### Eamcet - Physics - Magnetics

If two identical bar magnets, each of length l, pole strength m and magnetic moment  M are placed perpendicular to each other with their unlike poles in contact, the magnetic moment of the combination is :

A.  M / √2

B.  lm ( √2 )

C.  2 lm ( √2 )

D.  2M

Magnetic field induction at the centre of a circular coil of radius 5 cm and carrying a current 0.9 A is (in S.I. units) (ε0 = absolute permittivity of air in S.I. units; velocity of light = 3 x 108 ms-1)

A.  1 / ε0 1016

B.  1016 / ε0

C.  ε0 / 1016

D.  1016ε0

With a standard rectangular bar magnet of length (l), breadth (b; b < < l) and magnetic moment M, the time period of the magnet in a vibration magnetometer is 4 s. If the magnet is cut normal to its length into four equal pieces, the time period (in seconds) with one of the pieces is :

A.  16

B.  2

C.  1

D.  1/4

Which one of the following statement is true ?

A.  Paramagnetism ceases to exist below a certain temperature

B.  Ferromagnetism ceases to exist below a certain temperature

C.  Onset of paramagnetism requires the presence of paramagnet magnetic dipoles

D.  Ferromagnetism ceases to exist above a certain temperature

The magnetic induction at a point, distance x from the centre, on the axis of a circular current carrying coil is inversely proportional to (if x > > radius of coil) :

A.  x

B.  x2

C.  x3

D.  x3/2

A bar-magnet of moment of inertia 49 x 10-2 kg-m2 vibrates in a magnetic field of induction 0.5 x10-4 T. The time period of vibration is 8.8 s. The magnetic moment of the bar magnet is

A.  350 A-m2

B.  490 A-m2

C.  3300 A-m2

D.  5000 A-m2

A bar magnet of magnetic moment M and moment of inertia I is freely suspended such that the magnetic axial line is in the direction of magnetic meridian. If the magnet is displaced by a very small angle (θ), the angular acceleration is (Magnetic induction of earth's horizontal field = BH)

A.  MBH θ / I

B.  IBH θ /M

C.  M θ / IBH

D.  I θ /MBH

Two wires A and B are of lengths 40 cm and 30 cm, A is bent into a circle of radius r and B into an arc of radius r. A current i1 passed.through A and i2 through B. To have the same magnetic inductions at the centre, the ratio of    i1  :   i2  is

A.  3:4

B.  3:5

C.  2:3

D.  4:3

An electron beam travels with a velocity of 1.6 x 107 ms-1 perpendicularly to magnetic field of intensity 0.1 T. The radius of the path of the electron beam (me = 9 x 10-31 kg)

A.  9 x 10-5 m

B.  9 x 10-2 m

C.  9 x 10-4 m

D.  9 x 10-3 m

The effect due to uniform magnetic field on a freely suspended magnetic needle is as follows :

A.  both torque and net force are present

B.  torque is present but no net force

C.  both torque and net force are absent

D.  net force is present but not torque

Two short magnets AB and CD are in the X-Y plane and are parallel to X-axis and co-ordinates of their centres respectively are (0, 2) and (2, 0). Line joining the north-south poles of CD is opposite to that of AB and lies along the positive X-axis. The resultant field induction due to AB and CD at a point P (2, 2) is 100 x 10-7T. When the poles of the  magnet CD are reversed, the resultant field induction is 50 x 10-7 T. The value of magnetic moments of AB and CD (in Am2) are :

A.  300 ; 200

B.  600 ; 400

C.  200 ; 100

D.  300 ; 150

When a positively charged particle enters a uniform magnetic field with uniform velocity, its trajectory can be : 1) a straight line 2) a circle 3) a helix

A.  (1) only

B.  (1) or (2)

C.  (1) or (3)

D.  any one of (1) , (2) and (3)

Two bar magnets A, B are placed one over the other and are allowed to vibrate in a vibration magnetometer. They make 20 oscillations per minute when the similar poles of A and B are on the same side, while they make 15 oscillations per minute when their opposite poles lie on the same side. If MA and MB are the magnetic moments of A and B and if MA > MB, the ratio of MA and MB is

A.  4 : 3

B.  25 : 7

C.  7 :5

D.  25 : 16

A bar magnet of 10 cm long is kept with its north (N)-pole pointing North. A neutral point is formed at a distance of 15 cm from each pole. Given the horizontal component of earth's field is 0.4 Gauss, the pole strength of the magnet is

