Video Lecture
Theory For Notes Making
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Objective Assignment
1.
The magnetism of magnet is due to
(a) The spin motion of electron
(b) Earth
(c) Pressure of big magnet inside the earth
(d) Cosmic rays
Ans (a)
2.
The pole strength of a bar magnet is 48 ampere-metreand the distance between its poles is 25 cm. The moment of the couple by which it can be placed at an angle of 30o with the uniform magnetic intensity of flux density 0.15 Newton /ampere-metre will be
(a) 12 Newton × metre
(b) 18 Newton × metre
(c) 0.9 Newton × metre
(d) None of the above
Ans (c)
3.
The magnetic field at a point x on the axis of a small bar magnet is equal to the field at a point y on the equator of the same magnet. The ratio of the distances of x and y from the centre of the magnet is
(a) {{2}^{{-3}}}
(b) {{2}^{{-1/3}}}
(c) {{2}^{3}}
(d) {{2}^{{1/3}}}
Ans (d)
4.
A magnet of magnetic moment M is situated with its axis along the direction of a magnetic field of strength B. The work done in rotating it by an angle of 180o will be
(a) -MB
(b) +MB
(c) 0
(d) +2MB
Ans (d)
5.
A long magnet is cut in two parts in such a way that the ratio of their lengths is 2 : 1. The ratio of pole strengths of both the section is
(a) Equal
(b) In the ratio of 2 : 1
(c) In the ratio of 1 : 2
(d) In the ratio of 4 : 1
Ans (a)
6.
A long magnetic needle of length 2L, magnetic moment M and pole strength m units is broken into two pieces at the middle. The magnetic moment and pole strength of each piece will be
(a) \frac{M}{2},\frac{m}{2}
(b) M,\frac{m}{2}
(c) \frac{M}{2},m
(d) M,m
Ans (c)
7.
Force between two unit pole strength placed at a distance of one metre is
(a) 1 N
(b) \frac{{{{{10}}^{{-7}}}}}{{4\pi }}N
(c) {{10}^{{-7\,}}}\,N
(d) 4\pi \times {{10}^{{-7}}}N
Ans (c)
8.
Two equal bar magnets are kept as shown in the figure. The direction of resultant magnetic field, indicated by arrow head at the point P is (approximately)
Ans (b)
9.
A short bar magnet placed with its axis at 30° with a uniform external magnetic field of 0.16 Tesla experiences a torque of magnitude 0.032 Joule. The magnetic moment of the bar magnet will be
(a) 0.23 Joule/Tesla
(b) 0.40 Joule/Tesla
(c) 0.80Joule/Tesla
(d) Zero
Ans (b)
10.
Two magnets, each of magnetic moment ‘M’ are placed so as to form a cross at right angles to each other. The magnetic moment of the system will be
(a) 2 M
(b) \sqrt{2}\,M
(c) 0.5 M
(d) M
Ans (b)
11.
A bar magnet of magnetic moment 3.0 A-m2 is placed in a uniform magnetic induction field of 2 ´ 10–5 T. If each pole of the magnet experiences a force of 6 ´ 10–4N, the length of the magnet is
(a) 0.5 m
(b) 0.3 m
(c) 0.2 m
(d) 0.1 m
Ans (d)
12.
A bar magnet of length 3 cm has points A and B along its axis at distances of 24 cm and 48 cm on the opposite sides. Ratio of magnetic fields at these points will be
(a) 8
(b) 1/2\,\,\sqrt{2}
(c) 3
(d) 4
Ans (a)
Subjective Assignment
Q.1
Why do the magnetic field lines of a magnet form continuous closed loops ?
Q.2
A coil of N turns and radius R carries a current I. It is unwound and rewound to make another coil of radius R/2, current remaining the same. Calculate the ratio of magnetic moments of the new coil and the original coil.
Q.3
Why is a current loop considered a magnetic dipole?
Q.4
Does the length of an iron bar change when it is magnetized?
Q.5
What is tangent law in magnetism?
Q.6
The magnetic field at a point on the magnetic equator is found to be \displaystyle 3.1\,\times \,{{10}^{{-5\,T}}}. Taking the earth’s radius to be 6400 km, calculate the magnetic moment of the assumed dipole at the earth’s centre.
Q.7
A current of 5A is flowing through a 10 turn circular coil of radius 7 cm. The coil lies in XY plane. What is the magnitude and direction of magnetic dipole moment associated with it ?
Q.8
What is the magnitude of the equatorial and axial field due to a bar magnet of length 5.0 cm at a distance of 50 cm from its mid-point ? The magnetic moment of the bar magnet is 0.40 Am2.
