Science (English Medium) इयत्ता १२ - CBSE Important Questions for Physics

Write the expression, in a vector form, for the Lorentz magnetic force \[\vec{F}\] due to a charge moving with velocity \[\vec{V}\] in a magnetic field \[\vec{B}\]. What is the direction of the magnetic force? 

Two long coaxial insulated solenoids, S₁ and S₂ of equal lengths are wound one over the other as shown in the figure. A steady current "I" flow thought the inner solenoid S₁ to the other end B, which is connected to the outer solenoid S₂ through which the same current "I" flows in the opposite direction so as to come out at end A. If n₁ and n₂ are the number of turns per unit length, find the magnitude and direction of the net magnetic field at a point (i) inside on the axis and (ii) outside the combined system

Using the concept of force between two infinitely long parallel current carrying conductors, define one ampere of current.

Deduce an expression for the frequency of revolution of a charged particle in a magnetic field and show that it is independent of velocity or energy of the particle.

Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working.

Obtain the expression for mutual inductance of a pair of long coaxial solenoids each of length l and radii r₁ and r₂ (r₂ >> r₁). Total number of turns in the two solenoids are N₁ and N_2, respectively.

At a place, the horizontal component of earth's magnetic field is B and angle of dip is 60°. What is the value of horizontal component of the earth's magnetic field at equator?

Why is it necessary to introduce a radial magnetic field inside the coil of a galvanometer?

Derive the expression for the torque on a rectangular current carrying loop suspended in a uniform magnetic field.

A proton and a deuteron having equal momenta enter in a region of a uniform magnetic field at right angle to the direction of a the field. Depict their trajectories in the field.

A small compass needle of magnetic moment ‘m’ is free to turn about an axis perpendicular to the direction of uniform magnetic field ‘B’. The moment of inertia of the needle about the axis is ‘I’. The needle is slightly disturbed from its stable position and then released. Prove that it executes simple harmonic motion. Hence deduce the expression for its time period.

Define the current sensitivity of a galvanometer ?

Write current sensitivity of a galvanomete S.I. unit. 

Draw a schematic sketch of a cyclotron. Explain clearly the role of crossed electric and magnetic field in accelerating the charge. Hence derive the expression for the kinetic energy acquired by the particles.

An α-particle and a proton are released from the centre of the cyclotron and made to accelerate.

(i) Can both be accelerated at the same cyclotron frequency?

Give reason to justify your answer.

(ii) When they are accelerated in turn, which of the two will have higher velocity at the exit slit of the does?

Read the following paragraph and answer the questions.

1. Explain the reason for the jumping of the ring when the switch is closed in the circuit.
2. What will happen if the terminals of the battery are reversed and the switch is closed? Explain.
3. Explain the two laws that help us understand this phenomenon.

• Assertion (A): The deflecting torque acting on a current-carrying loop is zero when its plane is perpendicular to the direction of the magnetic field.
• Reason (R): The deflecting torque acting on a loop of the magnetic moment `vecm` in a magnetic field `vecB` is given by the dot product of `vecm` and `vecB`.

A galvanometer shows full-scale deflection for current I_g. A resistance R₁ is required to convert it into a voltmeter of range (0 - V) and a resistance R₂ to convert it into a voltmeter of range (0 - 2V). Find the resistance of the galvanometer.

Briefly explain various ways to increase the strength of the magnetic field produced by a given solenoid.

Out of the two magnetic materials, 'A' has relative permeability slightly greater than unity while 'B' has less than unity. Identify the nature of the materials 'A' and 'B'. Will their susceptibilities be positive or negative?