#neet
- The Sun rotates around its centre once in 27 days. What will be the period of revolution if the Sun were to expand to twice its present radius without any external influence? Assume the Sun to be a sphere of uniform density. Answer: 4. 108 days Key idea: no external torque, so angular momentum L=Iω is conserved. For a uniform sphere, I=2/5MR² . Mass stays the sam... 1
- A model for quantized motion of an electron in a uniform magnetic field B states that the flux passing through the orbit of the electron is n(h/e) where n is an integer, h is Planck's constant and e is the magnitude of electron's charge. According to… Answer: 2. he/2π m Key idea: the electron moves in a circle in the field, and the given flux quantization fixes the orbit area. From evB=mv²... 1
- Three identical heat conducting rods are connected in series as shown in the figure. The rods on the sides have thermal conductivity 2K while that in the middle has thermal conductivity K. The left end of the combination is maintained at temperature… Answer: 3. 5/3 Key idea: in steady state the same heat current flows through all three rods, so kA Δ T/l is equal for each. With equal A and... 1
- The plates of a parallel plate capacitor are separated by d. Two slabs of different dielectric constant K_1 and K_2 with thickness 3/8d and d/2, respectively are inserted in the capacitor. Due to this, the capacitance becomes two times larger than wh… Answer: 1. 2.66 Key idea: for slabs inserted in a capacitor, C=ε₀A/d-t₁-t₂+t₁/K₁+t₂/K₂ . Here t₁=3/8d , t₂=1/2d , and C=2C₀ , so the denomin... 1
- Two cities X and Y are connected by a regular bus service with a bus leaving in either direction every T min. A girl is driving scooty with a speed of 60 km/h in the direction X to Y notices that a bus goes past her every 30 minutes in the direction… Answer: 4. 15 min, 120 km/h Key idea: buses in one direction are spaced vT apart. The girl meets them at the rate set by the relative speed.... 1
- A uniform rod of mass 20 kg and length 5 m leans against a smooth vertical wall making an angle of 60^° with it. The other end rests on a rough horizontal floor. The friction force that the floor exerts on the rod is (take g = 10 m/s^2) Answer: 2. 100√(3) N Key idea: the wall is smooth, so the wall force N is horizontal. For equilibrium, floor friction f equals N , and torqu... 1
- In an oscillating spring mass system, a spring is connected to a box filled with sand. As the box oscillates, sand leaks slowly out of the box vertically so that the average frequency ω(t) and average amplitude A(t) of the system change with time t.… Answer: 2. Graph pair: ω(t) rises and saturates; A(t) decays Key idea: the frequency of a spring-mass system is ω=√(k/m) , and sand leaking... 1
- A balloon is made of a material of surface tension S and its inflation outlet (from where gas is filled in it) has small area A. It is filled with a gas of density ρ and takes a spherical shape of radius R. When the gas is allowed to flow freely out… Answer: 3. a=-1/2,α=-1/2,β=-1,γ=1/2,δ=7/2 Key idea: the excess pressure inside the balloon is proportional to S/r (it is 4S/r in the standar... 1
- Consider the diameter of a spherical object being measured with the help of a Vernier callipers. Suppose its 10 Vernier Scale Divisions (V.S.D.) are equal to its 9 Main Scale Divisions (M.S.D.). The least division in the M.S. is 0.1 cm and the zero o… Answer: 3. 4.98 cm Key idea: reading = MSR + (VSR × least count) - zero error. 10 VSD = 9 MSD, so the least count =1 MSD -1 VSD =0.1-0.09=0.... 1
- A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to displacement current is : Answer: 3. non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates Key idea: a changing e... 1
- An unpolarized light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster's angle. Then- Answer: 1. reflected light is completely polarized and the angle of reflection is close to 60^° Key idea: at Brewster's angle tan i_B=μ , an... 1
- Two identical charged conducting spheres A and B have their centres separated by a certain distance. Charge on each sphere is q and the force of repulsion between them is F. A third identical uncharged conducting sphere is brought in contact with sph… Answer: 4. 3F/8 Key idea: identical conducting spheres share charge equally on contact. Third sphere touches A: A and C each have q/2 . Then... 1
