3) increase in cross–sectional area and decrease in length. (4) decrease in cross–sectional area. 24. The temperature coefficient of resistance of a wire is 0.00125 per degree celcius. At 300 K its resistance is 1 ohm. The resistance of the wire will be 2 ohms at a temperature :– (1) 1154 K (2) 1127 K (3) 600 K (4) 1400 K 1 2 3 4 5 6 7 8 2 1 –1 –2 Time (in seconds) I A B C E D F a 4a 2a
NEET : Physics [ 246 ] www.allendigital.in  Digital 25. The current voltage graph for a given metallic conductor at two different temperatures T1 and T2 are as shown in the figure. Then :– (1) T1 > T2 (2) T1 = T2 (3) nothing can be said about T1 and T2 (4) T1 < T2 26. The effective resistance is 6 5 , when two wires are joined in parallel. When one of the wire breaks, the effective resistance is 2 ohms. The resistance of the broken wire was :– (1) 3 5  (2) 2  (3) 6 5  (4) 3  27. At what temperature will the resistance of a copper wire become three times its value at 0° C? [Temperature coefficient of resistance for copper = 4 × 10–3 per °C] :– (1) 400°C (2) 450° C (3) 500° C (4) 600° C 28. Copper and silicon are cooled from 300 K to 60 K; the specific resistance :– (1) decreases in copper but increases in silicon (2) increases in copper but decreases in silicon (3) increases in both (4) decreases in both 29. Two resistances R1 and R2 are made of different materials. The temperature coefficient of the material of R1 is  and that of the material of R2 is –. The resistance of the series combination of R1 and R2 does not change with temperature, then the ratio of resistances of the two wires at 0°C will be : (1)   (2)  +   − (3) 2 2  +   (4)   COMBINATION OF RESISTANCES & KIRCHHOFF'S LAW 30. A metal wire of resistance R is cut into three equal pieces which are then connected side by side to form a new wire, the length of which is equal to one third of the original length. The resistance of this new wire is :- (1) R (2) 3R (3) R 9 (4) R 3 31. Three resistances of values 2 , 3  and 6  are to be connected to yield an effective resistance of 4 . This can be done by connecting : (1) 3  resistance in series with a parallel combination of 2  and 6  (2) 6  resistance in series with a parallel combination of 2  and 3  (3) 2  resistance in series with a parallel combination of 3  and 6  (4) 2  resistance in parallel with a parallel combination of 3  and 6  32. What will be the equivalent resistance between the points A and D? (1) 10  (2) 20  (3) 30  (4) 40  33. In the circuit shown here, what is the value of the unknown resistance R so that the total resistance of the circuit between points 'P' and 'Q' is also equal to R :– (1) 3  (2) 39 (3) 69 (4) 10  T1 T2 I V A 10  B C D 10  10  10  10  10  10  10  P 3  R 3  10  Q
Current Electricity  Digital www.allendigital.in [ 247 ] 34. The resistance across P and Q in the given figure is (1) R 3 (2) R 2 (3) 2R (4) 6R 35. The resistance of the circuit between A and B is : (1) r (2) 0.5r (3) 2r (4) 3r 36. Thirteen resistances each of resistance R  are connected in the circuit as shown in the figure. The effective resistance between A and B is :- (1) 4R 3  (2) 2R (3) R  (4) 2R 3  37. The total resistance between x and y in ohms is:- (1) 1  (2) 4  (3) 4 3  (4) 2 3  38. The resultant resistance of n wires each of resistance r ohms is R, when they are connected in parallel. When these n resistances are connected in series, the resultant resistance will be :- (1) R n (2) 2 R n (3) nR (4) n2R 39. For the network of resistance shown in the fig. the equivalent resistance of the network between the points A and B is 18 . The value of unknown resistance R is :- (1) 8  (2) 10  (3) 16  (4) 24  40. In the arrangement of resistances shown below, the effective resistance between points A and B is (1) 20  (2) 30  (3) 90  (4) 110  41. In the circuit shown the equivalent resistance between A and B is (1) R (2) 2R/5 (3) R/3 (4) 2R 42. In the figure the numerical values denote resistances in SI units. The total resistance of the circuit between a & b will be: (1) 12 ohms (2) 24 ohms. (3) 15 ohms (4) 6 ohms A R R R R R P B R C Q A B r r r r r r r r r r R R R R R R R R R R R R R A B 4  x 4  2  8  6  4  y B 10  R A 10  10  10  10  10  5  A B 10  15  10  20  30  10  10  A R R R R R R B a b 50 4 20 30 16 3 2 20 15 