Page 1 JEE (MAIN)-2021 Phase-1 (24-02-2021)-E PART–A : PHYSICS SECTION - I Multiple Choice Questions: This section contains 20 multiple choice questions. Each question has 4 choices (1), (2), (3) and (4), out of which ONL Y ONE is correct. Choose the correct answer : 1. Zener breakdown occurs in a p-n junction having p and n both (1) Lightly doped and have narrow depletion layer (2) Heavily doped and have narrow depletion layer (3) Heavily doped and have wide depletion layer (4) Lightly doped and have wide depletion layer Answer (2) Sol. Zener diode has both p and n side highly doped and depletion layer is very thin. 2. If the source of light used in a Young’s double slit experiment is changed from red to violet (1) The fringes will become brighter (2) The central bright fringe will become a dark fringe (3) Consecutive fringe lines will come closer (4) The intensity of minima will increase Answer (3) Sol. ? D d ? ?? and ? v < ? red ? Fringe width will decrease. ? Fringe lines will come closer. 3. C A B The logic circuit shown above is equivalent to (1) C A B (2) C A B (3) C A B (4) C A B Answer (1) Sol. From gates diagram, (AB) AB AB ?? ? ? ? ? C A B 4. Which of the following equations represents a travelling wave? (1) y = Asin(15x – 2t) (2) y = Ae x cos( ?t – ?) (3) y = Asinx cos ?t (4) ? ??? 2 x yAe (vt ) Answer (1) Sol. Travelling wave equation, y = f(± kx ± ?t) ? y = A sin(15x – 2t) is a travelling wave. 5. Figure shows a circuit that contains four identical resistors with resistance R = 2.0 ?, two identical inductors with inductance L = 2.0 mH and an ideal battery with emf E = 9 V. The current ‘i’ just after the switch ‘S’ is closed will be A S R R R L R L i E = 9 V (1) 3.37 A (2) 3.0 A (3) 2.25 A (4) 9 A Answer (3) Page 2 JEE (MAIN)-2021 Phase-1 (24-02-2021)-E PART–A : PHYSICS SECTION - I Multiple Choice Questions: This section contains 20 multiple choice questions. Each question has 4 choices (1), (2), (3) and (4), out of which ONL Y ONE is correct. Choose the correct answer : 1. Zener breakdown occurs in a p-n junction having p and n both (1) Lightly doped and have narrow depletion layer (2) Heavily doped and have narrow depletion layer (3) Heavily doped and have wide depletion layer (4) Lightly doped and have wide depletion layer Answer (2) Sol. Zener diode has both p and n side highly doped and depletion layer is very thin. 2. If the source of light used in a Young’s double slit experiment is changed from red to violet (1) The fringes will become brighter (2) The central bright fringe will become a dark fringe (3) Consecutive fringe lines will come closer (4) The intensity of minima will increase Answer (3) Sol. ? D d ? ?? and ? v < ? red ? Fringe width will decrease. ? Fringe lines will come closer. 3. C A B The logic circuit shown above is equivalent to (1) C A B (2) C A B (3) C A B (4) C A B Answer (1) Sol. From gates diagram, (AB) AB AB ?? ? ? ? ? C A B 4. Which of the following equations represents a travelling wave? (1) y = Asin(15x – 2t) (2) y = Ae x cos( ?t – ?) (3) y = Asinx cos ?t (4) ? ??? 2 x yAe (vt ) Answer (1) Sol. Travelling wave equation, y = f(± kx ± ?t) ? y = A sin(15x – 2t) is a travelling wave. 5. Figure shows a circuit that contains four identical resistors with resistance R = 2.0 ?, two identical inductors with inductance L = 2.0 mH and an ideal battery with emf E = 9 V. The current ‘i’ just after the switch ‘S’ is closed will be A S R R R L R L i E = 9 V (1) 3.37 A (2) 3.0 A (3) 2.25 A (4) 9 A Answer (3) JEE (MAIN)-2021 Phase-1 (24-02-2021)-E Sol. At t = 0, inductor behaves as an open switch. ? 2 ? 9 V 2 ? ? 9 i2.25A 4 ?? 