Physics MCQs for NEET — Practice Questions with Answers

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Two infinitely long parallel wires having linear charge densities λ1 and λ2 respectively are placed at a distance of R meters. The force per unit length on either wire will be K=14πε0

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Explanation

When two infinitely long parallel wires with linear charge densities λ1 and λ2 are placed at a distance R, the force per unit length on either wire is given by the expression (2kλ1λ2)/R, where k = 1/(4πε0) is the Coulomb constant. This formula arises from the application of Coulomb's law and the principle of linear superposition.

The electric field in a region is radially outward with magnitude E=Aγ0. The charge contained in a sphere of radius γ0 centered at the origin is 

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Explanation

Flux linked with the given sphere φ=Qεo;

where Q = Charge enclosed by the sphere.

Hence Q = φε0 = (EA)ε0

Q = 4π (γ0)2 × 0ε0 = 4πε003.

Charge q is uniformly distributed over a thin half-ring of radius R. The electric field at the centre of the ring is 

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Two equal charges are separated by a distance d. A third charge placed on a perpendicular bisector at x distance will experience maximum coulomb force when 

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Explanation

When two equal charges are separated by a distance d, the maximum Coulomb force experienced by a third charge placed on the perpendicular bisector occurs when the third charge is at a distance x = d/(2√2) from the midpoint of the line joining the two charges. This position maximizes the net force due to the two equal charges.

An electric dipole is situated in an electric field of uniform intensity E whose dipole moment is p and moment of inertia is I. If the dipole is displaced slightly from the equilibrium position, then the angular frequency of its oscillations is 

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Explanation

When dipole is given a small angular displacement θ about it's equilibrium position, the restoring torque will be

τ=pEsinθ=pEθ   (as sinθ = θ)

or Id2θdt2=pEθ (as τ=Iα=Id2θdt2)

or d2θdt2=ω2θ with ω2=pEIω=pEI

An infinite number of electric charges each equal to 5 nano-coulomb (magnitude) are placed along x-axis at x = 1 cm, x = 2 cm, x = 4 cm, x = 8 cm ………. and so on. In the setup if the consecutive charges have opposite sign, then the electric field in Newton/Coulomb at x = 0 is 14πε0=9×109Nm2/c2

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Explanation

E=14πε0.5×109(1×102)25×109(2×102)2+5×109(4×102)2(5×109)(8×102)2+.....

 

E=9×109×5×10910411(2)2+1(4)21(8)2+...

 

E=45×1041+1(4)2+1(16)2+...45×1041(2)2+1(8)2+1(32)2+...

 

E=45×1041111645×104(2)21+142+1(16)2+...

 

E = 48 × 104 – 12 × 104 = 36 × 104 N/C  

Two-point charges +q and –q are held fixed at (–d, 0) and (d, 0) respectively of a (x, y) coordinate system. Then 

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Suppose the charge of a proton and an electron differ slightly. One of them is -e and the other is e+e. If the net of electrostatic force and gravitaional force between two hydrogen atoms placed at a distance d (much greater than atomic size) apart is zero,then e is of the order [Given mass of hydrogen, mh=1.67×10-27 kg]

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Explanation

(c) Net charge on one H-atom 

=-e+e+e

Net electrostatic repulsive force between two H-atoms

Fe = Ke2d2

FG=Gm12d2

It is given that 

Fe-FG=0

    Ke2d2-Gm12d2=0

    e2=6.67×10-111.67×10-2729×109

       e=1.437×10-37C

 

An electric dipole is place at an angle of 30 with an electric field intensity 2×105 N/C. It experiences a torque equal to 4 Nm. The charge on the dipole, if the dipole length is 2 cm, is

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Explanation

 

(b) Torque on an electric dipole in an electric field,

             τ=p×Eτ=pE sin θ

where θ is the angle between E and p

 4=ρ×2×105×12p=4×10-5cmp=q2l q2l =4×10-5

Where 2l=2cm=2×10-2 m

q=4×10-52×10-2

2×10-3C=2mC

What is the flux through a cube of side a if a point charge of q is a one of its corner?

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Explanation

Charge enclosed=q/8

Therefore,flux ϕ=qenclosedε0

                    ϕ=q8ε0

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