Magnetic Fields Illustration 6a14

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M AGNETIC F IELDS I LLUSTRATION

Magnetic Fields contains Magnetic Flux The space surrounding a magnet in which magnetic force is exerted is called a magnetic field B, and contains something which we call magnetic flux. Magnetic flux is a vector quantity and, like all vectors, has magnitude and direction. The presence of this invisible magnetic flux can be revealed by several methods. Let’s discover some of these methods.

Direction of magnetic flux · Simple experiment A bar magnet NS resting on the edge of a glass through containing water, while a magnetized knitting needle ns is pushed through a large cork and floats on the water with its N pole uppermost. When the needle is held near the North Pole of the magnet and then released it is repelled and travels towards the South Pole along a curved path which represents the direction of the magnetic flux. If the experiment is repeated with the S pole of the needle uppermost the needle travels in the reverse direction. Clearly, the direction of travel depends on which pole the needle is uppermost and one of these directions has to be chosen as a standard direction. It is conventional to choose the direction of the force of a N pole. Consequently, the direction of a magnetic flux is defined as the direction along which a N pole would move if free to travel.

Magnetic flux patterns by the plottingcom method

When a permanent record of a magnetic field is required the magnetic flux pattern may be traced out by means of a plotting com. This consists of a very small magnetic needle pivoted between two glass discs in a brass case. This image shows how the com is used.  NS is a bar magnet placed on a sheet of white paper.  Starting near one pole of the magnet, the position of the ends, n, s of the needle are marked by pencil dots A, B.  The com is now moved until the end n is exactly over the dot B and the new position of s is marked by a third dot C.  This process is continued, and the series of dots obtained are ed, thus giving a line which represents the direction of the magnetic flux.  Other lines are plotted in the same way, and every line is labeled with an arrow to indicate its direction.

If magnetic flux patterns are plotted on a sheet of paper when no magnets are in the vicinity a series of parallel straight lines are obtained directed approximately from S to N geographically. These represent the earth’s magnetic field in a horizontal plane. The main advantage of the plotting com is that it is sensitive and can be used for plotting comparatively weak fields. It is, however, unsuitable for fields in which the direction of the magnetic flux changes rapidly in a confined space, as, for example, in the neighborhood of two magnets placed close together. Fields such as these are best investigated by the iron-filings method, see below.

To plot magnetic flux patterns by the iron-filings method

The magnets whose fields is to be studied are arranged beneath a sheet of stiff white paper, over which a thin even layer of iron filings is sprinkled from a caster. On trapping the paper gently with a pencil the filings form into chains which reveal the flux pattern. Permanent records of these patterns may be obtained in two ways:  The first method is to use waxed paper and to fix the filings by playing a Bunsen flame quickly and evenly over the paper.  The second and better method is to perform the experiment with photographic paper in a dark room. When a good pattern has been obtained the paper is exposed to light and then developed and fixed in the usual way.

The principle of the method is as follows. Each filing becomes magnetized by induction. On trapping the paper the filings vibrate,

and consequently are able to turn in the direction of the magnetic flux. The Method FAILS with weak fields, which are unable to magnetize the filings sufficiently.

The earth as a magnet

The idea that the earth is magnetizes was first suggested towards the end of the sixteenth century by Dr. William Gilbert, who carried out experiments with spherical lodestones. Similarity between the magnetic field of a spherical lodestone and that of the earth led him to the conclusion that the earth was a magnet. The origin of the earth’s magnetism is still a matter of conjecture among scientists, but, broadly speaking, the earth behaves as though is contained a short bar magnet inclined at a small angle to its axis of rotation and with its S Pole in the northern hemisphere. The inclination of this supposed magnet to the earth’s axis is inferred from the fact that a magnetic com points towards the true north only at certain places on the earth’s surface. Elsewhere it points either east or west of the true north.

Magnetic declination In the previous post: Fact about magnetism we explained that the magnetic meridian at any place is a vertical plane containing the magnetic axis of a freely suspended magnet at rest under the action of the earth’s magnetic field. The geographic meridian at a place is a plane containing the place and the earth’s axis of rotation. The angle between the magnetic and geographic meridians is called the magnetic declination.

Inclination or dip

In 1576, Robert Norman, a com—maker living at Wapping, made an experiment with magnetic needle suspended at its center of gravity. He found that it dipped with its N Pole downwards and came to rest in the magnetic meridian with its axis making an angle of 71.5° with the horizontal. This angle is called the angle of dip or inclination, and is defined as the angle between the direction of the earth’s magnetic flux and the horizontal. When pivoted at its center of gravity the needle experiences no turning moment owing to the force of gravity on it. The ONLY other turning forces acting are magnetic forces on its poles, and these cause it to set along the true direction of the earth’s magnetic flux. An ordinary com needle shows no tendency to dip, since it is pivoted well above its center of gravity. Mechanically, it is in stable equilibrium when horizontal. The image shows, in general, how the dip varies over the earth’s surface. At earth’s magnetic poles the needle sets VERTICALLY, while along the magnetic equator it takes up a HORIZONTAL position. The N or S Pole of the needle dips according as the needle is north or south of the magnetic equator.

Simple dip needle: To measure the angle of dip

The following experiment illustrates the principle used in measuring the angle of dip.  

An un-magnetized knitting needle is ed by a length of cotton and the position of the cotton adjusted until the needle balances horizontally. Under these conditions the needle will be ed approximately at its center of gravity. Without distributing the position of attachment of the cotton, the needle is now magnetized by the solenoid method.

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On resuspending the needle, it dips with its N Pole downwards. The angle of dip may now be measured with the aid of a protractor placed with its 90° mark in the line with the cotton. Owing to the difficulty of ing the needle exactly at its center of gravity, the above method CANNOT be relied upon for very accurate values of the dip. Better results may be obtained with a dip circle, or other methods outside our range of discussion.

The dip circle formerly used for measuring angle of inclination or dip. Dip is now measured by electrical methods

To be continued..