6. An X-ray tube is operated at 1.24 million volt. The shortest wavelength of the produced photon will be (1) 10 –3 nm (2) 10 –2 nm (3) 10 –4 nm (4) 10 –1 nm Answer (1) Sol. min hc eV ?? = 34 8 19 6 6.63 10 3 10 1.6 10 1.24 10 ? ? ??? ?? ? = 1.0 × 10 –12 m = 10 –3 nm 7. The de Broglie wavelength of a proton and ?-particle are equal. The ratio of their velocities is (1) 1 : 4 (2) 4 : 3 (3) 4 : 1 (4) 4 : 2 Answer (3) Sol. p pp h mv ?? ? h mv ? ?? ?? ? ? ? p = ? ? ? m p v p = m ? v ? ? p p v m 4 vm 1 ? ? ?? 8. Given below are two statements : Statement I : PN junction diodes can be used to function as transistor, simply by connecting two diodes, back to back, which acts as the base terminal. Statement II : In the study of transistor, the amplification factor ? indicates ratio of the collector current to the base current. In the light of the above statements, choose the correct answer from the options given below. (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both statement I and Statement II are true Answer (3) Sol. Transistor is formed by formation of two p-n junctions by sandwiching a thin base layer. So, statement I is false. ? C B I I ? ?? ? and for DC, C B I I ?? So, statement II is true. 9. A circular hole of radius ?? ?? ?? a 2 is cut out of a circular disc of radius ‘a’ as shown in figure. The centroid of the remaining circular portion with respect to point ‘O’ will be a/2 a X-axis Y-axis O (1) 1 a 6 (2) 2 a 3 (3) 10 a 11 (4) 5 a 6 Answer (4) Sol. Let mass of hole be (–m), then mass of complete disc will be 4m. a/2 a O C ? Centre of mass w.r.t. point O is CM 3 4m a ( m) a 5 2 Xa (4m m) 6 ?? ?? ? ? ?? ?? ?? ? Page 3 JEE (MAIN)-2021 Phase-1 (24-02-2021)-E PART–A : PHYSICS SECTION - I Multiple Choice Questions: This section contains 20 multiple choice questions. Each question has 4 choices (1), (2), (3) and (4), out of which ONL Y ONE is correct. Choose the correct answer : 1. Zener breakdown occurs in a p-n junction having p and n both (1) Lightly doped and have narrow depletion layer (2) Heavily doped and have narrow depletion layer (3) Heavily doped and have wide depletion layer (4) Lightly doped and have wide depletion layer Answer (2) Sol. Zener diode has both p and n side highly doped and depletion layer is very thin. 2. If the source of light used in a Young’s double slit experiment is changed from red to violet (1) The fringes will become brighter (2) The central bright fringe will become a dark fringe (3) Consecutive fringe lines will come closer (4) The intensity of minima will increase Answer (3) Sol. ? D d ? ?? and ? v < ? red ? Fringe width will decrease. ? Fringe lines will come closer. 3. C A B The logic circuit shown above is equivalent to (1) C A B (2) C A B (3) C A B (4) C A B Answer (1) Sol. From gates diagram, (AB) AB AB ?? ? ? ? ? C A B 4. Which of the following equations represents a travelling wave? (1) y = Asin(15x – 2t) (2) y = Ae x cos( ?t – ?) (3) y = Asinx cos ?t (4) ? ??? 2 x yAe (vt ) Answer (1) Sol. Travelling wave equation, y = f(± kx ± ?t) ? y = A sin(15x – 2t) is a travelling wave. 5. Figure shows a circuit that contains four identical resistors with resistance R = 2.0 ?, two identical inductors with inductance L = 2.0 mH and an ideal battery with emf E = 9 V. The current ‘i’ just after the switch ‘S’ is closed will be A S R R R L R L i E = 9 V (1) 3.37 A (2) 3.0 A (3) 2.25 A (4) 9 A Answer (3) JEE (MAIN)-2021 Phase-1 (24-02-2021)-E Sol. At t = 0, inductor behaves as an open switch. ? 2 ? 9 V 2 ? ? 9 i2.25A 4 ?? 6. An X-ray tube is operated at 1.24 million volt. The shortest wavelength of the produced photon will be (1) 10 –3 nm (2) 10 –2 nm (3) 10 –4 nm (4) 10 –1 nm Answer (1) Sol. min hc eV ?? = 34 8 19 6 6.63 10 3 10 1.6 10 1.24 10 ? ? ??? ?? ? = 1.0 × 10 –12 m = 10 –3 nm 7. The de Broglie wavelength of a proton and ?