Physics MCQs for NEET — Practice Questions with Answers

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A vibration magnetometer consists of two identical bar magnets placed one over the other such that they are perpendicular and bisect each other. The time period of oscillation in a horizontal magnetic field is 254 seconds. One of the magnets is removed and if the other magnet oscillates in the same field, then the time period in seconds is :

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A cylindrical rod magnet has a length of 5 cm and a diameter of 1 cm. It has a uniform magnetisation of 5.30 × 103Amp/m3. What is its magnetic dipole moment ?

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If ϕ1 and ϕ2 be the angles of dip observed in two vertical planes at right angles to each other and ϕ be the true angle of dip, then

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A dip needle lies initially in the magnetic meridian when it shows an angle of dip θ at a place. The dip circle is rotated through an angle x in the horizontal plane and then it shows an angle of dip θ' . Then tan θ'tan θ is

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If θ1 and θ2 be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip θ is given by 

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Explanation

If θ1 and θ2 are the apparent angles of dip observed in two vertical planes at right angles, the true angle of dip θ is given by cot^2 θ = cot^2 θ1 + cot^2 θ2. This relation is derived from the vector addition of the horizontal and vertical components of the Earth's magnetic field.

The magnetic susceptibility is negative for

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Explanation

 

(d) As we know the relation between the magnetic permeability and susceptibility of material i.e.

                                    μr=1+xm    ...(i)

so, For diamagnetic substances, μr<1

so, according to equation (i), the magnetic susceptibility (xm) of diamagnetic substance will be negative.

While in case of para and ferromagnetic substances, magnetic susceptibility is positive.

A compass needle which is allowed to move in a horizontal plane is taken to a geomagnetic pole.It

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Explanation

It will stay in any position at geomagnetic north and south poles.

A magnetic needle suspended parallel to a 

magnetic field requires 3 J of work to turn

it through 60°. The torque needed to maintain 

the needle in this position will be

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Explanation

In this case, work done

W=MB(cosθ1-cosθ2)    =MB(cos 0°-cos 60°)    =MB1-12=MB2MB=23J                      (so, given W=3J)τ=MB sin 60°=(23)32J=3J

There are four light-weight-rod samples; A, B, C, D separately suspended by threads. A bar magnet is slowly brought near each sample and the following observations are noted:

(i) A is feebly repelled 

(ii) B is feebly attracted

(iii) C is strongly attracted

(iv) D remains unaffected

Which one of the following is true?

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Explanation

Paramagnetic material will be feebly attracted, diamagnetic material will be feebly repelled and Ferromagnetic material will be strongly attracted. 

A short bar magnet of magnetic moment 0.4 JT-1 is placed in a uniform magnetic field of 0.16 T. The magnet is stable equilibrium when the potential energy is

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Explanation

We know that 

      U=-MB cos θ

   For stable equilibrium θ=0°

   So, U=-MB

            =-(0.4)(0.16)=-0.064 J

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