Magnetic energy per unit volume is represented by from Phys

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 Multiple Choice QuestionsMultiple Choice Questions

241.

What is the dimensions of a magnetic field B in terms of C ( = coulomb ), M, L, T?

  • [ M1 L-1 T-2 C ]

  • [ M1 L0 T-1 C-1 ]

  • [ M1 L0 T-2 C ]

  • [ M1 L0 T-1 C ]


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242.

Magnetic energy per unit volume is represented by

  • B22 μ0

  • B22 μ0 2

  • 2 B2μ0

  • B2μ0


A.

B22 μ0

The emf induced in an inductor 

         Iε = I2 R + LI dIdt

This expression indicates that the rate at which energy is supplied by the battery (Iε ) equals the sum of the rate at which energy energy delivered to the resistor I2R and the rate at which energy is stored in the inductor L dIdt 

Now U is the energy stored in inductor at any time, rate of change of energy is given by

         dUdt = L I dIdt

To find the total energy stored in the inductor, dU = LI

         U =  dU 

             = 0I LI dI

         U = 12L I2             ....(i)

where L is constant.

This expression represents the energy stored in the magnetic field of the inductor when current is I.

The energy stored in the electric field of capacitor 

       U = Q22C

Now energy density of magnetic field 

       L = μ0n2 Al

The magenetic field of a  solenoid is given by

        B = μ0 nI

Substituting the expression for L and  I = Bμ0 n 

Put above value in eq. (i)

       U = 12μ0n2 Al Bμ0 n2

⇒    U = B22 μ0 Al   

Because Al is the volume of the solenoid, the energy stored per unit volume in the magnetic field surrounding the inductor is

    uBUA l

⇒  uBB22 μ0


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243.

Magnetic field at ' 𝑎' distance a from long currying current wire is proportional to

  • 1𝑎

  • 1𝑎2

  • 1𝑎

  • 1𝑎32


244.

When a positively charged particle enters into a uniform magnetic field with uniform velocity, its trajectory can be

(i) a straight line  (ii) a circle  (iii) a helix

  • (i) only

  • (i) or (ii)

  • (i) or (iii)

  • any one of (i), (ii) and (iii)


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245.

Assertion: Magnetic field lines are continuous and closed.

Reason: Magnetic monopole does not exist.

  • If both assertion and reason are true but reason is not the correct explanation of assertion.

  • If both assertion and reason are true but reason is not the correct explanation of assertion.

  • If assertion is true but reason is false.

  • If both assertion and reason are false.


246.

Assertion:  Magnetic force between two short magnets, when they are co-axial follows inverse square law of distance. 

Reason:  The magnetic forces between two poles do not follow inverse square law of distance.

  • If both assertion and reason are true and reaon is correct explanation of assertion.

  • If both assertion and reason are true and reaon is not the correct explanation of assertion.

  • If assertion is true but reason is false.

  • If both assertion and reason are false.


247.

Assertion:  If  a proton and an a-particle enter a uniform magnetic field perpendicularly with the same speed, the time period of revolution of a particle is double that of proton. 

Reason:  In a magnetic field, the period of revolution of a charged particle is directly proportional to the mass of the particle and is inversely proportional to charge of particle.

  • If both assertion and reason are true and reason is the correct explanation of assertion

  • If both assertion and reason are true but reason is not the correct explanation of assertion

  • If assertion is true but reason is false

  • If both assertion and reason are false.


248.

A  moving  coil  galvanometer  has  a resistance  of  900 Ω.  In order  to send  only 10 % of  the  main  current through  this  galvanometer,  the  resistance  of  the required  shunt  is

  • 0.9 Ω

  • 100 Ω

  • 405 Ω

  • 90 Ω


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249.

The intensity of magnetic field due to an isolated pole of strength mp at a point distant r from it will be

  • mp/r2

  • mp r2

  • r2/ mp

  • mp/r


250.

The  ratio of magnetic fields on the axis of a circular current carrying coil of radius a to the magnetic field at its centre will be

  • 11 + x2a232

  • 11 + a2x212

  • 11 + a2x22

  • 11 + a2x23


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