-particle are equal. The ratio of their velocities is (1) 1 : 4 (2) 4 : 3 (3) 4 : 1 (4) 4 : 2 Answer (3) Sol. p pp h mv ?? ? h mv ? ?? ?? ? ? ? p = ? ? ? m p v p = m ? v ? ? p p v m 4 vm 1 ? ? ?? 8. Given below are two statements : Statement I : PN junction diodes can be used to function as transistor, simply by connecting two diodes, back to back, which acts as the base terminal. Statement II : In the study of transistor, the amplification factor ? indicates ratio of the collector current to the base current. In the light of the above statements, choose the correct answer from the options given below. (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both statement I and Statement II are true Answer (3) Sol. Transistor is formed by formation of two p-n junctions by sandwiching a thin base layer. So, statement I is false. ? C B I I ? ?? ? and for DC, C B I I ?? So, statement II is true. 9. A circular hole of radius ?? ?? ?? a 2 is cut out of a circular disc of radius ‘a’ as shown in figure. The centroid of the remaining circular portion with respect to point ‘O’ will be a/2 a X-axis Y-axis O (1) 1 a 6 (2) 2 a 3 (3) 10 a 11 (4) 5 a 6 Answer (4) Sol. Let mass of hole be (–m), then mass of complete disc will be 4m. a/2 a O C ? Centre of mass w.r.t. point O is CM 3 4m a ( m) a 5 2 Xa (4m m) 6 ?? ?? ? ? ?? ?? ?? ? JEE (MAIN)-2021 Phase-1 (24-02-2021)-E 10. A particle is projected with velocity v 0 along x-axis. A damping force is acting on the particle which is proportional to the square of the distance from the origin i.e. ma = – ?x 2 . The distance at which the particle stops (1) ?? ?? ?? ? ?? 1 2 3 0 3v 2 (2) ?? ?? ?? ? ?? 1 2 2 0 3v 2 (3) ?? ?? ?? ? ?? 1 2 2 0 2v 3 (4) ?? ?? ? ?? 1 3 0 2v 3 Answer (1) Sol. Loss in KE = work against damping ? x 22 0 x0 1 mv x dx 2 ? ?? ? ? 3 2 0 1x mv 23 ? ? ? 1 2 3 0 3mv x 2 ?? ??? ?? ? ?? 11. When a particle executes SHM, the nature of graphical representation of velocity as a function of displacement is : (1) parabolic (2) straight line (3) circular (4) elliptical Answer (4) Sol. ?? ? 22 vA x ?? ? 2 22 2 v xA 12. The period of oscillation of a simple pendulum is ?? L T2 . g Measured value of ‘L’ is 1.0 m from meter scale having a minimum division of 1 mm and time of one complete oscillation is 1.95 s measured from stopwatch of 0.01 s resolution. The percentage error in the determination of ‘g’ will be : (1) 1.30% (2) 1.33% (3) 1.13% (4) 1.03% Answer (3) Sol. ?? 2 2 g4 T ?? dg d 2dT gT dg 1 2 0.01 1.13 g 1000 1.95 100 ? ?? ? 13. In the given figure, a body of mass M is held between two massless spring, on a smooth inclined plane. The free ends of the springs are attached to firm supports. If each spring has spring constant k, the frequency of oscillation of given body is : M k k ? (1) ? 12k 2M (2) ? 1k 22M (3) ?? 12k 2Mgsin (4) ?? 1k 2Mgsin Answer (1) Sol. ?? eq M T2 k ? ? eq k 1 f 2M ? eq k2k 14. Match List-I with List-II. List-I List-II (a) Source of (i) Radioactive decay microwave of nucleus frequency (b) Source of (ii) Magnetron infrared frequency (c) Source of (iii) Inner shell electrons Gamma Rays (d) Source of X-rays (iv) Vibration of atoms and molecules (v) LASER (vi) RC circuit Choose the correct answer from the options given below : Page 4 JEE (MAIN)-2021 Phase-1 (24-02-2021)-E PART–A : PHYSICS SECTION - I Multiple Choice Questions: This section contains 20 multiple choice questions. Each question has 4 choices (1), (2), (3) and (4), out of which ONL Y ONE is correct. Choose the