  WWW http://sajitsir.tripod.com           21. How will you identify whether the magnetic field at a point is due to earth or due to some current carrying conductor? I will identify whether the magnetic field at a point is due to earth or due to some current carrying conductor by knowing the nature of magnetic field whether it is uniform or non-uniform. If the magnetic field is uniform then the magnetic field at a point is due to the earth but of the magnetic field is non-uniform then the magnetic field is due to some current carrying conductor.   22. When three identical bar magnets are placed in the form of equilateral triangle with the North Pole of one touching the South Pole of the other. What will be the magnetic moment of the system? When three identical bar magnets are placed in the form of equilateral triangle with the North Pole of one touching the South Pole of the other. The net magnetic moment of the system becomes zero because no pole is free at either side of each pole as there is a force of attraction between each joining.   23. Is it possible to construct a magnet with three poles? Justify your answer. Basically, magnetic poles always occur in pair but it is possible to construct a magnet with three poles if two bar magnets are arranged to produced a magnetic field configuration of three poles. It can be done by keeping North poles or South poles of two magnets together.   24. Does magnetic monopole exist? Explain. The poles of a magnet  exits in pair and have equal strength. The magnet never exits having monopole i.e. an isolated magnetic pole doesn't exits. For example when a magnet is broken down into pieces, each small piece behaves as a magnet having two poles. Hence, magnetic monopole does not exits.   25. A piece of iron is in touch with a powerful magnet. The iron too develops magnetic property. Explain how. A piece of iron is in touch with a powerful magnet. The iron too develops magnetic property. The magnetic substances i.e. iron contains a large number of magnetic domains which are oriented randomly in all directions so there is no magnetic effect. When the powerful magnet touches iron then the magnetic field of the magnet causes alignment of magnetic domains of iron in one direction along the magnetic field so that the iron becomes temporary magnet. Hence, the iron too develops magnetic property when it is in touch with a powerful magnet.   26. When a magnet is broken down into pieces, each small piece behaves as a magnet. Why? According to domain theory, a magnet contains numerous tiny molecular magnets associated with a domain have their magnetic axes oriented in same direction. So, each domain behaves like a tiny magnet having two poles. When a magnet is broken down into pieces, each broken piece contains domain or group of domain oriented in same direction. As the domain behaves like magnet so, when a magnet is broken into pieces, each piece behaves as a magnet.   27. What may happen if a bar magnet is dropped on a floor or hammered? A bar magnet loses its magnetism via shock such as being dropped or hit with a hammer. This really only applies to older materials such as magnetic steels and Alnico materials; the mechanism that creates coercivity means that they are susceptible to being demagnetized if enough energy is transmitted through the material via a shock, such as being dropped or hit with a hammer. Modern materials do not suffer this type of problem. Hence, if a bar magnet is dropped on a floor or hammered then a bar magnet loses its magnetism.   28. Why is the amplitude of oscillation magnetometer small? The oscillation magnetometer works on the principle of simple harmonic motion principle. When the amplitude of oscillation magnetometer is taken small then the sine of the angular displacement of the magnet from its mean position is equal to its angular displacement but if we take large amplitude of oscillation magnetometer then sine of the angular displacement of the magnet from its mean position cannot be equalize to its angular displacement.  For accuracy in the measurement of time period using simple harmonic motion principle, the amplitude of oscillation magnetometer is small. Hence, the amplitude of oscillation magnetometer is small.   29. Will the time period of a vibration magnetometer change if the magnets are of larger dimension? Why? Yes, if the magnets are of larger dimension then the time period of a vibration magnetometer surely changes i.e. it increases. The time period for the vibration magnetometer is given as The time period varies directly to the square root of moment of inertia. The momentum of inertia increases in the dimensions so, there is change in time period of a vibration magnetometer if the magnets are of larger dimensions.   