Unit 3bb tủ tài liệu training

VERY SHORT AND SHORT-ANSWERS QUESTIONS 46 Name the law on which the tangent galvanometer works 47 Where are the null points obtained when a bar magnet is placed horizontally with its north pole towards earths’ magnetic south pole? 48 Where are the null points obtained when a bar magnet is placed horizontally with its north pole towards earths’ magnetic north pole? 49 What is the basic difference between magnetic lines of force and electric lines of force ? 50 Write the unit of magnetic dipole moment in terms of joule 51 Does a magnetic monopole exist ? 52 Write two basic properties of a magnet 53 What you mean by the poles of a magnet ? 54 Write the S.I unit of pole strength 55 What is magnetic length ? 56 Why does a paramagnetic sample display greater magnetization (for the same magnetizing field) when cooled ? 57 Why are diamagnetic substances not affected by temperature ? 58 Two iron bars A and B are given, one of which is definitely known to be magnetized, but which one we not know How would you ascertain whether or not both are magnetized ? If only one is magnetized, how does one ascertain which one ? Use only the two bars A and B 59 Magnetic field arises due to charges in motion Can a system have magnetic moment even though its net charge is zero ? 60 Calculate the dipole moment of a magnetic dipole of pole strength 30 A -m and of length 0.2 m 61 What is the magnetic moment associated with a coil of one turn, having area of cross section (AISSCE Delhi 1993) 10–4 m2, carrying a current of 2A? S Chand & Company Limited 62 At a place the angle of dip is 30° Find the horizontal and the vertical components of earth’s magnetic field Given the magnitude of the earth’s field = 0.4 gauss 63 A bar magnet of magnetic moment 5.0 Am2 has poles 20 cm apart Calculate its pole strength 64 How can the magnetic moments of two bar magnets of the same size and mass be compared using a vibration magnetometer ? 65 Describe how the horizontal components of earths’ magnetic field at two places may be compared by using a vibration magnetometer 66 What are the differences between the magnetic properties of soft iron and steel ? 67 Draw lines of force of two magnets in the space between them, if (a) the N pole of one magnet is directed towards the S pole of the other, their axes coinciding (b) the N pole of one magnet is directed towards the N pole of the other, their axes coinciding 68 Draw pattern of lines of force of a bar magnet when the magnet is placed in the magnetic meridian and the N pole of the magnet points towards the north pole of the earth 69 Draw pattern of lines of force of a bar magnet when the magnet is placed in the magnetic meridian and N pole of the magnet points towards the south pole of the earth 70 Define retentivity 71 Does an iron bar magnet retain its magnetisation when melted ? Explain (AISSCE 1990 C) 72 Does the magnetisation of a paramagnetic salt depend on temperature ? Explain (AISSCE 1990) 73 Interstellar space has an extermely weak magnetic field of the order of 10–12 T Can such a weak field be of any significant consequence ? Explain 74 Write some properties of diamagnetic substances 75 Write some properties of paramagnetic substances 76 Define coercivity S Chand & Company Limited What are the uses of ferromagnetic substances ? Which direction would a compass point to if located near the north or the south pole of the earth ? Why we use steel or ALNICO for making permanent magnets ? Name two alloys which can be used for making electromagnets or chokes Write an expression for the work done in rotating a bar magnet of moment M through an angle θ from the equilibrium position in a magnetic field B 82 What is the angle of dip at a place where the horizontal component of earth’s magnetic field is equal to the vertical component? 83 A magnetic needle of magnetic moment M and pole strength m is broken into two pieces at the middle What are the magnetic moment and the pole strength of each piece ? 84 A freely suspended bar magnet oscillates with a period T If it is cut into two equal parts parallel to its length, what is the time period of each part ? 