correct answer : 1. Zener breakdown occurs in a p-n junction having p and n both (1) Lightly doped and have narrow depletion layer (2) Heavily doped and have narrow depletion layer (3) Heavily doped and have wide depletion layer (4) Lightly doped and have wide depletion layer Answer (2) Sol. Zener diode has both p and n side highly doped and depletion layer is very thin. 2. If the source of light used in a Young’s double slit experiment is changed from red to violet (1) The fringes will become brighter (2) The central bright fringe will become a dark fringe (3) Consecutive fringe lines will come closer (4) The intensity of minima will increase Answer (3) Sol. ? D d ? ?? and ? v < ? red ? Fringe width will decrease. ? Fringe lines will come closer. 3. C A B The logic circuit shown above is equivalent to (1) C A B (2) C A B (3) C A B (4) C A B Answer (1) Sol. From gates diagram, (AB) AB AB ?? ? ? ? ? C A B 4. Which of the following equations represents a travelling wave? (1) y = Asin(15x – 2t) (2) y = Ae x cos( ?t – ?) (3) y = Asinx cos ?t (4) ? ??? 2 x yAe (vt ) Answer (1) Sol. Travelling wave equation, y = f(± kx ± ?t) ? y = A sin(15x – 2t) is a travelling wave. 5. Figure shows a circuit that contains four identical resistors with resistance R = 2.0 ?, two identical inductors with inductance L = 2.0 mH and an ideal battery with emf E = 9 V. The current ‘i’ just after the switch ‘S’ is closed will be A S R R R L R L i E = 9 V (1) 3.37 A (2) 3.0 A (3) 2.25 A (4) 9 A Answer (3) JEE (MAIN)-2021 Phase-1 (24-02-2021)-E Sol. At t = 0, inductor behaves as an open switch. ? 2 ? 9 V 2 ? ? 9 i2.25A 4 ?? 6. An X-ray tube is operated at 1.24 million volt. The shortest wavelength of the produced photon will be (1) 10 –3 nm (2) 10 –2 nm (3) 10 –4 nm (4) 10 –1 nm Answer (1) Sol. min hc eV ?? = 34 8 19 6 6.63 10 3 10 1.6 10 1.24 10 ? ? ??? ?? ? = 1.0 × 10 –12 m = 10 –3 nm 7. The de Broglie wavelength of a proton and ?-particle are equal. The ratio of their velocities is (1) 1 : 4 (2) 4 : 3 (3) 4 : 1 (4) 4 : 2 Answer (3) Sol. p pp h mv ?? ? h mv ? ?? ?? ? ? ? p = ? ? ? m p v p = m ? v ? ? p p v m 4 vm 1 ? ? ?? 8. Given below are two statements : Statement I : PN junction diodes can be used to function as transistor, simply by connecting two diodes, back to back, which acts as the base terminal. Statement II : In the study of transistor, the amplification factor ? indicates ratio of the collector current to the base current. In the light of the above statements, choose the correct answer from the options given below. (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both statement I and Statement II are true Answer (3) Sol. Transistor is formed by formation of two p-n junctions by sandwiching a thin base layer. So, statement I is false. ? C B I I ? ?? ? and for DC, C B I I ?? So, statement II is true. 9. A circular hole of radius ?? ?? ?? a 2 is cut out of a circular disc of radius ‘a’ as shown in figure. The centroid of the remaining circular portion with respect to point ‘O’ will be a/2 a X-axis Y-axis O (1) 1 a 6 (2) 2 a 3 (3) 10 a 11 (4) 5 a 6 Answer (4) Sol. Let mass of hole be (–m), then mass of complete disc will be 4m. a/2 a O C ? Centre of mass w.r.t. point O is CM 3 4m a ( m) a 5 2 Xa (4m m) 6 ?? ?? ? ? ?? ?? ?? ? JEE (MAIN)-2021 Phase-1 (24-02-2021)-E 10. A particle is projected with velocity v 0 along x-axis. A damping force is acting on the particle which is proportional to the square of the distance from the origin i.e. ma = – ?x 2 . The distance at which the particle stops (1) ?? ?? ?? ? ?? 1 2 3 0 3v 2 (2) ?? ?? ?? ? ?? 1 2 2 0 3v 2 (3) ?? ?? ?? ? ?? 1 2 2 0 2v 3 (4) ?? ?? ? ?? 