Practice Questions

Q9. A stationary observer receives sound from two identical tuning forks, one of which approaches and the other one recedes with the same speed (much less than the speed of sound). The observer hears 2 beats/sec . The oscillation frequency of each tuning fork is v0 = 1400 Hz and the velocity of sound in air is 350 m/s . The speed of each tuning fork is close to: (1) 1 m/s (2) 1m/s 2 (3) 4 1 m/s (4) 81 m/s →

Q9. Two identical cylindrical vessels are kept on the ground and each contain the same liquid of density d. The area of the base of both vessels is S but the height of liquid in one vessel is x1 and in the other x2 . When both cylinders are connected through a pipe of negligible volume very close to the bottom, the liquid flows from one vessel to the other until it comes to equilibrium at a new height. The change in energy of the system in the process is : (1) gdS(x22 + x21) (2) gdS(x2 + x1)2 (3) 4 3 gdS(x2 −x1)2 (4) 41 gdS(x2 −x1)2

Q9. Two infinite planes each with uniform surface charge density +σ are kept in such a way that the angle between them is 30o . The electric field in the region shown between them is given by: (1) + 2∈0 σ [(1 √3)ˆy −ˆx2 ] (2) ∈0σ [(1 + √32 )ˆy + ˆx2 ] (3) + 2∈0 σ [(1 √3)ˆy + ˆx2 ] (4) 2∈0σ [(1 −√32 )ˆy −ˆx2 ]

Q9. For a transvers wave travelling, along a straight line, the distance between two peaks (crests) is 5 m , while the distance between one crest and one trough is 1. 5 m. The possible wavelengths (in m ) of the waves are: (1) 1, 3, 5 (2) 11 , 13 , 51 , . . . . . (3) 1, 2, 3, . . . . (4) 12 , 14 , 61 , . . . . .

Q10.A 10 μF capacitor is fully charged to a potential difference of 50 V After removing the source voltage it is connected to an uncharged capacitor in parallel. Now the potential difference across them becomes 20 V . The capacitance of the second capacitor is : (1) 15 μF (2) 30 μF (3) 20 μF (4) 10 μF

Q10.Two gases - argon (atomic radius 0.07nm, atomic weight 40 ) and xenon (atomic radius 0.1nm, atomic weight 140 ) have the same number density and are at the same temperature. The ratio of their respective mean free times is closest to: (1) 3.67 (2) 1.83 (3) 2.3 (4) 4.67

Q10.Charges Q1 and Q2 are at points A and B of a right-angled triangle OAB. The resultant electric field at point O is perpendicular to the hypotenuse, then Q1/Q2 is proportional to: (1) x31 (2) x2 x32 x1 (3) x1 (4) x22 x2 x21

Q10.Three charged particles A , B and C with charges −4q, 2q and −2q are present on the circumference of a circle of radius d. The charged particles A, C and centre O of the circle formed an equilateral triangle as shown JEE Main 2020 (08 Jan Shift 1) JEE Main Previous Year Paper in the figure. The electric field at the point O is (1) √3q (2) 2√3q πε0d2 πε0d2 (3) √3q (4) 3√3q 4πε0d2 4πε0d2 →

Q10.A block of mass m attached to a massless spring is performing oscillatory motion of amplitude 'A' on a frictionless horizontal plane. If half of the mass of the block breaks off when it is passing through its equilibrium point, the amplitude of oscillation for the remaining system become ƒA. The value of ƒ is: (1) 1 (2) 1 √2 (3) 1 (4) √2 2

Q10.Two charged thin infinite plane sheets of uniform charge density σ+ and σ− , where |σ+| > |σ−|, intersect at the right angle. Which of the following best represents the electric field lines for the system: (1) (2) (3) (4)

Q11.In a building there are 15 bulbs of 45W , 15 bulbs of 100W, 15 small fans of 10W and 2 heaters of 1kW . The voltage of electric main supply is 220V . The minimum fuse capacity (rated value) of the building will be: (1) 5A (2) 25A (3) 15A (4) 20A

Q11.An electric dipole of moment →p = (−ˆi −3ˆj 2ˆk) to this dipole at→r= +ˆi + 3ˆj + 5ˆk (note that →r∙→p = 0 ) is parallel to: (1) (+ˆi −3ˆj −2ˆk) (2) (−ˆi + 3ˆj −2ˆk) + (3) (+ˆi + 3ˆj −2ˆk) (4) (−ˆi −3ˆj 2ˆk)

