NEET Physics Tutor Doubt 9 - NEET Physics Special Practice Questions

Correct answer: Option 4. Zero

In the absence of an external electric field, free electrons move randomly in all directions.

Their vector average velocity is therefore zero.

Correct answer: Option 4. Ohm's law

At constant temperature, current density is directly proportional to the applied electric field.

This linear relation is Ohm's law.

Correct answer: Option 3. 83 Ω

Points B, C, D and L are at the same potential, so the circuit reduces to a 2 Ω series resistance with three 2 Ω resistors in parallel.

R = 2 + 23 = 83 Ω.

Correct answer: Option 4. V₁ = V₂ = 3 V

Current in the circuit is i = 4 - 22 + 2 = 0.5 A.

For the 4 V cell, V₁ = 4 - (0.5)(2) = 3 V.

For the 2 V cell while charging, V₂ = 2 + (0.5)(2) = 3 V.

Correct answer: Option 3. 32v

For the same material, I = neAv_d, so drift velocity is proportional to I/A.

v_newv = 10050 × d²(d/4)² = 2 × 16 = 32.

Correct answer: Option 2. Charge

At a junction, charge cannot accumulate in steady state.

Hence total current entering the junction equals total current leaving it.

Correct answer: Option 3. 3 A

Applying Kirchhoff's laws to the loops gives the branch current through the central 4 V battery.

The current through the 4 V battery is 3 A.

Correct answer: Option 2. Increases non-linearly with increase in temperature

For copper, resistivity increases with temperature and the experimental curve is not perfectly linear over a wide range.

Correct answer: Option 4. Unitless

Relative permeability is μ_r = μ/μ₀.

It is a ratio of two quantities with the same unit, so it is unitless.

Correct answer: Option 4. Both statements I and II are incorrect

Magnetic force on a charge is F = q(v × B).

Here the particle is kept at rest, so v = 0 and magnetic force is zero. Both statements are incorrect.

Correct answer: Option 2. 1 : √2

For circular motion in a magnetic field, R = √(2mK)/(qB).

With the same kinetic energy and charge, radius is proportional to √m, so R_p : R_d = 1 : √2.

Correct answer: Option 1. Straight line

The magnetic field and velocity are parallel, hence v × B = 0.

The electron is affected only by the electric field, so the given solution treats the trajectory as rectilinear.

Correct answer: Option 2. μ₀2I4πr

For a long straight wire, B = μ₀I2πr, which is the same as option 2.

Correct answer: Option 4. μ₀I24a, out of the plane

The magnetic fields due to the two arcs are in opposite directions.

The resultant is B = B₁ - B₂ = μ₀I24a, directed out of the plane.

Correct answer: Option 4. 1232 N m

τ_max = NBIA.

Here A = πr² = π(0.07)², so τ_max = 100 × 20 × 40 × π(0.07)² = 1232 N m.

Correct answer: Option 4. Deflection per unit voltage

Voltage sensitivity is the deflection produced per unit voltage applied across the galvanometer.

Correct answer: Option 4. 2 T

Retentivity is the residual magnetic flux density B when H = 0.

From the graph, B at H = 0 is 2 T.

Correct answer: Option 2. Infinite

An ideal voltmeter should draw no current from the circuit.

Therefore its resistance is infinite.

Correct answer: Option 3. Magnetic field intensity due to a short bar magnet at equatorial line is double than that at axial line

For a short bar magnet, the magnetic field at the axial point is double that at the equatorial point.

Hence statement 3 is incorrect.

Correct answer: Option 4. 0.5 × 10^-3 A m²

At stable equilibrium, U = -MB.

M = 2 × 10^-34 = 0.5 × 10^-3 A m².

Correct answer: Option 1. 0.01 Ω

Required shunt S = I_gGI - I_g ≈ (0.002)(10)2 = 0.01 Ω.

The effective resistance of the ammeter is nearly 0.01 Ω.

Correct answer: Option 4. [M⁰L²T⁰A]

Bohr magneton has the dimensions of magnetic moment.

Magnetic moment = current × area, so its dimensional formula is [M⁰L²T⁰A].

Correct answer: Option 4. None of the above

Magnetic permeability is positive for ordinary magnetic materials.