1 3 0 2v 3 Answer (1) Sol. Loss in KE = work against damping ? x 22 0 x0 1 mv x dx 2 ? ?? ? ? 3 2 0 1x mv 23 ? ? ? 1 2 3 0 3mv x 2 ?? ??? ?? ? ?? 11. When a particle executes SHM, the nature of graphical representation of velocity as a function of displacement is : (1) parabolic (2) straight line (3) circular (4) elliptical Answer (4) Sol. ?? ? 22 vA x ?? ? 2 22 2 v xA 12. The period of oscillation of a simple pendulum is ?? L T2 . g Measured value of ‘L’ is 1.0 m from meter scale having a minimum division of 1 mm and time of one complete oscillation is 1.95 s measured from stopwatch of 0.01 s resolution. The percentage error in the determination of ‘g’ will be : (1) 1.30% (2) 1.33% (3) 1.13% (4) 1.03% Answer (3) Sol. ?? 2 2 g4 T ?? dg d 2dT gT dg 1 2 0.01 1.13 g 1000 1.95 100 ? ?? ? 13. In the given figure, a body of mass M is held between two massless spring, on a smooth inclined plane. The free ends of the springs are attached to firm supports. If each spring has spring constant k, the frequency of oscillation of given body is : M k k ? (1) ? 12k 2M (2) ? 1k 22M (3) ?? 12k 2Mgsin (4) ?? 1k 2Mgsin Answer (1) Sol. ?? eq M T2 k ? ? eq k 1 f 2M ? eq k2k 14. Match List-I with List-II. List-I List-II (a) Source of (i) Radioactive decay microwave of nucleus frequency (b) Source of (ii) Magnetron infrared frequency (c) Source of (iii) Inner shell electrons Gamma Rays (d) Source of X-rays (iv) Vibration of atoms and molecules (v) LASER (vi) RC circuit Choose the correct answer from the options given below : JEE (MAIN)-2021 Phase-1 (24-02-2021)-E (1) (a)-(ii), (b)-(iv), (c)-(vi), (d)-(iii) (2) (a)-(vi), (b)-(v), (c)-(i), (d)-(iv) (3) (a)-(vi), (b)-(iv), (c)-(i), (d)-(v) (4) (a)-(ii), (b)-(iv), (c)-(i), (d)-(iii) Answer (4) Sol. Source of Gamma Rays is radioactive decay source X-Ray is transition of inner shell electron and that of microwave is magnetron. 15. If one mole of an ideal gas at (P 1 , V 1 ) is allowed to expand reversibly and isothermally (A to B) its pressure is reduced to one-half of the original pressure (see figure). This is followed by a constant volume cooling till its pressure is reduced to one-fourth of the initial value (B ? C). Then it is restored to its initial state by a reversible adiabatic compression (C to A). The net work done by the gas is equal to : P P 1 P 1 2 P 1 4 A B C V 2V 1 V 1 (1) ?? ? ? ? RT 21 (2) ?? ? ?? ?? ?? 1 RT ln2 2( 1) (3) 0 (4) RTln2 Answer (2) Sol. ? ? AB 1 1 WPVln(2) ? ? BC W0 ? ? ? ? ? 11 CA PV W 2( 1) 16. A soft ferromagnetic material is placed in an external magnetic field. The magnetic domains: (1) decrease in size and changes orientation. (2) may increase or decrease in size and change its orientation. (3) increase in size but no change in orientation. (4) have no relation with external magnetic field Answer (2) Sol. Domains aligned in the direction of field increases in size and those aligned in the direction opposite to the field reduces in size. 17. A body weighs 49 N on a spring balance at the north pole. What will be its weight recorded on the same weighing machine, if it is shifted to the equator? [ ? 2 GM Useg R = 9.8 ms –2 and radius of earth, R = 6400 km.] (1) 49.83 N (2) 49 N (3) 48.83 N (4) 49.17 N Answer (3) Sol. g ? = g –?? 2 R Weight at equator will decrease. 18. According to Bohr atom model, in which of the following transitions will the frequency be maximum? (1) n = 3 to n = 2 (2) n = 5 to n = 4 (3) n = 4 to n = 3 (4) n = 2 to n = 1 Answer (4) Sol. Energy in transition is maximum for n = 2 to n = 1, hence the frequency. 