Class XII Subject Physics

  1. State two reasons why a galvanometer can not be used as such to measure current in a given circuits.
  2. Draw diagrams to depict the behavior of magnetic field lines near a ‘bar’ of.

(a) copper (b) aluminium (c) mercury cooled to a very law temperature (4.2K)

  1. A charge q moving in a straight line is accelerated by a potential difference V. It enters a uniform magnetic field

B perpendicular to its path. Deduce in terms of V an expression for radius of the circular path in which it travels.

  1. The horizontal component of earth’s magnetic field at a given place is 0.4 x 10 -4wb/m2 and angle of dip is 30. Calculate the value of (i) vertical component (ii) total intensity of earth’s magnetic field.
  2. A galvanometer has a resistance 30 and gives a full scale deflection for a current of 2mA. How much resistance and in what way must be connected to convert it into

(i) an ammeter of range 0.3 A.(ii) a voltmeter of range 0.20V.

  1. Using Ampere’s circuital law find an expression for the magnetic field at a point on the axis of a long solenoid

with closely wound turns.

  1. Deduce an expression for magnetic dipole moment of an electron revolving around a nucleus I a circular orbit.

Use the expression to derive the relation between the magnetic moment of an electron moving in a circle and

its related angular momentum?

  1. Distinguish the magnetic properties of dia, para and ferro magnetic substances in terms of

(i) susceptibility (ii) magnetic permeability (iii) coercively

Give an examples of each of these materials.

  1. Find an expression for mutual inductance of two co-axial solenoid.
  2. What is meant by impedance? Give its unit. Using phase diagram or otherwise derive an expression for the impedance and instantaneous current I of an a.c circuit containing L, C and R in series. Find the expression for resonant frequency.
  3. A step up transformer converts a law voltage into high voltage. Does it not violate the principle of conservation

of energy? Explain.

  1. An inductor 200 mH, capacitor 500F, resistor 10are connected in series with a 100V variable frequency a.c source. Calculate the (i) frequency at which the power factor of the circuit is unity. (ii) current amplitude at this frequency (iii) Q factor
  2. What are eddy currents? How are they produced? Give two applications of eddy currents.
  3. Draw the graphs showing variations of inductive reactance and capacitive reactance with frequency of applied

a.c source.

  1. An electric lamp having coil of negligible inductance connected in series with a capacitor and an a.c. source is glowing with certain brightness. How does the brightness of the lamp change on reducing.

(i) the capacitance (ii) the frequency ? Justify your answer?

  1. Prove that an idea capacitor in an a.c. circuit does not dissipate power.