30. How can you determine which one is stronger if you are given two magnets of the same size and mass using the vibration magnetometer? The time period for the vibration magnetometer is given as From above relation, the time period of vibration magnetometer varies inversely to the square root of magnetic moment. Stronger the magnet, larger will be its magnetic moment and vice-versa. From above relation, stronger the magnet less will be its time period. By determining the time period from vibration magnetometer, we are able to determine the stronger magnet from given two magnets. 31. Can you plot the lines of the earth’s magnetic field? What are their shapes? Yes, we can plot the lines of the earth's magnetic field. It is believed that the earth behaves as a huge magnet with its magnetic north pole towards the geographical south & the south pole towards the geographical north pole. The earth contains the magnetic lines of force whose direction is from the geographical south to the geographical north. The magnetic poles of the earth's magnet are very far apart from each other so, the magnetic lines of force over any small portion of the earth's surface are parallel to each other i.e. of magnetic lines of force are plotted on a paper, parallel lines are obtained.   32. Do the magnetic elements remain the same at every place with time? As the magnetic elements varies with the time so, the magnetic elements do  not remain the same at every place with time because there are some periodic variation that are secular change, daily variations, actual changes and irregular changes.   33. What are the factors, which are responsible to make a permanent magnet? Substances that are used for making permanent magnets should have high retentivity and high coercivity. For example: steel & alnico are used to make permanent magnet. High retentivity and high coercivity are the factors that a substance must possess to make a permanent magnet.   34. What type of substance will you prefer for the core of transformer? Why? Substances having high retentivity, low coercivity and low hysterisis loss are used to make the core of transformer as these materials can easily lose the magnetism when the current is switched off. For example: soft iron, radio-metal, etc are used for the core of transformer.   35. Why is it easier to demagnetize soft iron than the steel? It is easier to demagnetize soft iron than the steel because the coercivity of soft iron is less than that of the steel. As a result, the area of the hysteresis loop for soft iron is smaller than that for the steel. Thus, the loss of energy per cycle is less for iron than for steel and if loss of energy is less it results easier to demagnetize. So, it is easier to demagnetize soft iron than the steel.   36. When a magnetic needle is located right on geo-magnetic north or South Pole, which direction does it point? The horizontal components of the earth's magnetic field vanish at the North or South poles. hence only vertical magnetic field of the earth will be there. Due to this, a magnetic needle is free to move in a horizontal planes. So, a magnetic needle will point any direction on the geo-magnetic North or South pole.   37. Can the earth’s magnetic field be vertical at a place? What will happen to a freely suspended at such a place? What is the value of dip there? Yes, the earth's magnetic field can be vertical at a place i.e. at poles. At poles, the value of angle of dip is 900. Using tangent law, the magnet placed at poles will make angle of 900 i. e. the magnet will be vertical at such place.   38. The angle of dip in Britain is greater than that in Kathmandu. Why? The angle of dip at a place is defined as the angle made between the direction of the resultant intensity of the earth's magnetic field and the horizontal line at that place. As the latitude increases, the value of angle of dip also increases. The latitude in Britain is higher than that in Kathmandu. So, the angle of dip in Britain is greater than that in Kathmandu.   39. Why does a bar magnet not retain its magnetism when it is melted? If a bar magnet is heated, due to the heat energy, the tiny molecular magnet regains the kinetic energy and orient themselves in any directions. And when it melts, the orientation of the molecular magnets are completely changed and they are arranged in random or haphazard fashion and cannot arrange the position of molecular magnets in same direction as initially. Hence, a bar magnet does not retain its magnetism when it is melted.   40. A bar magnet made up of iron is heated to 10000C and then cooled in a magnetic field free space. Will it retain magnetism? If a bar magnet made up of iron is heated to 10000C which is quite above the curie's temperature i.e. 7500C. Due to the fall in susceptibility above curie temperature all its atomic magnets are randomly oriented so, these materials becomes paramagnetic materials above curie's temperature. Thermal motion gained due to thermal energy at this temperature destroys its magnetism. Again, if it is cooled, it will not retain or gain its magnetism.           If you want certain questions answered, please write to
Magnetism 1