#neet
- A long solenoid of diameter 0.1 m has 2×10^4 turns per meter. At the centre of the solenoid, a coil of 100 turns and radius 0.01 m is placed with its axis coinciding with the solenoid axis. The current in the solenoid reduces at a constant rate to 0A… Answer: 2. 32 μ C Key idea: charge q=ΔΦ/R (independent of the time taken). Field inside solenoid B=μ₀nI . Change: Δ B=4π×10⁻⁷×2×10⁴×4=0.032π... 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
- At what temperature will the rms speed of oxygen molecules become just sufficient for escaping from the Earth's atmosphere ?(Given :Mass of oxygen molecule (m) = 2.76×10^-26 kgBoltzmann's constant k_B = 1.38×10^-23 J K^-1) Answer: 3. 8.360×10⁴ K The molecules escape only if their rms speed equals the escape speed of the Earth, v_e = 11.2\ mathrmkm s⁻¹ = 1.12×10... 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
- 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
- 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 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
- An inductor 20 mH, a capacitor 100 μF and a resistor 50 Ω are connected in series across a source of emf, V = 10 sin 314 t. The power loss in the circuit is Answer: 1. 0.79 W Power is lost only in the resistor: P=I_rms²R , with I_rms=V_rms/Z . X_L=ω L=314×0.02=6.28\ Ω ; X_C=1/ω C=dfrac1314×10⁻⁴=3... 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 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
- 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
- A 250 - Turn rectangular coil of length 2.1 cm and width 1.25 cm carries a current of 85 μA and subjected to a magnetic field of strength 0.85 T. Work done for rotating the coil by 180^° against the torque is : Answer: 4. 9.1 μ J Key idea: work done in rotating a coil from θ=0 to 180^° is W=MB(cos0-cos180)=2MB , with M=NIA . A=2.1×10⁻²×1.25×10⁻²=2.6... 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
- 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 ratio of resolving powers of an optical microscope for two wavelengths λ_1 = 4000 and λ_2 = 6000 is : Answer: 2. 3 : 2 Key idea: resolving power of a microscope is ∝dfrac1λ . R₁/R₂=λ₂/λ₁=6000/4000=3/2 , i.e. 3 : 2. 1
- A thin prism having refracting angle 10^° is made of glass of refractive index 1.42. This prism is combined with another thin prism of glass of refractive index 1.7. This combination produces dispersion without deviation. The refracting angle of seco… Answer: 1. 6 ^° Key idea: dispersion without deviation means the net deviation is zero: (μ₁-1)A₁=(μ₂-1)A₂ . (1.42-1)×10=(1.7-1)A₂⇒ 4.2=0.7A₂... 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
- 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
- Thermodynamic processes are indicated in the following diagram.Match the following :Column-1: P. Process I; Q. Process II; R. Process III; S. Process IVColumn-2: a. Adiabatic; b. Isobaric; c. Isochoric; d. Isothermal Answer: 1. P → c, Q → a, R → d, S → b Key idea: identify each process from the P-V diagram. I: vertical line, volume constant, isochoric (c)... 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
- The two nearest harmonics of a tube closed at one end and open at other end are 220 Hz and 260 Hz. What is the fundamental frequency of the system ? Answer: 1. 20 Hz Key idea: a tube closed at one end has only odd harmonics: f₀,3f₀,5f₀,dots . Consecutive harmonics differ by 2f₀ . 260-220=... 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
- A spherical black body with a radius of 12 cm radiates 450 watt power at 500 K. If the radius were halved and the temperature doubled, the power radiated in watt would be : Answer: 3. 1800 Key idea: power radiated P=σ AT⁴∝ r²T⁴ . Radius halved gives factor 1/4 ; temperature doubled gives factor 2⁴=16 . P'=450×1/... 1