Science (English Medium) Class 12 - CBSE Important Questions

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

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.

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

The current is drawn from a cell of emf E and internal resistance r connected to the network of resistors each of resistance r as shown in the figure. Obtain the expression for

1. the current draw from the cell and
2. the power consumed in the network.

State Kirchhoff's rules for an electric network. Using Kirchhoff's rules, obtain the balance condition in terms of the resistances of four arms of Wheatstone bridge.

Define the current sensitivity of a galvanometer ?

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

Using Kirchhoff’s rules determine the value of unknown resistance R in the circuit so that no current flows through 4 Ω resistance. Also find the potential difference between A and D.

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?

Show diagrammatically the behaviour of magnetic field lines in the presence of (i) paramagnetic and (ii) diamagnetic substances. How does one explain this distinguishing feature?

In an experiment, two coils c₁ and c₂ are placed close to each other. Find out the expression for the emf induced in the coil c₁ due to a change in the current through the coil c₂.

Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it.

The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of

Magnetic flux versus the current

The current flowing through an inductor of self inductance L is continuously increasing. Plot a graph showing the variation of Magnetic potential energy stored versus the current.

Draw a schematic sketch of an ac generator describing its basic elements. State briefly its working principle. Show a plot of variation of

(i) Magnetic flux and

(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.