Download Common Practice Test-8 Answer key with solutions

Common Practice Test-8
Class - XII (Batch: C2, G2, C, G)
Answer key with solutions
Time : 3 Hours
20.07.2014
Maximum marks : 360
PART - I: PHYSICS
1.
Graph of force per unit length between two long parallel current carrying conductors and
the distance between them is:
(a) straight line
(b) parabola
(c) ellipse
(d) rectangular hyperbola
Sol. (d)
2. A circular conductor of radius 5 cm produces a magnetic field of 7 × 10–6 T. The current
flowing through the conductor is:
(a) 0.26 A
(b) 0.36 A
(c) 0.46 A
(d) 0.56 A
Sol. (d)
3. A long wire carrying a steady current is bent into a circle of single turn. The magnetic
field at the centre of the coil is B. If it is bent into a circular loop of n turns, the magnetic
field at the centre of the coil for the same current is:
(a) 2nB
(b) 2n2B
(c) n2B
(d) nB
Sol. (c)
4. The oscillating frequency of a cyclotron is 10 MHz. If the radius of its D is 0.5 m, the
kinetic fenergy of a proton, which is accelerated by the cyclotron is
(a) 10.2 MeV
(b) 2.55 MeV
(c) 20.4 Me V
(d) none of these
Sol. (d)
KE of charged particle in a cyclortron.
Ek 
5.
q2B2r2
2m
A horizontal overhead power line is at a height of 4 m from the ground and carries a
current of 100 A from east to west. The magnetic field directly below it on the ground

7
–1
is 0  4 10 TmA
(a)
2.5 × 10
–7

T southward
(b)
5 × 10–6 T northward
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(c) 5 × 10–6 T southward
Sol. (c)
(d)
2.5 × 10–7 T northward
6.
A long solenoid has 20 turns/cm. The current necessary to produce a magnetic field of 20
mT inside the solenoid is approximately:
(a) 1 A
(b) 2A
(c) 4A
(d) 8A
Sol. (d)
7. A charged particle with charge q enters in a region of constant, uniform and mutually





orthogonal field E and B with a velocity v perpendicular to both E and B and comes out

without any change in magnitude or direction of v . Then :
(a)
Sol. (b)
8.
  
v  B  E / E2
(b)
  
v  E  B / B2
(c)
  
v  B  E / B2
(d)
  
v  E  B / E2
A long straight wire of radius a carries a steady current i. The current is uniformly
distributed across its cross-section. The ratio of the magnetic field a a/2 and 2a is:
(a)
1
2
(b)
1
4
(c)
4
(d)
1
Sol. (d)
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9.
Force acting on electron (charge q) entering in the magnetic field of H wb/m2 along the
field, with velocity v is given by:
(a) Hqv Newton in the direction of magnetic field
(b) Hqv Dynes in the direction of magnetic field
(c) Hqv Newton perpendicular to the direction of magnetic field
(d) zero
Sol. (d)
10. Two similar coils are kept mutually perpendicular such that their centres coincide. At the
centre, find the ratio of the magnetic field due to one and the resultant magnetic field
through both coils, if the same current is flows:
(a)
1: 2
(b)
1:2
(c)
1:3
(d)
3 :1
Sol. (a)
11. A charged particle (charge q) is moving in a circle of radius R with uniform speed v. The
associate magnetic moment  is given by:
(a) qvR2
(b) qvR2/2
(c) qvR
(d) qvR/2
Sol. (d)
12. In the figure shown, the magnetic field induction at the point O will be:
(a)
 0i
2 r
(b)
 0   i 

