NEET UG

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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. Series and parallel combination of resistors 1 answer
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... Einstein's photoelectric equation, particle nature of light 1 answer
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⇒... Matter waves, de Broglie relation 1 answer
An inductor of inductance L, a capacitor of capacitance C and a resistor of resistance 'R' are connected in series to an ac source of potential difference 'V' volts as shown in figure.Potential difference across L, C and R is 40 V, 10 V and 40 V, res… Answer: 4. 5\ Ω Key idea: voltages in series LCR add as phasors: V=√(V_R²+(V_L-V_C)²) , and Z=V/I with both in the same (rms) form. V=√(40²+... Reactance, impedance, LCR series circuit, resonance 1 answer
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... Reactance, impedance, LCR series circuit, resonance 1 answer
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... Self and mutual inductance 1 answer
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(... Logic gates (OR, AND, NOT, NAND, NOR) 1 answer
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... Semiconductors 1 answer
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... Conductors, insulators, dielectrics and polarization 1 answer
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/... Electric potential, potential difference, equipotential surfaces 1 answer
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... Electric potential, potential difference, equipotential surfaces 1 answer
A screw gauge gives the following readings when used to measure the diameter of a wireMain scale reading : 0 mmCircular scale reading : 52 divisionsGiven that 1 mm on main scale corresponds to 100 divisions on the circular scale. The diameter of the… Answer: 4. 0.052 cm Key idea: diameter = main scale reading + circular reading × least count. Least count =1\ mm/100=0.01 mm. Diameter =0+52... Vernier calipers and screw gauge 1 answer
The escape velocity from the Earth's surface is v. The escape velocity from the surface of another planet having a radius, four times that of Earth and same mass density is : Answer: 4. 4v Key idea: v_e=√(2GM/R) and M=4/3π R³ρ , so v_e∝ Rsqrtρ . Same density and radius four times as large gives v_e'=4v . Option 4. Escape velocity 1 answer
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... Projectile motion 1 answer
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)=... Uniformly accelerated motion relations 1 answer
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... Ampere's law: straight wire and solenoid 1 answer
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... Current loop and bar magnet as magnetic dipole, field along axis and equator 1 answer
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... Force on moving charge in magnetic and electric fields 1 answer
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... Refraction at plane and spherical surfaces, thin lens and lens maker formula 1 answer
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 .... Dimensions and dimensional analysis 1 answer
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):... Equilibrium of rigid bodies, rigid body rotation, equations of rotational motion 1 answer
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... Power 1 answer
Column - I gives certain physical terms associated with flow of current through a metallic conductor. Column - II gives some mathematical relations involving electrical quantities. Match Column - I and Column - II with appropriate relations.Column -… Answer: 1. (A)-(R), (B)-(S), (C)-(P), (D)-(Q) (A) Drift velocity v_d=aτ=eE/mτ : R. (B) Resistivity ρ=E/J : S. (C) From ρ=m/ne²τ , relaxation... Electric current, drift velocity, mobility 1 answer
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... Displacement current 1 answer

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