Thursday 19 January 2012



Class – XII
Subject –
Physics

Time: 2 Hours                                                               Max. Marks: 45

Q 1        An electron and a proton moving with same speed enter the same magnetic field region at right angles to the   
            direction of the field. For which of the two particles will the radius of the circular path be smaller?             1
Q 2.     If a wire is stretched to double its original length without loss of mass, how will the resistivity of the
             wire be influenced?                                                                                                                       1
Q 3.     Why do magnetic lines of force prefer to pass through iron than through air?                             1
Q 4.     Name the device used for measuring the internal resistance of a secondary cell?             (1 mark)
Q5.      A set of n identical resistors, each of resistance R ohm, when connected in series have an effective resistance X ohm and when the resistors are connected in parallel, their effective resistance is Y ohm. Find the relation between R, X and Y.                                                                                 (2 marks)
 Q6.       State Kirchhoff's rules for electrical networks.                                                                  (2 marks)
Q7       The electron drift speed in metals is small (~mms-1) and the charge of the electron is also very small (~10-19C),  
              but   we can still obtain a large amount of current in a metal. Why?                                                  (2 marks)       
Q:8        An electron travelling west to east enters a chamber having a uniform electrostatic field in north to    
            south direction. Specify the direction in which a uniform magnetic field should be set up to prevent the  
             electron from deflecting from its straight line path.                                                             (2 marks)
Q:9       The given figure shows the experimental set up of a metre bridge. The null point is found to be 60 cm away from the end A  
                with  X and Y in position as shown.







When a resistance of 15Ω is connected in series with ‘Y’, the null point is found to shift by 10 cm towards the end A of the wire. Find the position of null point if a resistance of 30Ω were connected in parallel with ‘Y’.                             (2 marks)
Q:10     A galvanometer coil has a resistance of 15 Ω and the metre shows full scale deflection for a current of 4 mA.    
             How will you convert the metre into an ammeter of range 0 to 6 A?                                                          (2 marks)
Q:11    The voltage –current graph for two resisters of same material and same radii with lengths L1 and L2 are shown. If  
             L1 > L2, state with reason , which reason which of the graphs represents voltage –current change for L1
                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
                                                        (2 marks)
Q:12    You are given 8 W resistor. What length of constantan wire of resistance 120 W m‑ 1 should be joined in parallel  
             with it to get a value of 6W?                                                                                                              (2 marks)
Q:13    Two long parallel straight wires X and Y separated by a distance of 5 cm in air carry currents of 10 A and 5A   
             respectively in opposite direction. Calculate the magnitude and the direction of force on a 20 cm length of the
             wireY. (3 marks)
                                                            OR
A straight wire of mass 200 g and length 1.5 m carries a current of 2 A. It is suspended in mid-air by a uniform horizontal magnetic field B. What is the magnitude of the magnetic field?

 















Q:14         Determine the currents I1 , I2 , I3 in the following network using Kirchhoff’s laws. (3 marks)


Q:15   Derive the expression for the torque on a rectangular coil of area A carrying current I placed in a magnetic field B.  
            The angle b/w direction of B and the area vector of the coil is θ.                                                             (3 marks)
Q:16. Derive an expression for the equivalent emf and internal resistance of a parallel combination of 2
          primary cells. Under what condition is the potential difference across a cell equal to its emf? (3 marks)
Q:17. Explain with the help of a circuit diagram how the value of an unknown resistance can be determined  
           using a Wheatstone bridge. Give the formula used.                                                                (3 marks)
Q18 . A straight wire carries a current of 3A.Calulate the magnitude of the magnetic field at a point 10cm
          away from the wire. Draw diagram to show the direction of the magnetic field.                    (3 marks)
Q:19  Why is a potentiometer preferred over a voltmeter for determining the emf of a cell? (3 marks)
                                                                     OR
           Two cells of emf E1 and E2 are   connected together in two ways shown below
     
     


          The balance points in a given potentiometer experiment for these two combinations of cells are found to be at 351   
          cm and 70.2 cm respectively. Calculate the ratio of the emfs of the two cells.
msoDB8D Q:20 Derive an expression for the magnetic field at a point
           on the axis of a current carrying circular loop.

Two coaxial circular loops L1 and L2 of radii 3 cm and 4 cm are placed as shown. What should be the magnitude and direction of the current in the loop L2 so that the net magnetic field at the point O is zero?                                           
(4 marks)

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