     2 
 4   r 
(c)
 0  i 

     1
 4   r 
(d)
0i
   2
4 r
Sol. (b)
13. An electron and proton having same kinetic energy enter into a magnetic field perpendicular
to it. Then:
(a) the path of electron is less curved
(b) the path of proton is less curved
(c) both have equal curvel paths
(d) both have straight line paths
Sol. (b)
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14. A conducting rod of 1 m length and 1kg mass is suspended by two vertical wires through
its ends. An external magnetic field of 2T is applied normal to the rod. Now the current to
be passed through the rod so as to make the tension in the wires zero is: (Take g = 10 ms–2)
(a) 0.5 A
(b) 15 A
(c) 5 A
(d) 1.5 A
Sol. (c)
15. A wire oriented in the east-west direction carries a current east-ward. Direction of the
magnetic field at a point to the south of wire is (due to current) :
(a) vertically down
(b) vertically up
(c) north-east
(d) south-west
Sol. (a)
16. An electron revolves in a circle of radius 0.4 Å with a speed of 105 ms–1. The magnitude of
the magnetic field produced at the centre of the circular path due to the motion of the
electron, in Weber/metre2, is:
(a) 0.01
(b) 10.0
(c) 1.0
(d) 0.005
Sol. (c)
17. An electric current passes through a long straight copper wire. At a distance 5 cm from
the straight wire, the magnetic field is B. The magnetic field at 20 cm from the straight
wire would be :
(a)
B
6
(b)
B
4
(c)
B
3
(d)
B
2
Sol. (b)
18. A solenoid of 0.4 m length with 500 turns carries a current of 3 A. A coil of 10 turns and of
radius 0.01 m carries a current of 0.4 A. The torque requires to hold the coil with its axis
at right angles to that of solenoid in the middle part of it is:
(a)
Sol. (a)
6 2  10 –7 Nm
(b)
3 2  10 –7 Nm
(c)
9 2  10 –7 Nm
(d)
12 2  10 –7 Nm
UG–1 & 2, Concorde Complex, Above OBCBank. R.C. Dutt Road., Alkapuri Baroda. Ph. 6625979, 6534152 Cell 9426503864
19. A straight wire of mass 200 g and length 1.5 m carries a current of 2 A. It is suspended in
med-air by a uniform horizontal magnetic field B. The magnitude of B (in tesla) is :
(assume g = 9.8 ms–2 )
(a) 2
(b) 1.5
(c) 0.55
(d) 0.66
Sol. (d)
20. The graph between B and distance (r) from the axis of cylinder (radius R) for a current
flowing through the cylinder is best shown by
(a)
(b)
(c)
(d)
Sol. (a)
21. An electron beam is moving south to north just above a magnetic needle then the north
pole of the magnetic needle will point to wards
(a) west
(b) east
(c) south
(d) north-east
Sol. (d)
22. The order of magnitude of the earth’s magnetic field is
(a) 10 T
(b) 100 T
(c) 1000 T
(d) 1/10,000 T
Sol. (a)
23. The magnetic field strength due to a short bar magnet direction along its axial line at a
distance r is B. What is its value at the same distance along the equatorial line?
B
B
(d)
(a) B
(b) 2B
(c)
2
4
Sol. (a)
24. The most suitable metal for making permanent magnet is
(a) iron
(b) steel
(c) copper
(d) aluminium
Sol. (d)
25. A rectangular piece of soft iron is kept in a uniform magnetic field. Which of the following
correctly represents the lines of force in the region
(a)
(b)
(c)
(d)
Sol. (b)
26. How much work is done to rotate a magnet of dipole moment p along the field in a magnetic
field B through 180º?
(a)
pB
(b)
pB
2
(c)
2pB
(d)
Zero
Sol. (c)
UG–1 & 2, Concorde Complex, Above OBCBank. R.C. Dutt Road., Alkapuri Baroda. Ph. 6625979, 6534152 Cell 9426503864
27. Two identical bar magnets each of dipole moment p and length l are perpendicular to each
other. The dipole moment of the combination is
(a)
2p
(b)
P
2
(c)
P
(d)
2P
Sol. (a)
28. What is the angle of dip at the magnetic poles?
(a) 30º
(b) 0º
(c) 45º
(d) none of these
Sol. (d)
 

 

29. Out of F,v and B in the relation F  q v  B , which of the following pairs can have any


angle between them?






(a) v and B
(b) F and v
(c) F and B
(d) none of the above
Sol. (a)
30. The dipole moment of a current loop does not depend on the
(a) number of turns
(b) area of the loop
(c) current in the loop
(d) magnetic field in which it is kept
Sol. (d)
UG–1 & 2, Concorde Complex, Above OBCBank. R.C. Dutt Road., Alkapuri Baroda. Ph. 6625979, 6534152 Cell 9426503864