Q11.In a resonance tube experiment when the tube is filled with water up to a height of 17. 0 cm , from bottom, it resonates with a given tuning fork. When the water level is raised the next resonance with the same tuning fork occurs at a height of 24. 5 cm . If the velocity of sound in air is 330 m s−1, the tuning fork frequency is : (1) 2200 Hz (2) 550 Hz (3) 1100 Hz (4) 3300 Hz

Q11.A wire of length L and mass per unit length 6.0 × 10−3 kg m−1 is put under tension of 540 N. Two consecutive frequencies that it resonates at are: 420 Hz and 490 Hz. Then L in meters is : (1) 2.1m (2) 1.1m (3) 8.1m (4) 5.1m

Q11.Two identical strings X and Z made of same material have tension TX and TZ in then if their fundamental frequencies are 450 Hz and 300 Hz, respectively, then the ratio TX/TZ is : (1) 2. 25 (2) 0. 44 (3) 1. 25 (4) 1. 5

Q11.In finding the electric field using Gauss law the formula E = qenc is applicable. In the formula ε0 is ε0|A| permittivity of free space, A is the area of Gaussian surface and qenc is charge enclosed by the Gaussian surface. This equation can be used in which of the following situation? (1) Only when the Gaussian surface is an (2) Only when the Gaussian surface is an → equipotential surface equipotential surface and E is constant on the surface. → (3) (4) For any choice of Gaussian surface. Only when E = constant on the surface.

Q11.Consider two charged metallic spheres S1 and S2 of radii R1 and R2, respectively. The electric fields E1 (on S1 ) and E2 (on S2 ) on their surfaces are such that E1 = R1 . Then the ratio V1 (on S1 ) /V2 (on S2 ) of the E2 R2 electrostatic potentials on each sphere is: (1) R1 (2) R1 2 R2 ( R2 ) (3) 3 ( R2R1 ) (4) ( R1R2 )

Q11.The driver of bus approaching a big wall notices that the frequency of his bus's horn changes from 420 Hz to 490 Hz when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is 330 ms–1 : (1) 91 kmh−1 (2) 81 kmh−1 (3) 61 kmh−1 (4) 71 kmh−1

Q11.Model a torch battery of length l to be made up of a thin cylindrical bar of radius a and a concentric thin cylindrical shell of radius b filled in between with an electrolyte of resistivity ρ (see figure). If the battery is connected to a resistance of value R, the maximum Joule heating in R will take place for : JEE Main 2020 (03 Sep Shift 1) JEE Main Previous Year Paper (1) R = 2πℓ(ρ ab ) (2) R = 2πℓln(ρ ba ) (3) R = πℓIn(ρ ab ) (4) R = 2ρπℓln( ba )

Q11.The current I1 (in A ) flowing through 1Ω resistor in the following circuit is: (1) 0.4 (2) 0.5 (3) 0.2 (4) 0.25

Q11.A two point charges 4q and –q are fixed on the x -axis at x = −d2 and x = d2 , respectively. If the third point charge ‘ q ’ is taken from the origin to x = d along the semicircle as shown in the figure, the energy of the charge will: JEE Main 2020 (04 Sep Shift 1) JEE Main Previous Year Paper (1) Increase by 3q2 (2) Increase by 2q2 4π∈0d 3π∈0d (3) decrease by q2 (4) decrease by 4q2 4π∈0d 3π∈0d

Q11.In the circuit shown, charge on the 5μF capacitor is : (1) 18. 00μC (2) 10. 90μC (3) 16. 36μC (4) 5. 45μC

Q11.For the given input voltage waveform Vin (t), the output voltage waveform V0(t), across the capacitor is correctly depicted by : (1) (2) (3) (4) towards a large screen placed in the

Q11.Concentric metallic hollow spheres of radii R and 4R hold charges Q1 and Q2 respectively. Given that surface charge densities of the concentric spheres are equal, the potential difference V(R) −V(4R) is: (1) Q2 (2) 3Q2 4πε0R 4πε0R (3) 3Q1 (4) 3Q1 16πε0R 4πε0R

Q12.A parallel plate capacitor has plate of length l, width w and separation of plates is d. It is connected to a battery of emf V . A dielectric slab of the same thickness d and of dielectric constant K = 4 is being inserted between the plates of the capacitor. At what length of the slab inside plates, will the energy stored in the capacitor be two times the initial energy stored? (1) 2l (2) l 3 3 (3) l (4) l 4 2