Video Lecture
Theory For Notes Making
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Objective Assignment
1.
At a place, if the earth’s horizontal and vertical components of magnetic fields are equal, then the angle of dip will be
(a) 30{}^\circ
(b) 90{}^\circ
(c) \displaystyle 45{}^\circ
(d) 0°
Ans (c)
2.
At a certain place, the horizontal component of earth’s magnetic field is \sqrt{3}times the vertical component. The angle of dip at that place is
(a) 60{}^\circ
(b) 45{}^\circ
(c) 90{}^\circ
(d) {{30}^{0}}
Ans (d)
3.
The angle between the magnetic meridian and geographical meridian is called
(a) Angle of dip
(b) Angle of declination
(c) Magnetic moment
(d) Power of magnetic field
Ans (b)
4.
At a place the earth’s horizontal component of magnetic field is 0.36\times {{10}^{{-4}}}weber/{{m}^{2}}. If the angle of dip at that place is 60o, then the vertical component of earth’s field at that place in weber/m2 will be approximately
(a) 0.12\times {{10}^{{-4}}}
(b) 0.24\times {{10}^{{-4}}}
(c) 0.40\times {{10}^{{-4}}}
(d) 0.62\times {{10}^{{-4}}}
Ans (d)
5.
The lines joining the places of the same horizontal intensity are known as
(a) Isogonic lines
(b) Aclinic lines
(c) Isoclinic lines
(d) Agonic lines
(e) Isodynamic lines
Ans (e)
6.
A line passing through places having zero value of magnetic dip is called
(a) Isoclinic line
(b) Agonic line
(c) Isogonic line
(d) Aclinic line
Ans (d)
7.
The magnetic compass is not useful for navigation near the magnetic poles because
(a) The magnetic field near the poles is zero
(b) The magnetic field near the poles is almost vertical
(c) At low temperature, the compass needle looses its magnetic properties
(d) Neither of the above
Ans (b)
8.
At a place, the magnitudes of the horizontal component and total intensity of the magnetic field of the earth are 0.3 and 0.6 Oersted The value of the angle of dip at this place will be
(a) 60°
(b) 45°
(c) 30°
(d) 0°
Ans (a)
9.
A compass needle will show which one of the following directions at the earth’s magnetic pole
(a) Vertical
(b) No particular direction
(c) Bent at 45° to the vertical
(d) Horizontal
Ans (a)
10.
The north pole of the earth’s magnet is near the geographical
(a) South
(b) East
(c) West
(d) North
Ans (a)
Subjective Assignment
Q.1
The horizontal component of the earth’s magnetic field at a place is B and angle of dip is 60º. What is the value of vertical component of earth’s magnetic field at equator ?
Q.2
Where on the surface of Earth is the vertical component of Earth’s magnetic field zero ?
Q.3
Define the term ‘magnetic declination’.
Q.4
Where on the surface of Earth is the angle of dip 90º?
Q.5
Where on the surface of Earth is the angle of dip zero ?
Q.6
If the horizontal and vertical components of the Earth’s magnetic field are equal at a certain place, what would be the angle of dip at that place ?
Q.7
The angle of dip at a location in southern India is about 18º. Would you expect a greater or lesser dip angle in Britain?
Q.8
The horizontal component, of the earth’s magnetic field, at a place, is \displaystyle \frac{1}{{\sqrt{3}}} times its vertical component there. Find the value of the angle of dip at that place. What is the ratio of the horizontal component to the total magnetic field of the earth at that place ?
Q.9
The horizontal component, of the earth’s magnetic field, at a place, equals its vertical component there. Find the value of the angle of dip at that place. What is the ratio of the horizontal component to the total magnetic field of the earth at that place ?
Q.10
A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip down at 60º with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.4 G. Determine the magnitude of the earth’s magnetic field at the place.
Q.11
At a certain location in Africa, a compass points 12º west of the geographic north. The north tip of the magnetic needle of a dip circle placed in the plane of magnetic meridian points 60º above the horizontal. The horizontal component of the earth’s field is measured to be 0.16 G. Specify the direction and magnitude of the earth’s field at the location.
Q.12
A telephone cable at a place has four long straight horizontal wires carrying a current of 1.0 A in the same direction east to west earth’s magnetic field at the place is 0.39 G, and the angle of dip is 35º. The magnetic declination is nearly zero. What are the resultant magnetic fields at points 4.0 cm below the cable?
Q.13
A magnetic needle free to rotate in a vertical plane parallel to magnetic meridian has its north tip down at 60º with the horizontal. The horizontal component of earth’s magnetic field at that place is 0.4 G. Determine the magnitude of earth’s magnetic field at the place.
Q.14
A very short bar magnet has magnetic moment 1.4175 \displaystyle J{{T}^{{-1}}}. It is placed (i) with its north pole pointing towards geographic north (ii) with its north pole pointing towards geographic south. If horizontal component of earth’s field at the place is 0.42 gauss, calculate the distance of the neutral points from the magnet.
Q.15
Answer the following questions:
(a) The angle of dip at a location in southern India is about 18º. Would you expect a greater or smaller dip angle in Britain?
(b) If you made a map of magnetic field lines at Melbourne in Australia, would the lines seem to go into the ground or come out of the ground?
(c) In which direction would a compass free to move in the vertical plane point to, if located right on the geomagnetic north or south pole?
(d) The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment 8 × 1022 J T–1 located at its centre. Check the order of magnitude of this number in some way.
(e) Geologists claim that besides the main magnetic N-S poles, there are several local poles on the earth’s surface oriented in different directions. How is such a thing possible at all?
(f) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?
(g) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents?
(h) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such distant past?
(i) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?
(j) Interstellar space has an extremely weak magnetic field of the order of 10–12 T. Can such a weak field be of any significant consequence? Explain.
Q.16
What are the magnetic elements of earth? Explain them briefly.
Q.17
A compass needle, pivoted about the horizontal axis and forced to move in the magnetic meridian is observed to point along.
(i) Vertical direction at place A,
(ii) horizontal direction at place B. What are the angles of dip at the two places?
Q.18
A compass box and a dip circle were taken to magnetic north pole of earth. What would one observe with regard to the direction of their needles and why?
Q.19
A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22º with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.35 G. Determine the magnitude of the earth’s magnetic field at the place.
Q.20
A short bar magnet of magnetic moment 5.25 × 10–2 J T–1 is placed with its axis perpendicular to the earth’s field direction. At what distance from the centre of the magnet, the resultant field is inclined at 45º with earth’s field on (a) its normal bisector and (b) its axis. Magnitude of the earth’s field at the place is given to be 0.42 G. Ignore the length of the magnet in comparison to the distances involved.
Q.21
A long straight horizontal cable carries a current of 2.5 A in the direction 10º south of west to 10º north of east. The magnetic meridian of the place happens to be 10º west of the geographic meridian. The earth’s magnetic field at the location is 0.33 G, and the angle of dip is zero. Locate the line of neutral points (ignore the thickness of the cable). (At neutral points, magnetic field due to a current-carrying cable is equal and opposite to the horizontal component of earth’s magnetic field.)
(a) A vector needs three quantities for its specification. Name the three independent quantities conventionally used to specify the earth’s magnetic field.