Physics - Magnetism and Matter
Exam Duration: 45 Mins Total Questions : 20
A toroid of n turn, mean radius R and cross- sectional radius a carries current I. It is placed on a horizontal table taken as XY-plane. Its magnetic moment m
- (a)
is non-zero and points in the Z direction by symmetry
- (b)
points along the axis of the toroid
- (c)
is zero
- (d)
is pointing radially outwards
The inner and the outer radii of a toroid are 9 cm and 11 cm respectively and the number of turns in it is 3140. A magnetic field of 2.5 T is produced in it when a current of 0.5 A is passed in it. The permeability of core material is (in H/m)
- (a)
10-1
- (b)
10-2
- (c)
10-3
- (d)
10-4
The magnetic field of the earth can be modelled by that of a point dipole placed at the centre of the earth. The dipole axis makes an angle of 11.3o with the axis of the earth. At Mumbai, declination is nearly zero. Then,
- (a)
the declination varies between 11.3o W to 11.3oE
- (b)
the least declination is 0o
- (c)
the plane defined by dipole axis and earth axis passes through greenwich
- (d)
declination averaged over the earth always be negative
The horizontal component of the earth's magnetic field at any place is 0.36X10-4 Wb/m2 . If the angle of dip at that place is 60o , then the value of vertical component of the earth's magnetic field will be (in Wb/m2)
- (a)
0.12X10-4
- (b)
0.24X10-4
- (c)
0.40X10-4
- (d)
0.622X10-4
The dipole moment of each molecule of a paramagnetic gas is 1.5X10-23 A-m2. The temperature of gas is 27o C and the number of molecules per unit volume in it is 2.0X1026 m-3 m-3 . The maximum possible intensity of magnetisation in the gas will be (in A/m)
- (a)
3X103
- (b)
4X10-3
- (c)
5X105
- (d)
6X10-4
In a permanent magnet at room temperature
- (a)
magnetic moment of each molecule is zero
- (b)
the individual molecules have non-zero magnetic moment which are all perfectly aligned.
- (c)
domains are partially aligned
- (d)
domains are all perfectly aligned
The cause of paramagnetism is
- (a)
unpaired electrons
- (b)
excess electron and spin motion of electrons
- (c)
paired electrons and orbital motion of electrons
- (d)
excess electrons and orbital motion of electron
The magnetic susceptibility for paramagnetic materials depends on temperature as
- (a)
\(\chi \propto \frac { 1 }{ T } \)
- (b)
\(\chi \propto T\)
- (c)
\(\chi \propto { T }^{ 2 }\)
- (d)
\(\chi \propto \frac { 1 }{ { T }^{ 2 } } \)
The coercivity of a bar magnet is 100 A/m is to be demagnetised by placing it inside a solenoid of length 100 cm and number of turns 50. The current flowing in the solenoid will be
- (a)
4 A
- (b)
2 A
- (c)
1 A
- (d)
zero
A short bar magnet of magnetic dipole moment m=0.32JT-1 is placed in a uniform external magnetic field of 0.15 T. if bar is free to rotate in the field, then
- (a)
when m is antiparallel to B, U=4.8X10-2 J
- (b)
when m is perpendicular to B, U=0
- (c)
when m is perpendicular to B, U=4.8X10-2J
- (d)
None of the above
Choose the correct alternative.
- (a)
Spin magnetic moment of an electron is \({ \mu }_{ s }=-\left( \frac { e }{ m } \right) s,\) where s=intrinsic spin angular momentum of electron
- (b)
Orbital magnetic moment \({ \mu }_{ e }=-\left( \frac { e }{ 2m } \right) .l,\) when l= angular momentum of electron
- (c)
Protons also has a spin magnetic moment associated with them
- (d)
All of the above
What will be the energy loss per hour in the iron core of a transformer, if the area of its hysteresis loop is equivalent to 2500 erg/ cm3 . The frequency of alternating current is 50 Hz. The mass of core is 10 Kg and the density of iron is 7.5 g/cm3 .
- (a)
2X102 J
- (b)
4X103 J
- (c)
6X104 J
- (d)
8X105 J
A rod of ferromagnetic material with dimensions 10 cm X 0.5 cm X 0.2 cm is placed in a magnetic field of strength 0.5 X 104 A/m as a result of which a magnetic moment of 5 A-m2 is produced in the rod. The value of magnetic induction will be
- (a)
0.54 T
- (b)
0.358 T
- (c)
2.591 T
- (d)
6.28 T
The coercivity of a small magnet where the ferromagnet gets demagnetised is 3X103 Am-1 . The current required to be passed in a solenoid of length 10 cm and number of turns 100, so that the magnet gets demagnetised when inside the solenoid is
- (a)
30 mA
- (b)
60 mA
- (c)
3 A
- (d)
6 A
Two short bar magnets of length 1 cm each have magnetic moments 1.20 Am2 and 1.00 Am2 , respectively. They are placed on a horizontal table parallel to each other with their N pole pointing towards the south. They have a common magnetic equator and are separated by a distance of 20.0 cm
The value of the resultant horizontal magnetic induction at the mid-point O of the line joining their centres is close to (horizontal component of the earth's magnetic induction is 3.6 x 10-5 Wb/m2)
- (a)
3.6X10-5 Wb/m2
- (b)
2.56X10-4 Wb/m2
- (c)
3.50X10-4 Wb/m2
- (d)
5.80X10-4 Wb/m2
Relative permittivity and permeability of a material are \({ \varepsilon }_{ r }\quad and\quad { \mu }_{ r }\) respectively. Which of the following values of these quantities are allowed for a diamagnetic material?
- (a)
\({ \varepsilon }_{ r }=0.5,\quad { \mu }_{ r }=1.5\)
- (b)
\({ \varepsilon }_{ r }=1.5,\quad { \mu }_{ r }=0.5\)
- (c)
\({ \varepsilon }_{ r }=0.5,\quad { \mu }_{ r }=0.5\)
- (d)
\({ \varepsilon }_{ r }=1.5,\quad { \mu }_{ r }=1.5\)
Needles N1, N2 and N3 are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will
- (a)
attract N1 and N2 strongly but repel N3
- (b)
attract N1 strongly, N2 weakly and repel N3 weakly
- (c)
attract N1 strongly but repel N2 and N3 weakly
- (d)
attract all three of them
A magnetic needle is kept in a non- uniform magnetic field. It experiences
- (a)
a torque but not a force
- (b)
neither a force nor a torque
- (c)
a force and a torque
- (d)
a force but not a torque
A thin rectangular magnet suspended freely has a period of oscillation equal to T. Now, it is broken into two equal halves (each having half of the original length) and one piece is made to oscillate freely in the same field. If its period of oscillation is T'. the ratio T'/T is
- (a)
\(\frac { 1 }{ 2\sqrt { 2 } } \)
- (b)
\(\frac { 1 }{ 2 } \)
- (c)
\(2\)
- (d)
\(\frac { 1 }{ 4 } \)
The magnetic lines of force inside a bar magnet
- (a)
are from north-pole to south-pole of the magnet
- (b)
do not exist
- (c)
depend upon the area of cross section of the bar magnet
- (d)
are from south-pole to north-pole of the magnet.