19. On the basis of kinetic theory of gases, the gas exerts pressure because its molecules : (1) suffer change in momentum when impinge on the walls of container. (2) continuously stick to the walls of container. (3) continuously lose their energy till it reaches wall. (4) are attracted by the walls of container. Answer (1) Sol. Pressure is due to force exerted by molecules on wall 20. Two electrons each are fixed at a distance ‘2d’. A third charge proton placed at the midpoint is displaced slightly by a distance x(x << d) perpendicular to the line joining the two fixed charges. Proton will execute simple harmonic motion having angular frequency: (m = mass of charged particle) (1) ?? ?? ?? ?? ?? 1 2 2 3 0 q 2md (2) ?? ?? ?? ?? ?? 1 3 2 0 2 2md q (3) ?? ?? ?? ?? ?? 1 2 2 3 0 2q md (4) ?? ?? ?? ?? ?? 1 3 2 0 2 md 2q Answer (1) Page 5 JEE (MAIN)-2021 Phase-1 (24-02-2021)-E PART–A : PHYSICS SECTION - I Multiple Choice Questions: This section contains 20 multiple choice questions. Each question has 4 choices (1), (2), (3) and (4), out of which ONL Y ONE is correct. Choose the correct answer : 1. Zener breakdown occurs in a p-n junction having p and n both (1) Lightly doped and have narrow depletion layer (2) Heavily doped and have narrow depletion layer (3) Heavily doped and have wide depletion layer (4) Lightly doped and have wide depletion layer Answer (2) Sol. Zener diode has both p and n side highly doped and depletion layer is very thin. 2. If the source of light used in a Young’s double slit experiment is changed from red to violet (1) The fringes will become brighter (2) The central bright fringe will become a dark fringe (3) Consecutive fringe lines will come closer (4) The intensity of minima will increase Answer (3) Sol. ? D d ? ?? and ? v < ? red ? Fringe width will decrease. ? Fringe lines will come closer. 3. C A B The logic circuit shown above is equivalent to (1) C A B (2) C A B (3) C A B (4) C A B Answer (1) Sol. From gates diagram, (AB) AB AB ?? ? ? ? ? C A B 4. Which of the following equations represents a travelling wave? (1) y = Asin(15x – 2t) (2) y = Ae x cos( ?t – ?) (3) y = Asinx cos ?t (4) ? ??? 2 x yAe (vt ) Answer (1) Sol. Travelling wave equation, y = f(± kx ± ?t) ? y = A sin(15x – 2t) is a travelling wave. 5. Figure shows a circuit that contains four identical resistors with resistance R = 2.0 ?, two identical inductors with inductance L = 2.0 mH and an ideal battery with emf E = 9 V. The current ‘i’ just after the switch ‘S’ is closed will be A S R R R L R L i E = 9 V (1) 3.37 A (2) 3.0 A (3) 2.25 A (4) 9 A Answer (3) JEE (MAIN)-2021 Phase-1 (24-02-2021)-E Sol. At t = 0, inductor behaves as an open switch. ? 2 ? 9 V 2 ? ? 9 i2.25A 4 ?? 6. An X-ray tube is operated at 1.24 million volt. The shortest wavelength of the produced photon will be (1) 10 –3 nm (2) 10 –2 nm (3) 10 –4 nm (4) 10 –1 nm Answer (1) Sol. min hc eV ?? = 34 8 19 6 6.63 10 3 10 1.6 10 1.24 10 ? ? ??? ?? ? = 1.0 × 10 –12 m = 10 –3 nm 7. The de Broglie wavelength of a proton and ?-particle are equal. The ratio of their velocities is (1) 1 : 4 (2) 4 : 3 (3) 4 : 1 (4) 4 : 2 Answer (3) Sol. p pp h mv ?? ? h mv ? ?? ?? ? ? ? p = ? ? ? m p v p = m ? v ? ? p p v m 4 vm 1 ? ? ?? 