#neet
- Water falls from a height of 60 m at the rate of 15 kg/s to operate a turbine. The losses due to frictional force are 10% of the input energy. How much power is generated by the turbine ? (g = 10 m/s^2) Answer: 2. 8.1 kW Key idea: power input = (mass per second) × g × h; the turbine gets 90% of it. P_in=15×10×60=9000\ mathrm W P_out=0.9×9000... 1
- A small block slides down on a smooth inclined plane, starting from rest at time t = 0. Let S_n be the distance travelled by the block in the interval t = n − 1 to t = n. Then, the ratio S_nS_n+1 is : Answer: 2. 2n-1/2n+1 Key idea: distance in the n-th second from rest is S_n=1/2a(2n-1) . S_n∝(2n-1) and S_(n+1)∝(2n+1) , so dfracS_nS_(n+1)=... 1
- Polar molecules are the molecules : Answer: 4. having a permanent electric dipole moment. Polar molecules (such as H₂O , HCl) have centres of positive and negative charge that... 1
- A thick current carrying cable of radius 'R' carries current 'I' uniformly distributed across its cross-section. The variation of magnetic field B(r) due to the cable with the distance 'r' from the axis of the cable is represented by : Answer: 3. Graph (3): B vs r, linear rise to a peak then decreasing Key idea: use Ampere's law inside and outside the cable. Inside ( r Outs... 1
- In a potentiometer circuit a cell of EMF 1.5 V gives balance point at 36 cm length of wire. If another cell of EMF 2.5 V replaces the first cell, then at what length of the wire, the balance point occurs ? Answer: 1. 60 cm Key idea: in a potentiometer, EMF ∝ balancing length. E₂/E₁=l₂/l₁⇒ l₂=36×2.5/1.5=60 cm. Option 1. 1
- Two charged spherical conductors of radius R_1 and R_2 are connected by a wire. Then the ratio of surface charge densities of the spheres (σ_1/σ_2) is : Answer: 2. R₂/R₁ Key idea: connected conductors are at the same potential. V=kQ/R=kσ4π R²/R=4π kσ R . Equal V gives σ₁R₁=σ₂R₂ , so σ₁/σ₂=R₂/... 1
- An electromagnetic wave of wavelength 'λ' is incident on a photosensitive surface of negligible work function. If 'm' mass is of photoelectron emitted from the surface has de-Broglie wavelength λ_d, then : Answer: 3. λ = (2mc/h)λ_d² Key idea: with negligible work function, the photoelectron's kinetic energy equals the photon energy. hc/λ=p²/2m⇒... 1
- The electron concentration in an n-type semiconductor is the same as hole concentration in a p-type semiconductor. An external field (electric) is applied across each of them. Compare the currents in them. Answer: 3. current in n-type > current in p-type. Key idea: I∝ n μ E , and the same concentration means the current depends on mobility. In... 1
- The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of 3.3×10^-3 watt will be : (h = 6.6×10^-34 Js) Answer: 3. 10¹⁶ Key idea: number of photons per second N=P/hc/λ=Pλ/hc . N=dfrac3.3×10⁻³×600×10⁻⁹6.6×10⁻³⁴×3×10⁸=dfrac1.98×10⁻⁹1.98×10⁻²⁵=10¹... 1
- If E and G respectively denote energy and gravitational constant, then E/G has the dimensions of : Answer: 1. [M²][L⁻¹][T⁰] Key idea: get the dimensions of G from F=Gm₁m₂/r² . [G]=M⁻¹L³T⁻² and [E]=ML²T⁻² . E/G=dfracML²T⁻²M⁻¹L³T⁻²=M²L⁻¹T⁰ .... 1
- The effective resistance of a parallel connection that consists of four wires of equal length, equal area of cross-section and same material is 0.25 Ω. What will be the effective resistance if they are connected in series ? Answer: 4. 4 Ω Key idea: four equal resistors R: parallel gives R/4 , series gives 4R . R/4=0.25\ Ω⇒ R=1\ Ω . In series: 4R=4\ Ω . Option 4. 1
- The half-life of a radioactive nuclide is 100 hours. The fraction of original activity that will remain after 150 hours would be : Answer: 2. 1/2√(2) Key idea: activity decays as A=A₀(1/2)^t/T_(1/2) . dfrac AA₀=(1/2)^(150/100)=(1/2)^(3/2)=dfrac12sqrt2 . Option 2. 1
- A nucleus with mass number 240 breaks into two fragments each of mass number 120, the binding energy per nucleon of unfragmented nuclei is 7.6 MeV while that of fragments is 8.5 MeV. The total gain in the Binding Energy in the process is : Answer: 4. 216 MeV Key idea: gain in binding energy = total B.E. of fragments - B.E. of parent. 240×7.6=1824 MeV; fragments: 240×8.5=2040 Me... 1
