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Magnetic Field

Experiments have shown that a magnet or a current carrying conductor can deflect a magnetic needle placed nearby. This implies that the needle experiences a magnetic force in regions around a magnet or a current carrying conductor. The space in which magnetic effect is felt is said to contain a magnetic field. Usually magnetic field is represented by a set of lines. These lines are called ‘magnetic lines of force’. A line of force in a magnetic field is the path along which an isolated free north pole of a magnetic needle moves.
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Tracing Magnetic Lines of Force
Place a compass near the north pole of a bar magnet. Mark a dot against the north pole of the compass. Then move the compass so that the south pole of the compass coincides with the dot that is marked. Again mark a dot against the north pole of the compass. Continue this process till the south pole of the bar magnet is reached. Draw a smooth curve through these dots. Similarly, draw some more lines of force from different points near the north pole. [These lines represent magnetic lines of force.
Observe the overall pattern of these lines of force.
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Magnetic Lines of Force Around a Bar Magnet
By using iron filings and a bar magnet we can obtain magnetic lines of force as shown in Figure.
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Iron Filings Arranged Along Magnetic Lines of Force
Place the given bar magnet on a cardboard and sprinkle iron filings around a magnet. Tap the card board lightly. The iron filings arrange themselves in a regular pattern. These represent the set of lines of force around the bar magnet.

Properties of Magnetic Lines of Force

The study of magnetic lines of force around a powerful bar magnet reveals the following properties:
  1. Magnetic lines of force are relatively compact wherever the field strength is strong (near poles) and less in number wherever the field strength is weak.
  2. The magnetic lines of force start at one pole and end at the other. But, conventionally the lines of force are considered to start at the north pole and end at the south pole.
  3. The magnetic lines of force do not intersect each other.
  4. Lines of force can penetrate substance such as paper, glass, etc.
Diamagnetic, Paramagnetic and Ferromagnetic Substances
In general, all substances have magnetic properties. However, the degree of magnetism exhibited by them differs. Based on their behaviour in an external magnetic field, they are classified into three groups namely, diamagnetic, paramagnetic and ferromagnetic substances.
  1. Diamagnetic materials are feebly repelled by a strong magnetic field. For example, bismuth, antimony, lead, gold and water.
  2. Paramagnetic materials are feebly attracted in a strong magnetic field. For example, aluminium, platinum, chromium, copper sulphate, etc.
  3. Ferromagnetic materials are strongly attracted by an external magnetic field. For example, iron, nickel, cobalt, etc.
Earth’s Magnetism
The earth behaves like a huge but a weak bar magnet. It has a magnetic south pole and a magnetic north pole. However, they do not coincide with geographic north and south poles. In fact, the earth’s magnetic south pole is actually near the earth’s geographic north pole and its magnetic north pole is near the geographic south pole. The earth’s geographic north pole is called the true north. Hence, any freely suspended magnet or magnetic needle comes to rest in the north–south direction.
Since the earth acts as a magnet, it has its own magnetic field. One of the effects of earth’s magnetic field is the colourful displays in the atmosphere at north and south pole called auroral displays.
The earth’s magnetic field consists of parallel lines pointing northwards. The density of lines is uniform and we say that earth’s field is a uniform field.

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