8. Given below are two statements : Statement I : PN junction diodes can be used to function as transistor, simply by connecting two diodes, back to back, which acts as the base terminal. Statement II : In the study of transistor, the amplification factor ? indicates ratio of the collector current to the base current. In the light of the above statements, choose the correct answer from the options given below. (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both statement I and Statement II are true Answer (3) Sol. Transistor is formed by formation of two p-n junctions by sandwiching a thin base layer. So, statement I is false. ? C B I I ? ?? ? and for DC, C B I I ?? So, statement II is true. 9. A circular hole of radius ?? ?? ?? a 2 is cut out of a circular disc of radius ‘a’ as shown in figure. The centroid of the remaining circular portion with respect to point ‘O’ will be a/2 a X-axis Y-axis O (1) 1 a 6 (2) 2 a 3 (3) 10 a 11 (4) 5 a 6 Answer (4) Sol. Let mass of hole be (–m), then mass of complete disc will be 4m. a/2 a O C ? Centre of mass w.r.t. point O is CM 3 4m a ( m) a 5 2 Xa (4m m) 6 ?? ?? ? ? ?? ?? ?? ? JEE (MAIN)-2021 Phase-1 (24-02-2021)-E 10. A particle is projected with velocity v 0 along x-axis. A damping force is acting on the particle which is proportional to the square of the distance from the origin i.e. ma = – ?x 2 . The distance at which the particle stops (1) ?? ?? ?? ? ?? 1 2 3 0 3v 2 (2) ?? ?? ?? ? ?? 1 2 2 0 3v 2 (3) ?? ?? ?? ? ?? 1 2 2 0 2v 3 (4) ?? ?? ? ?? 1 3 0 2v 3 Answer (1) Sol. Loss in KE = work against damping ? x 22 0 x0 1 mv x dx 2 ? ?? ? ? 3 2 0 1x mv 23 ? ? ? 1 2 3 0 3mv x 2 ?? ??? ?? ? ?? 11. When a particle executes SHM, the nature of graphical representation of velocity as a function of displacement is : (1) parabolic (2) straight line (3) circular (4) elliptical Answer (4) Sol. ?? ? 22 vA x ?? ? 2 22 2 v xA 12. The period of oscillation of a simple pendulum is ?? L T2 . g Measured value of ‘L’ is 1.0 m from meter scale having a minimum division of 1 mm and time of one complete oscillation is 1.95 s measured from stopwatch of 0.01 s resolution. The percentage error in the determination of ‘g’ will be : (1) 1.30% (2) 1.33% (3) 1.13% (4) 1.03% Answer (3) Sol. ?? 2 2 g4 T ?? dg d 2dT gT dg 1 2 0.01 1.13 g 1000 1.95 100 ? ?? ? 13. In the given figure, a body of mass M is held between two massless spring, on a smooth inclined plane. The free ends of the springs are attached to firm supports. If each spring has spring constant k, the frequency of oscillation of given body is : M k k ? (1) ? 12k 2M (2) ? 1k 22M (3) ?? 12k 2Mgsin (4) ?? 1k 2Mgsin Answer (1) Sol. ?? eq M T2 k ? ? eq k 1 f 2M ? eq k2k 14. Match List-I with List-II. List-I List-II (a) Source of (i) Radioactive decay microwave of nucleus frequency (b) Source of (ii) Magnetron infrared frequency (c) Source of (iii) Inner shell electrons Gamma Rays (d) Source of X-rays (iv) Vibration of atoms and molecules (v) LASER (vi) RC circuit Choose the correct answer from the options given below : JEE (MAIN)-2021 Phase-1 (24-02-2021)-E (1) (a)-(ii), (b)-(iv), (c)-(vi), (d)-(iii) (2) (a)-(vi), (b)-(v), (c)-(i), (d)-(iv) (3) (a)-(vi), (b)-(iv), (c)-(i), (d)-(v) (4) (a)-(ii), (b)-(iv), (c)-(i), (d)-(iii) Answer (4) Sol. Source of Gamma Rays is radioactive decay source X-Ray is transition of inner shell electron and that of microwave is magnetron. 15. If one mole of an ideal gas at (P 1 , V 1 ) is allowed to expand reversibly and isothermally (A to B) its pressure is reduced to one-half of the original pressure (see figure). This is followed by a constant volume cooling till its pressure is reduced to one-fourth of the initial value (B ? C). Then it is restored to its initial state by a reversible adiabatic compression (C to A). The net work done by the gas is equal to : P P 1 P 1 2 P 1 4 A B C V 2V 1 V 1 (1) ?? ? ? ? RT 21 (2) ?? ? ?? ?? ?? 1 RT ln2 2( 1) (3) 0 (4) RTln2 Answer (2) Sol. ? ? AB 1 1 WPVln(2) ? ? BC W0 ? ? ? ? ? 