NEET UG
- A metallic rod of mass per unit length 0.5 kg m^-1 is lying horizontally on a smooth inclined plane which makes an angle of 30^° with the horizontal. The rod is not allowed to slide down by flowing a current through it when a magnetic field of induct… Answer: 4. 11.32 A For the rod to stay at rest on a smooth incline, the horizontal magnetic force must balance the component of weight down... 1
- A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational po… Answer: 1. the current source A static magnetic field exerts a force perpendicular to the motion of charges, so it does no net work itself.... 1
- Current sensitivity of a moving coil galvanometer is 5 div/mA and its voltage sensitivity (angular deflection per unit voltage applied) is 20 div/V. The resistance of the galvanometer is Answer: 2. 250 Ω Current sensitivity I_s=θ/I and voltage sensitivity V_s=θ/V . Since V=IG , V_s=I_s/G . G=I_s/V_s=5\ div/mA/20\ div/V=frac5\... 1
- A particle executes linear simple harmonic motion with an amplitude of 3 cm. When the particle is at 2 cm from the mean position, the magnitude of its velocity is equal to that of its acceleration. Then its time period in seconds is : Answer: 2. 4π/sqrt5 Key idea: in SHM v=ω√(A²-x²) and a=ω²x . At x=2 cm, A=3 cm: ω√(9-4)=ω²×2⇒ω=sqrt5/2 s ⁻¹ . T=2π/ω=4π/sqrt5 s. 1
- The fundamental frequency in an open organ pipe is equal to the third harmonic of a closed organ pipe. If the length of the closed organ pipe is 20 cm, the length of the open organ pipe is Answer: 1. 13.2 cm Closed pipe: odd harmonics only, f_n=nv/4L . Third harmonic: f=3v/4× 20 . Open pipe fundamental: f₁=v/2L' . Equating: v/2... 1
- A U tube with both ends open to the atmosphere, is partially filled with water. Oil, which is immiscible with water, is poured into one side until it stands at a distance of 10 mm above the water level on the other side. Meanwhile the water rises by… Answer: 3. 928 kg m ⁻³ Key idea: pressure at the same level in the same liquid is equal. Take points B (oil-water interface, left) and C (sa... 1
- The volume (V) of a monatomic gas varies with its temperature (T), as shown in the graph. The ratio of work done by the gas, to the heat absorbed by it, when it undergoes a change from state A to state B, is Answer: 1. 2/5 The graph is a straight line through the origin, so V∝ T : the process is at constant pressure. For a monatomic gas, W=nRΔ T... 1
- A carnot engine having an efficiency of 1/10 as heat engine, is used as a refrigerator. If the work done on the system is 10 J, the amount of energy absorbed from the reservoir at lower temperature is : Answer: 1. 90 J Key idea: as a refrigerator, coefficient of performance =Q₂/W=1-η/η . η=tfrac110⇒Q₂/W=9/10/1/10=9 . Q₂=9×10=90 J absorbed fr... 1
- The efficiency of an ideal heat engine working between the freezing point and boiling point of water, is Answer: 1. 26.8% Efficiency of an ideal (Carnot) engine is η=1-T₂/T₁ , with temperatures in kelvin. T₂=273 K (freezing point), T₁=373 K (boi... 1
- Consider a drop of rain water having mass 1 g falling from a height of 1 km. It hits the ground with a speed of 50 m/s. Take 'g' constant with a value 10 m/s^2. The work done by the (i) gravitational force and the (ii) resistive force of air is : Answer: 3. (i) 10 J (ii) - 8.75 J Key idea: use work-energy theorem: W_g+W_air=Δ KE . (i) W_g=mgh=0.001×10×1000=10 J. Δ KE=1/2(0.001)(50)²=1... 1
- The ratio of wavelengths of the last line of Balmer series and the last line of Lyman series is : Answer: 2. 4 Key idea: the last line of a series has n₂=∞ : dfrac1λ=RZ²(dfrac1n₁²-0) . Balmer ( n₁=2 ): dfrac1λ_B=R/4 . Lyman ( n₁=1 ): dfra... 1
- A potentiometer is an accurate and versatile device to make electrical measurements of E.M.F. because the method involves : Answer: 2. a condition of no current flow through the galvanometer Key idea: at the balance point no current flows through the galvanometer,... 1