77 78 79 80 81 85 Horizontal component of earth’s magnetic field at a place is times the vertical component What is the value of the angle of dip at this place ? (AISSCE Delhi 1997) 86 What is the angle of dip at a place where the horizontal and the vertical components of the earth’s magnetic field are equal ? 87 What is the use of dip circle ? 88 What is the magnetic axis of the earth ? 89 A bar magnet is placed in a uniform magnetic field In which position the potential energy of the magnet is (i) maximum (ii) zero 90 Angles of dip at two places on the surface of the earth are 0° and 90°, respectively Where are these places located ? 91 What material is used in making the core of a moving coil galvanometer ? (AISSCE 1996) 92 Which type of magnetic material is used in making permanent magnets ? (AISSCE 1996) S Chand & Company Limited ANSWERS 46 47 48 49 A tangent galvanometer works on the tangent law The null points are obtained on the equatorial line of the magnet The null points are obtained on the axial line of the magnet An electric line of force starts at one point (positive charge) and ends at some other point (negative charge) A magnetic line of force has no beginning and end It forms closed loop 50 joule/testa (JT–1) 51 No, the most elementary magnetic entity is a dipole 52 (i) A magnet attracts certain substances like iron, nickel, cobalt etc (ii) A freely suspended magnet always aligns itself nearly in the geographic North-South direction 53 Poles are two points near the two ends of a magnet where the magnetic force is maximum 54 The S I unit of pole strength is Ampere–meter (Am) 55 The distance between the two poles of a magnet is called its magnetic length The magnetic length is always less then the actual length of a magnet 56 At lower temperatures, the tendency to disrupt the alignment of atomic dipoles arising from random thermal motion is reduced 57 Diamagnetism is due to induced dipole moment in a sample which is always opposite to the magnetizing field, no matter what the internal motion of the atom is 58 Bring different ends of the bars closer A repulsive force in some situations shows that both are magnetized If the force is always attractive then only one bar is magnetized To see which one, pick up one, say A, and lower one of its ends first on one of the ends of B, and then on the middle of B If you find that in the middle of B, A experiences no force, then B is magnetized If you not find any change from the end to the middle of B, then A is magnetized S Chand & Company Limited 59 Yes, It is possible The net charge in a system may be zero but it is not necessary that the net magnetic moment due to all the current loops will also be zero For example, a neutron has zero charge, but non-zero magnetic moment 60 Dipole moment M = Pole strength × Length of magnet = 30 Am × 0.2 m = 6.0 Am2 61 Magnetic dipole moment M = niA Here n = 1, i = 2, A = 10–4 m2 M = × × 10–4 = × 10–4 Am2 62 B = 0.4 Gauss = × 10–4 T Horizontal component BH = B cos θ = 0.4 × cos 30° = 0.4 × = 0.35 gauss Vertical component BV = B sin θ = 0.4 × sin 30° = 0.4 × = 0.2 gauss S Chand & Company Limited 63 We have M = Pole strength × Length of magnet Here M = 5.0 Am2 Length of magnet = 0.2 m 5.0 = 25 Am 0.2 64 The time period of oscillation of a magnet in a vibration magnetometer is ∴ Pole strength = I T = 2π MBH or M= 4π I .(1) T BH In order to compare the magnetic moments of two magnets, they are made to oscillate in turn in the magnetometer If M1, M2 are their magnetic moments, I1, I2, are their respective moments of inertia, and T1, T2 are their respective time periods of oscillation, then from Eq (1) S Chand & Company Limited 4π I1 M1 = .(2) T1 BH M2 = 4π I .(3) T2 BH Dividing Eq (2) by Eq (3) M1 M2 = I1 T2 2 I2 T If the mass and size are same, then I1 = I2 In that case M1 M2 = T2 T1 2 65 A given magnet is made to oscillate in a vibration magnetometer at the two places, in turn If T1 and T2 are the time periods and B1 and B2 are the horizontal components of the earth’s field at the two places, then T1 = 2π T2 = 2π I , and .(1) , .