11 CA PV W 2( 1) 16. A soft ferromagnetic material is placed in an external magnetic field. The magnetic domains: (1) decrease in size and changes orientation. (2) may increase or decrease in size and change its orientation. (3) increase in size but no change in orientation. (4) have no relation with external magnetic field Answer (2) Sol. Domains aligned in the direction of field increases in size and those aligned in the direction opposite to the field reduces in size. 17. A body weighs 49 N on a spring balance at the north pole. What will be its weight recorded on the same weighing machine, if it is shifted to the equator? [ ? 2 GM Useg R = 9.8 ms –2 and radius of earth, R = 6400 km.] (1) 49.83 N (2) 49 N (3) 48.83 N (4) 49.17 N Answer (3) Sol. g ? = g –?? 2 R Weight at equator will decrease. 18. According to Bohr atom model, in which of the following transitions will the frequency be maximum? (1) n = 3 to n = 2 (2) n = 5 to n = 4 (3) n = 4 to n = 3 (4) n = 2 to n = 1 Answer (4) Sol. Energy in transition is maximum for n = 2 to n = 1, hence the frequency. 19. On the basis of kinetic theory of gases, the gas exerts pressure because its molecules : (1) suffer change in momentum when impinge on the walls of container. (2) continuously stick to the walls of container. (3) continuously lose their energy till it reaches wall. (4) are attracted by the walls of container. Answer (1) Sol. Pressure is due to force exerted by molecules on wall 20. Two electrons each are fixed at a distance ‘2d’. A third charge proton placed at the midpoint is displaced slightly by a distance x(x << d) perpendicular to the line joining the two fixed charges. Proton will execute simple harmonic motion having angular frequency: (m = mass of charged particle) (1) ?? ?? ?? ?? ?? 1 2 2 3 0 q 2md (2) ?? ?? ?? ?? ?? 1 3 2 0 2 2md q (3) ?? ?? ?? ?? ?? 1 2 2 3 0 2q md (4) ?? ?? ?? ?? ?? 1 3 2 0 2 md 2q Answer (1) JEE (MAIN)-2021 Phase-1 (24-02-2021)-E Sol. F net = 2F cos ? ? F d +q F x d +q ? ?? ? 2 22 0 q F 4(x d) ? ?? ? 2 net 223/2 0 2q x F 4(x d) ?? ?? 2 2 3 0 qx –mx 2d ?? ?? 2 2 3 0 q 2md SECTION - II Numerical Value Type Questions: This section contains 10 questions. In Section II, attempt any five questions out of 10. The answer to each question is a NUMERICAL VALUE. For each question, enter the correct numerical value (in decimal notation, truncated/rounded-off to the second decimal place; e.g. 06.25, 07.00, –00.33, –00.30, 30.27, –27.30) using the mouse and the on-screen virtual numeric keypad in the place designated to enter the answer. 1. The root mean square speed of molecules of a given mass of a gas at 27°C and 1 atmosphere pressure is 200 ms –1 . The root mean square speed of molecules of the gas at 127°C and 2 atmosphere pressure is –1 x ms 3 . The value of x will be_______. Answer (400) Sol. u rms = 3RT M 2 1 u u = 2 1 T T u 2 = ? ? 1 273 127 .u 273 27 = 2 .200 3 = 400 m/s 3 2. An electromagnetic wave of frequency 3 GHz enters a dielectric medium of relative electric permittivity 2.25 from vacuum. The wavelength of this wave in that medium will be ______× 10 –2 cm. Answer (667) Sol. Assuming relative permeability of the medium to be 1. n = ?? rr = 1.5 u = ?? 8 C 210m/s n ? = 8 9 u210 m f 310 ? ? ? = 666.67 × 10 –2 cm 3. A point charge of + 12 ?C is at a distance 6 cm vertically above the centre of a square of side 12 cm as shown in figure. The magnitude of the electric flux through the square will be ______ × 10 3 Nm 2 /C. 6 cm 12 cm 12 cm +q Answer (226) Sol. r r a/2 a Q ? = ? 0 Q 6 = ? ??? ? –6 9 12 10 4910 6 = 226.28 × 10 3 2 N-m C 4. Two solids A and B of mass 1 kg and 2 kg respectively are moving with equal linear momentum. The ratio of their kinetic energies (K.E) A : (K.E.) B will be A 1 , so the value of A will be________. Answer (2)Read More

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