- The photoelectric threshold wavelength of silver is 3250×10^-10 m. The velocity of the electron ejected from a silver surface by ultraviolet light of wavelength 2536×10^-10 m is :(Given h = 4.14×10^-15 eVs and c = 3×10^8 ms^-1) Answer: Dropped in the official answer key: full marks to every candidate Key idea: 1/2mv²=hc/λ-hc/λ₀ . hc=4.14×10⁻¹⁵×3×10⁸=1242 eV nm. Phot... 1
- Figure shows a circuit that contains three identical resistors with resistance R = 9.0 Ω each, two identical inductors with inductance L = 2.0 mH each, and an ideal battery with emf ε = 18 V. The current 'i' through the battery just after the switch… Answer: Dropped in the official answer key: full marks to every candidate Key idea: just after the switch closes, an inductor acts as an ope... 1
- Which one of the following represents forward bias diode ? Answer: 4. diode with 0 V on left, R, -2 V on right Key idea: a diode is forward biased when its p-side (the triangle base) is at higher pot... 1
- Suppose the charge of a proton and an electron differ slightly. One of them is -e, the other is (e+Δ e). If the net of electrostatic force and gravitational force between two hydrogen atoms placed at a distance d (much greater than atomic size) apart… Answer: 2. 10⁻³⁷ C Key idea: each hydrogen atom then has a net charge Δ e , and the electric repulsion balances gravity. (Δ e)²/4πε₀d²=Gm_h²... 1
- The diagrams below show regions of equipotentials.A positive charge is moved from A to B in each diagram. Answer: 1. In all the four cases the work done is the same. Key idea: work done by an external agent moving a charge slowly is W=q(V_B-V_A)... 1
- A capacitor is charged by a battery. The battery is removed and another identical uncharged capacitor is connected in parallel. The total electrostatic energy of resulting system : Answer: 1. decreases by a factor of 2 Key idea: charge is conserved when the second capacitor is connected, but energy is not. Initially U=Q... 1
- The acceleration due to gravity at a height 1 km above the earth is the same as at a depth d below the surface of earth. Then : Answer: 3. d = 2 km Key idea: at height h (small): g'=g(1-2h/R) . At depth d : g'=g(1-d/R) . Equating: 2h/R=d/R⇒ d=2h=2 km. 1
- Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t_1. On other days, if she remains stationary on the moving escalator, then the escalator takes her up in time t_2. The time… Answer: 2. t₁t₂/t₂+t₁ Key idea: add speeds. Let the stair length be L . Her walking speed is L/t₁ , escalator speed is L/t₂ . Combined speed... 1
- A gas mixture consists of 2 moles of O_2 and 4 moles of Ar at temperature T. Neglecting all vibrational modes, the total internal energy of the system is : Answer: 3. 11 RT Key idea: internal energy U=f/2nRT . O₂ is diatomic ( f=5 , vibration neglected): U=5/2×2RT=5RT . Ar is monatomic ( f=3 ):... 1
- One end of string of length l is connected to a particle of mass 'm' and the other end is connected to a small peg on a smooth horizontal table. If the particle moves in circle with speed 'v', the net force on the particle (directed towards center) w… Answer: 4. T Key idea: only the string tension acts in the horizontal plane. Weight and normal reaction cancel vertically. So the net force... 1
- Two blocks A and B of masses 3m and m respectively are connected by a massless and inextensible string. The whole system is suspended by a massless spring as shown in figure. The magnitudes of acceleration of A and B immediately after the string is c… Answer: 1. g/3 , g Key idea: the spring force cannot change instantly, but the string tension vanishes at once. Before cutting, the spring h... 1
- If θ_1 and θ_2 be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip θ is given by : Answer: 4. cot²θ=cot²θ₁+cot²θ₂ Key idea: in a vertical plane at angle α to the magnetic meridian, tanθ'=tanθ/cosα , i.e. cotθ'=cotθcosα . Fo... 1