Diamagnetic substances have negative susceptibility, but their permeability is not negative.

Correct answer: Option 4. 10⁴ A m^-1

Intensity of magnetisation I = mA = 1010 × 10^-4 = 10⁴ A m^-1.

Correct answer: Option 2. Non-existence of magnetic monopoles

Gauss law for magnetism is ∮B · dS = 0.

It means magnetic monopoles do not exist and magnetic field lines form closed loops.

Correct answer: Option 3. A is true but R is false

Magnetic force is perpendicular to velocity, so it does no work and speed remains constant.

However, it can change the direction of velocity and hence the momentum. Thus A is true and R is false.

Correct answer: Option 2. Only statement II is correct

For a paramagnetic substance, susceptibility follows Curie's law and decreases with rise in temperature.

Above Curie temperature, a ferromagnetic material becomes paramagnetic.

Correct answer: Option 1. 4π mT

Current in the solenoid is I = V/R = 1000/100 = 10 A.

B = μ₀nI = (4π × 10^-7)(1000)(10) = 4π × 10^-3 T = 4π mT.

Correct answer: Option 3. 2Mπ

Pole strength remains the same. When length L is bent into a semicircle, L = πr.

The new pole separation is 2r = 2Lπ, so M' = 2Mπ.

Correct answer: Option 2. 160 N

Net magnetic force on a bent wire in a uniform field depends on the effective end-to-end length.

Here the effective length is 8 m, so F = IBl = 10 × 2 × 8 = 160 N.

Correct answer: Option 4. Zero

The area vector of a loop in the x-y plane is along the z-axis.

For the given field direction, the angle with the area vector is 90°, hence flux is zero.

Correct answer: Option 4. Insufficient information

Motional emf depends on the relative orientation of the rod, velocity and magnetic field.

The necessary angle information is not given, so the emf cannot be determined uniquely.

Correct answer: Option 2. B

Eddy current is reduced when the path for circulating current is interrupted and its resistance is increased.

The sheet marked B offers the greatest interruption to the current path, so eddy current is minimum.

Correct answer: Option 1. a → T, b → S, c → P, d → R, e → Q

Ampere's law is ∮B · dl = μ₀i.

For a semicircular loop B = μ₀i/(4r), Lorentz force is q(E + v × B), long solenoid field is μ₀ni, and field on the axis of a thin long current carrying pipe is zero.

Correct answer: Option 3. V_O - V_A = V_O - V_D

The symmetric arms acquire equal motional potential differences with respect to O.

Therefore V_O - V_A = V_O - V_D.

Correct answer: Option 3. 5 A

The bridge is balanced, so no current flows through the central branch.

The equivalent resistance is 185 Ω, hence I = 1818/5 = 5 A.

Correct answer: Option 2. 4999

B = μ₀μ_rnI.

μ_r = 5000, so magnetic susceptibility χ = μ_r - 1 = 4999.

Correct answer: Option 4. 1 : 8

Self inductance of a solenoid is proportional to N²/l for the same core and area.

L_A/L_B = (1/2)² × (1/2) = 1/8.

Correct answer: Option 4. 8 V

The inductor contributes induced potential L(di/dt) = 0.4 × 20 = 8 V.

Applying KVL across the shown segment gives V_A - V_B = 8 V.

Correct answer: Option 2. 160 μC

Charge q = ΔΦR = BAR.

q = (0.4)(40 × 10^-4)/10 = 160 μC.

Correct answer: Option 2. 9 times

Energy stored in an inductor is U = (1/2)LI².

When current becomes 3I, energy becomes 9 times.

Correct answer: Option 3. Both A and C

Mutual inductance depends on geometry, number of turns, separation, area, medium and orientation.

It does not depend on the current flowing through the coils.

Correct answer: Option 2. Circuit 2

Null deflection in a bridge is possible when the balance condition can be satisfied.

Among the shown circuits, only circuit 2 can satisfy the required balance condition for R ≠ 0.

Correct answer: Option 4. Zero

The net magnetic force on any closed current loop in a uniform magnetic field is zero.

Correct answer: Option 2. B and C only

Paramagnetic substances are weakly attracted by an external magnetic field.

They have small positive susceptibility and tend to move from weaker to stronger magnetic field regions.