- Two conducting circular loops of radii R_1 and R_2 are placed in the same plane with their centres coinciding. If R_1 >> R_2, the mutual inductance M between them will be directly proportional to : Answer: 4. R₂²/R₁ Key idea: the large loop R₁ produces a nearly uniform field at its centre, which passes through the small loop. B=μ₀I/2R₁... 1
- In the productF = q(v×B) = q v×(Bi+Bj+B_0k)For q = 1 and v = 2i+4j+6k and F = 4i-20j+12kWhat will be the complete expression for B ? Answer: 2. -6i-6j-8k Key idea: expand vec v×vec B as a determinant and match components. Here vec v=(2,4,6) , vec B=(B_x,B_y,B_z) , vec F=(4... 1
- For the given circuit, the input digital signals are applied at the terminals A, B and C. What would be the output at the terminal y ? Answer: 3. Output waveform (3): constant 5 V Key idea: from the circuit, y=A· B+B· C (AND of A,B ORed with NAND of B,C). IntervalABCA.BNAND(... 1
- A series LCR circuit containing 5.0 H inductor, 80 μF capacitor and 40 Ω resistor is connected to 230 V variable frequency ac source. The angular frequencies of the source at which power transferred to the circuit is half the power at the resonant an… Answer: 3. 46 rad/s and 54 rad/s Key idea: power falls to half of its resonance value when |X_L-X_C|=R , which happens at ω=ω₀±dfrac R2L (fo... 1
- A uniform rod of length 200 cm and mass 500 g is balanced on a wedge placed at 40 cm mark. A mass of 2 kg is suspended from the rod at 20 cm and another unknown mass 'm' is suspended from the rod at 160 cm mark as shown in the figure. Find the value… Answer: 4. 1/12 kg Key idea: take torques about the wedge (40 cm mark). The rod's weight acts at its centre (100 cm). Anticlockwise (left):... 1
- A point object is placed at a distance of 60 cm from a convex lens of focal length 30 cm. If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of 40 cm from it, the final image would be formed at a distance of… Answer: 4. 20 cm from the plane mirror, it would be a virtual image. Key idea: lens, then mirror, then lens again. Lens: u=-60,\ f=30⇒ v=60... 1
- A car starts from rest and accelerates at 5 m/s^2. At t = 4 s, a ball is dropped out of a window by a person sitting in the car. What is the velocity and acceleration of the ball at t = 6 s ? (Take g = 10 m/s^2) Answer: 4. 20√(2) m/s, 10 m/s ² Key idea: once the ball is dropped, its horizontal velocity stays equal to the car's velocity at that instan... 1
- Twenty seven drops of same size are charged at 220 V each. They combine to form a bigger drop. Calculate the potential of the bigger drop. Answer: 4. 1980 V Key idea: total charge and volume are conserved when drops merge. Volume: 4/3π R³=27·4/3π r³⇒ R=3r . Charge: Q=27q . V_big... 1
- A uniform conducting wire of length 12a and resistance 'R' is wound up as a current carrying coil in the shape of,(i) an equilateral triangle of side 'a'.(ii) a square of side 'a'.The magnetic dipole moments of the coil in each case respectively are… Answer: 1. √(3) Ia² and 3 Ia² Key idea: magnetic moment M=NIA , where N is the number of turns the wire of length 12a gives. Triangle (side... 1
- Consider the following statements (A) and (B) and identify the correct answer.(A) A zener diode is connected in reverse bias, when used as a voltage regulator.(B) The potential barrier of p-n junction lies between 0.1 V to 0.3 V. Answer: 3. (A) is correct and (B) is incorrect. Key idea: a Zener diode regulates voltage only in reverse-bias breakdown, and the barrier po... 1
- A capacitor of capacitance 'C', is connected across an ac source of voltage V, given byV = V_0 ω tThe displacement current between the plates of the capacitor, would then be given by : Answer: 1. I_d = V₀ω Ccosω t Key idea: displacement current equals the conduction current that charges the capacitor. q = CV = CV₀sinω t I_d... 1