Science (English Medium) इयत्ता १२ - CBSE Important Questions

Three cells, each of emf E but internal resistances 2r, 3r and 6r are connected in parallel across a resistor R.

Obtain expressions for (i) current flowing in the circuit, and (ii) the terminal potential differences across the equivalent cell.

A galvanometer of resistance G is converted into a voltmeter to measure upto V volts by connecting a resistance R₁ in series with the coil. If a resistance R₂ is connected in series with it, then it can measures upto V/2 volts. Find the resistance, in terms of R₁ and R₂, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance G of the galvanometer in terms of R₁ and R₂

Use this law to obtain the expression for the magnetic field inside an air cored toroid of average radius 'r', having 'n' turns per unit length and carrying a steady current I.

An observer to the left of a solenoid of N turns each of cross section area 'A' observes that a steady current I in it flows in the clockwise direction. Depict the magnetic field lines due to the solenoid specifying its polarity and show that it acts as a bar magnet of magnetic moment m = NIA.

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.

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?

Define the current sensitivity of a galvanometer ?

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

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