Q.9
A closely wound solenoid of 800 turns and area of cross section 2.5 × 10–4 m2 carries a current of 3.0 A. Explain the sense in which the solenoid acts like a bar magnet. What is its associated magnetic moment?
Q.10
A magnetic dipole is under the influence of two magnetic fields. The angle between the field directions is 60º, and one of the fields has a magnitude of 1.2 × 10–2 T. If the dipole comes to stable equilibrium at an angle of 15º with this field, what is the magnitude of the other field?
Q.11
Why are 2, 50 and 500 turns provided in the coil of tangent galvanometer?
Q.12
An electric current of 0.25 A flows in a loop of radius 0.2 cm. What is the magnitude of dipole moment?
Q.13
Two circular loops of radii r and 2r have currents I and I/2 flowing through them in clockwise and anticlockwise sense respectively. If their equivalent magnetic moments are \displaystyle {{M}_{1}} and \displaystyle {{M}_{2}}, what is the relation between \displaystyle {{M}_{1}} and \displaystyle {{M}_{2}}?
Q.14
Define the term : Magnetic dipole moment of a current loop. Write the expression for the magnetic moment when an electron revolves at a speed \displaystyle \upsilon around an orbit of radius r in hydrogen atom.
Q.15
An electron is revolving in a hydrogen atom in a circular orbit of radius 4Å making \displaystyle {{10}^{{15}}} rps. What is the magnetic moment associated with this electron?
Q.16
A short bar magnet has a magnetic moment of 0.48 J T–1. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10 cm from the centre of the magnet on (a) the axis, (b) the equatorial lines (normal bisector) of the magnet.
Q.17
(a) A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at 14 cm from the centre of the magnet. The earth’s magnetic field at the place is 0.36 G and the angle of dip is zero. What is the total magnetic field on the normal bisector of the magnet at the same distance as the null–point (i.e., 14 cm) from the centre of the magnet? (At null points, field due to a magnet is equal and opposite to the horizontal component of earth’s magnetic field.)
(b) If the above bar magnet is turned around by 180º, where will the new null points be located?
Q.18
Two tangent galvanometers differ only in the matter of number of turns in the coil. On passing current through the two joined in series, the first shows a deflection of 35º and the other shows 45º deflection. Compute the ratio of their number of turns. Take tan 35º=0.7.
Q.19
(a) Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?
(b) If magnetic monopoles existed, how would Gauss’s law of magnetism be modified?
(c) Mangnetic field arises due to charges in motion. Can a system have magnetic moment even though its net charge is zero?
(d) What happens if a bar magnet is cut into two pieces (i) transverse to its length (ii) along its length?
(e) Must every magnetic field configuration have a north pole and a south pole? What about the field due to a toroid?
(f) Can you think of magnetic field configuration with three poles?
Q.20
Depict the field line pattern due to a current carrying solenoid of finite length.
(i) In what way do these lines differ from those due to an electric dipole.
(ii) Why can’t two magnetic field lines intersect each other?
Q.21
State and prove tangent law in magnetism. Give two factors by switch tangent galvanometer can be made more sensitive.
Q.22
Describe the principle, construction, theory and working of a tangent galvanometer.
Q.23
A bar magnet is placed in a uniform magnetic field with its magnetic moment making an angle with the field. Write expressions for torque on the bar magnet and potential energy of magnet in this orientation. When is this energy minimum ?
Q.24
(a) Magnetic force is always normal to the velocity of a charge and therefore does no work. An iron nail held near a magnet, when released, increased its kinetic energy as it moves to cling to the magnet. What agency is responsible for this increase in kinetic energy if not the magnetic field?
Q.25
Two identical magnets with a length 10 cm and weight 50 kgf each are arranged freely with their like poles facing in a vertical glass tube. The upper magnet hangs in air above the lower one so that the distance between the nearest poles of the magnets is 3 mm. Determine the pole strength of the poles of these magnets.
Q.26
Deduce an expression for the magnetic dipole moment of an electron orbiting around the central nucleus.
Q.27
(a) Magnetic field lines show the directions (at every point) which a small magnetized needle takes up (at that point). Do the magnetic field lines also represent the lines of force of a moving charged particle at every point?
(b) Two identical looking iron bars A and B are given, one of which is definitely known to be magnetized. How would one ascertain whether or not both are magnetised ? If only one is magnetized how does one ascertain which one ? Use nothing else but the bars A and B.
Q.28
A bar magnet AB is cut into two equal parts one part is kept over the other so that pole \displaystyle {{C}_{2}} is above \displaystyle {{C}_{1}}. If M is magnetic moment of original magnet, what will be the magnetic moment of the combination so formed?