- A container has two chambers of volumes V_1 = 2 litres and V_2 = 3 litres separated by a partition made of a thermal insulator. The chambers contains n_1 = 5 and n_2 = 4 moles of ideal gas at pressures p_1 = 1 atm and p_2 = 2 atm, respectively. When… Answer: 2. 1.6 atm Key idea: no heat or work leaves the container, so the total internal energy is conserved. For the same gas, this gives n... 1
- A particle of mass m is moving around the origin with a constant force F pulling it towards the origin. If Bohr model is used to describe its motion, the radius r of the n^th orbit and the particle's speed v in the orbit depend on n as Answer: 3. r ∝ n^(2/3);\ v ∝ n^(1/3) Key idea: apply Newton's second law with the constant force as the centripetal force, together with Boh... 1
- The radius of Martian orbit around the Sun is about 4 times the radius of the orbit of Mercury. The Martian year is 687 Earth days. Then which of the following is the length of 1 year on Mercury ? Answer: 1. 88 earth days Key idea: Kepler's third law, T²∝ r³ . fracT_MarsT_Mercury=(fracr_Marsr_Mercury)^(3/2)=4^(3/2)=8 T_Mercury=687/8≈86... 1
- A body weighs 48 N on the surface of the earth. The gravitational force experienced by the body due to the earth at a height equal to one-third the radius of the earth from its surface is : Answer: 2. 27 N Key idea: weight at height h varies as 1/(R+h)² . F=48(R/R+R/3)²=48×(3/4)²=48×frac916=27\ mathrm N 1
- A wire of resistance R is cut into 8 equal pieces. From these pieces two equivalent resistances are made by adding four of these together in parallel. Then these two sets are added in series. The net effective resistance of the combination is : Answer: 3. R/16 Key idea: cutting a wire into equal parts divides its resistance equally. Each piece has resistance R/8 . Four in parallel g... 1
- De-Broglie wavelength of an electron orbiting in the n = 2 state of hydrogen atom is close to (Given Bohr radius = 0.052 nm) Answer: 2. 0.67 nm Key idea: Bohr's quantization 2π r_n=nλ (electron wave fits the orbit). Radius of the n=2 orbit: r₂=n²a₀=4×0.052=0.208 nm... 1
- An electric dipole with dipole moment 5×10^-6 Cm is aligned with the direction of a uniform electric field of magnitude 4×10^5 N/C. The dipole is then rotated through an angle of 60^° with respect to the electric field. The change in the potential en… Answer: 2. 1.0 J Key idea: potential energy of a dipole is U=-pEcosθ . Initially θ=0^° and then 60^° : Δ U=pE(cos0^°-cos60^°)=5×10⁻⁶×4×10⁵×1... 1
- A constant voltage of 50 V is maintained between the points A and B of the circuit shown in the figure. The current through the branch CD of the circuit is : Answer: 2. 2.0 A Key idea: C and D are joined by a wire, so they are at the same potential. The circuit becomes two parallel pairs in series... 1
- A photon and an electron (mass m) have the same energy E. The ratio ≤ft(λ_photon/λ_electron) of their de Broglie wavelengths is: (c is the speed of light) Answer: 3. c√(2m/E) Key idea: λ=h/p , with different momentum-energy relations for photon and electron. Photon: E=pc⇒λ_ph=hc/E . Electron: E... 1
- Which of the following options represent the variation of photoelectric current with property of light shown on the x-axis?A. Photoelectric current vs Intensity of light: straight line through origin risingB. Photoelectric current vs Intensity of lig… Answer: 1. A only Key idea: at a fixed frequency above the threshold, the photoelectric current is directly proportional to the intensity. A... 1
- A sphere of radius R is cut from a larger solid sphere of radius 2R as shown in the figure. The ratio of the moment of inertia of the smaller sphere to that of the rest part of the sphere about the Y-axis is : Answer: 3. 7/57 Key idea: moment of inertia of the remaining part = full sphere - removed sphere, using the parallel-axis theorem for the cu... 1
- Consider a water tank shown in the figure. It has one wall at x = L and can be taken to be very wide in the z direction. When filled with a liquid of surface tension S and density ρ, the liquid surface makes angle θ_0(θ_0 1) with the x-axis at x = L.… Answer: 2. d²y/dx²=ρ g/Sy Key idea: the surface tension pulls the curved surface so that, for each small piece, the net vertical force balan... 1