(2) MB1 I MB2 S Chand & Company Limited where I and M are the moments of inertia and the magnetic moment of the magnet Dividing Eq (2) by Eq (1), B1 B2 = T2 T1 2 66 Soft iron has low coercivity and small hysteresis loss It can be easily magnetized and demagnetized Steel has high coercivity and large hysteresis loss It cannot be easily magnetized and demagnetized 67 (a) S N Fig B.2 (b) S N N N S Fig.3 B.3 S Chand & Company Limited 68 N N X X S S Fig B.4 69 N S N S Fig B.5 S Chand & Company Limited 70 When a piece of ferromagnetic material is magnetised to saturation and then the magnetising field is removed, some magnetic induction remains in the material This remaining field is called remanence or retentivity This is due to the tendency of domains to stay together once they have been aligned 71 Iron melts at a temperature higher than its curie temperature Therefore a bar magnet loses its magnetisation on melting as the alignment of its atomic magnets is destroyed 72 Yes, the magnetisation of a paramagnetic salt depends on temperature The atoms of a paramagnetic salt have non-zero magnetic moments As the temperature of such a substances is increased, the alignment of these atomic magnets decreases due to thermal agitation and hence the magnetisation also decreases 73 From the relation r = mv/Bq, an extermely weak field bends charged particles in circles of very large radii Over a small distance this bending is not noticeable, but over large interstellar distances, there can be significant deflection in the path of, e.g., cosmic rays 74 Properties of diamagnetic substances: (a) When placed in a non-uniform magnetic field, they move from stronger to weaker parts of the field (b) They are feebly repelled by a magnet (c) Diamagnetic behaviour is independent of temperature (d) Their relative permeability is slightly less than (e) Their susceptibility is small and negative 75 Properties of paramagnetic substances : (a) They are feebly attracted by a magnet (b) When placed in a non-uniform magnetic field, they move from weaker to stronger parts of the field (c) Paramagnetic property decreases with rise in temperature (d) Their relative permeability is slightly greater than (e) Their susceptibility is small and positive S Chand & Company Limited 76 The reverse magnetising field required to destroy the residual magnetic induction in a material is called coercivity It is a measure of the difficulty of breaking up the alignment of the domains 77 Ferro-magnetic substances are mainly used in making permanent magnets, electromagnets and transformer cores 78 The compass can point in any direction, because at the poles, earth’s field is vertical and compass is free to move in a horizontal plane 79 For steel and ALNICO, the values of coercivity and retentivity are high 80 (a) Mu-metal (alloy of nickel, iron, copper and chromium) (b) Radio-metal (alloy of nickel, iron, copper and manganese) 81 W = MB (1 – cos θ) 82 We have tan δ = BV BH =1 ⇒ δ = 45° 83 Magnetic moment of each piece = M Pole strength of each piece = m 84 We know T = 2π I MH When the magnet is cut parallel to its length the moment of inertia and the magnetic moment of each part will become half The new time period S Chand & Company Limited T ′ = 2π I /2 ( M / 2) H = 2π I MH =T Thus the time period remains the same 85 86 tan δ = BV BH = ⇒ δ = 30° BH = BV or B cos δ = B sin δ or tan δ = or δ = 45° 87 It is used to measure the angle of dip at a place 88 The straight line passing through the magnetic north and south poles of the earth is called the magnetic axis of the earth 89 Potential energy U = – MB cos θ (i) U is maximum when θ = 180° Umax = – MB cos 180° = MB → → (ii) U is zero when θ = 90°, i.e., M & B are perpendicular to each other 90 Angle of dip is zero at the magnetic equator and 90° is at the magnetic poles 91 Soft iron (It has high magnetic permeability) 92 Materials having high remanence and high coercivity are used for making permanent magnet, e.g., steel S Chand & Company Limited ... Poles are two points near the two ends of a magnet where the magnetic force is maximum 54 The S I unit of pole strength is Ampere–meter (Am) 55 The distance between the two poles of a magnet is