NEET UG
- A long straight wire of radius a carries a steady current I. The current is uniformly distributed over its cross - section. The ratio of the magnetic fields B and B', at radial distances a/2 and 2a respectively, from the axis of the wire is : Answer: 3. 1 Key idea: use Ampere's law. Inside the wire, B=μ₀Ir/2π a² ; outside, B=μ₀I/2π r . At r=a/2 : B=μ₀I/4π a . At r=2a : B'=μ₀I/4π a... 1
- A square loop ABCD carrying a current i, is placed near and coplanar with a long straight conductor XY carrying a current I, the net force on the loop will be : Answer: 1. 2μ₀ Ii/3π Key idea: forces on the two sides parallel to the wire add up; forces on the other two sides cancel. Side nearest the w... 1
- The magnetic susceptibility is negative for : Answer: 1. diamagnetic material only Key idea: only diamagnetic materials get magnetised opposite to the applied field. Diamagnetic: chi sma... 1
- In a diffraction pattern due to a single slit of width 'a', the first minimum is observed at an angle 30^° when light of wavelength 5000 Å is incident on the slit. The first secondary maximum is observed at an angle of : Answer: 4. sin⁻¹(3/4) Key idea: single-slit minima satisfy asinθ=nλ ; secondary maxima are approximately at asinθ=(n+1/2)λ . First minimum a... 1
- The intensity at the maximum in a Young's double slit experiment is I_0. Distance between two slits is d = 5λ, where λ is the wavelength of light used in the experiment. What will be the intensity in front of one of the slits on the screen placed at… Answer: 4. I₀/2 Key idea: find the phase difference at the point in front of one slit, then use I=I₀cos²(φ/2) . The point is d/2 from the ce... 1
- The angle of incidence for a ray of light at a refracting surface of a prism is 45^°. The angle of prism is 60^°. If the ray suffers minimum deviation through the prism, the angle of minimum deviation and refractive index of the material of the prism… Answer: 2. 30^°;\ √(2) Key idea: at minimum deviation the ray passes symmetrically, so r₁=r₂=A/2 and i=A+δ_m/2 . r=30^° ; μ=sin45^°/sin30^°=... 1
- An air column, closed at one end and open at the other, resonates with a tuning fork when the smallest length of the column is 50 cm. The next larger length of the column resonating with the same tuning fork is : Answer: 3. 150 cm Key idea: a closed pipe resonates at odd multiples of λ/4 : lengths L, 3L, 5L,dots Smallest length =λ/4=50 cm. The next re... 1
- A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15\ ms^-1. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is : (Take velocity of sound in air = 330\ ms^… Answer: 3. 838 Hz Key idea: the cliff is a stationary reflector. The sound reaches it with a Doppler shift because the siren moves toward th... 1
- A uniform rope of length L and mass m_1 hangs vertically from a rigid support. A block of mass m_2 is attached to the free end of the rope. A transverse pulse of wavelength λ_1 is produced at the lower end of the rope. The wavelength of the pulse whe… Answer: 2. √(m₁+m₂/m₂) Key idea: the frequency of a pulse stays the same, so wavelength is proportional to wave speed v=√(T/μ) . At the lowe... 1
- Coefficient of linear expansion of brass and steel rods are α_1 and α_2. Lengths of brass and steel rods are l_1 and l_2 respectively. If (l_2 - l_1) is maintained same at all temperatures, which one of the following relations holds good ? Answer: 4. α₁ l₁ = α₂ l₂ Key idea: the difference of lengths stays constant only if both rods expand by the same amount. Δ l₁=Δ l₂⇒ l₁α₁Δ T=... 1
- A black body is at a temperature of 5760 K. The energy of radiation emitted by the body at wavelength 250 nm is U_1, at wavelength 500 nm is U_2 and that at 1000 nm is U_3. Wien's constant, b = 2.88×10^6 nmK. Which of the following is correct ? Answer: 4. U₂ > U₁ Key idea: Wien's law λ_m T=b gives the wavelength of peak emission. λ_m=dfrac2.88×10⁶5760=500 nm. Emission is maximum at... 1
- A uniform circular disc of radius 50 cm at rest is free to turn about an axis which is perpendicular to its plane and passes through its centre. It is subjected to a torque which produces a constant angular acceleration of 2.0\ rad\ s^-2. Its net acc… Answer: 1. 8.0 Key idea: net acceleration combines tangential and centripetal parts. ω=α t=2×2=4 rad/s. a_t=α r=2×0.5=1 m/s ² ; a_c=ω²r=16×0... 1
- A disk and a sphere of same radius but different masses roll off on two inclined planes of the same altitude and length. Which one of the two objects gets to the bottom of the plane first ? Answer: 2. Sphere Key idea: for rolling down an incline, a=gsinθ/1+k²/R² . Smaller k²/R² means larger acceleration and less time. Disk: k²/R... 1
- From a disc of radius R and mass M, a circular hole of diameter R, whose rim passes through the centre is cut. What is the moment of inertia of the remaining part of the disc about a perpendicular axis, passing through the centre ? Answer: 2. 13 MR²/32 Key idea: remaining part = full disc minus the cut-out disc, using the parallel axis theorem for the hole. Full disc: I... 1
- A refrigerator works between 4^° C and 30^° C. It is required to remove 600 calories of heat every second in order to keep the temperature of the refrigerated space constant. The power required is : (Take 1 cal = 4.2 Joules) Answer: 3. 236.5 W Key idea: for a refrigerator, COP=T₂/T₁-T₂=Q₂/W . T₂=277 K, T₁=303 K, so COP =277/26≈10.65 . Heat removed per second =600... 1
- What is the minimum velocity with which a body of mass m must enter a vertical loop of radius R so that it can complete the loop ? Answer: 4. √(5gR) Key idea: at the top the minimum speed satisfies mg=mv_t²/R , so v_t²=gR . Energy conservation from bottom to top (height... 1