A.  9 amp-m

B.  6.75 amp-m

C.  27 amp-m

D.  13.5 amp-m

A wire of length ‘l’ is bent into a circular loop of radius R and carries a current I. The magnetic field at the centre of the loop is 'B'. The same wire is now bent into a double loop of equal radii. If both loops carry the same current I and it is in the same direction, the magnetic field at the centre of the double loop will be

A.  0

B.  2B

C.  4B

D.  8B

The direction of the magnetic field produced by straight current carrying conductor is given by

A.  Right hand thumb rule

B.  Flemings left hand rule

C.  Joules law

D.  amperes law

A long straight wire carries an electric current of 2A.The magnetic induction at a perpendicular distance of 5m from the wire is (μ0=4πx10-7 Hm-1)

A.  4x10-8T

B.  8 x10-8T

C.  12 x10-8T

D.  16 x10-8T

The coil in a MCG has an area of 4cm2 and 500 turns.The intensity of magnetic induction is 2T.when a current of 10-4A is passed through it,the deflection is 200.The couple per unit twist is(N-m)

A.  5 x10-6

B.  4 x10-6

C.  2x10-6

D.  3x10-6

There is no couple acting when two bar magnets are placed coaxially separated by a distance because:

A.  there are no forces on the poles

B.  the forces are parallel and their lines of action do not coincide

C.  the forces are perpendicular to each other

D.  the forces act along the same line

A magnetic needle lying parallel to a magnetic field is turned through 600.The work does on it is w. The torque required to maintain the magnetic needle in the position mentioned above is

A.  √3w

B.  (√3/2)w

C.  w/2

D.  2w

The magnetic induction at the centre due to motion of electron in first Bohr orbit is B. magnetic filed due to the motion of electron in second Bohr orbit at the centre will bw

A.  B/64

B.  B/32

C.  B/16

D.  B/4

A proton takes 10-12 to complete one revolution in uniform magnetic field.The time taken in another orbit of double the radius in the same field is

A.  10-12 sec

B.  4x10-12sec

C.  2x10-12sec

D.  0.5x10-12sec

A wire of length 44cm is bent into a circle and a current of 9.8A is passed through it.The intensity of magnetic induction at the centre of the circle is

A.  8.8x10-6T

B.  8.8 x10-5T

C.  8.4 x10-6T

D.  61.5 x10-5T

A proton is projected with a velocity 107ms-1 ,at right angles to a uniform magnetic field of induction 100mT.The time(in seconds)taken by the proton to traverse 900 arc is:(Mass of proton=1.65x10-27kg and charge of proton = 1.6x10-19C

A.  3.24x10-7

B.  2.43 x10-7

C.  1.62 x10-7

D.  0.81 x10-7

A long horizontal rigidly supported wire carries ia=96A.Directly above it and parallel to it at a distance, another wire of 0.144N weight per meter carrying a current ib=24A, in a direction opposite to that of ia is placed. If the upper wire is to float in air due to magnetic repulsion, then tits distance (in mm) from the lower wire is:

A.  9.6

B.  4.8

C.  3.2

D.  1.6

To measure the field B between the poles of an electromagnet,a small test loop of area 1cm2,resistance 10Ω and 20 turns is pulled out of it.A galvanometer shows that the total charge of 2μC passed through the loop.The value of resistance connected in series is

A.  1.0 T

B.  0.1 T

C.  0.01 T

D.  0.001 T

The coils in two moving coil galvanometer have their areas in the ratio of 2:3 and number of turns in the ratio 4:5. These two coils carry the same current and are situated in the same field. The deflections produced by these coil will be in the ratio of

A.  8:15

B.  15:8

C.  8:1

D.  1:4

The area of the coil in moving coil galvanometer is 80 cm2 and its has 200 turns. The magnetic induction of the radial field is 0.2T and the couple per unit twist of the suspension wire is 2 x 10-6Nm per degree. If the deflection is 40,the current passing through it is

A.  0.25mA

B.  2.5 mA

C.  0.025 mA

D.  250 mA

Two long straight conductors are held parallel to each other at a distance 0.3 m. If the conductors carry currents 2A and 8A in opposite directions, the distance of the neutral point from the conductor carrying smaller current is:

A.  0.06 m

B.  0.24 m

C.  0.18 m

D.  0.1 m

Two concentric circular coils A and B have radii 25cm and 15cm and carry currents 10A and 15A respectively.A has 24 turns and B has 18 turns.The direction of currents are in opposite order.The magnetic induction at the common centre of the coil is

A.  μo

B.  420μoT

C.  480μoT

D.  120μoT