Magnetism - Part II

Magnetic field and magnetic lines of force
When a magnet is kept in a place, it influences the space surrounding itself. We have seen earlier how iron filings or nails get attracted towards a magnet. If you place a bar magnet on a piece of paper and sprinkle some iron filings, you will notice that the iron filings form a particular pattern around the magnet.

The pattern is called the magnetic lines of force. The effect the magnet has around its surrounding is called the magnetic field.  The magnetic force always functions along the tangent to the line of force. The lines of force around a magnetic pole are all pervasive and in principle extend up to infinity.  The strength of a magnetic force, similar to an electric force field, is inversely proportional to the square of the distance from the magnet (or centre of a magnetic dipole).  

The properties of lines of force can be enumerated as :
1. They are said to originate from the north pole and end at the south pole. This is only a convention.
2. The lines of forces of a particular magnet do not intersect with each other.
3. Tangent to the line of force gives the direction of the magnetic field acting at that point.
4. A line of force is continuous : starts from the north pole and ends at the south pole.
5. There is no line of force within the magnet.

Magnetic effects can be induced on substances that can then become magnetic themselves.  The substances have to be made of magnetically favourable materials like iron, nickel, steel, etc. 

Handling or breaking, heating, etc may destroy magnetism. Such effects are called demagnetizing effects. In order to avoid demagnetizing effects, a permanent magnet is always kept stuck to soft iron materials called keepers. These keepers help the poles to stay apart and not get stuck to each other or bang against each other.

2. Electricity and magnetism
An electric current also produces a magnetic field. This was first discovered by Oersted in 1820.  His experiment is shown below.  

Take a wire and connect it to a batter and a key. Keep a compass needle in the centre. Note its initial position. As soon as you pass a current through the wire, the compass needle will show a deflection. As long as the current is passing through the wire, the compass needle will stay deflected. This clearly demonstrates that an electric current induces magnetic field around itself.  Reverse the current, the deflection of the needle will be in the opposite direction.

Magnetic Field due to a straight conductor
In the arrangement shown above, keep the magnetic compass outside the circuit, you will still see a deflection as you switch on the current. As you go away from the circuit, the deflection will decrease and as you come closer to the circuit, the deflection will increase. Increase the current passing through the circuit by putting another battery in series, you will notice that the deflection is stronger now. Thus we can conclude that higher the current, higher is the magnetic field.

Now take a cardboard piece and sprinkle some iron filings on it. Tap gently so that the iron filings spread evenly on the cardboard. Pass a wire through the middle of the cardboard. Connect the wire to a battery and a key. As you pass a current through the wire, the iron filings will arrange themselves in concentric circles. These circles are the magnetic lines of force.  

From all the above observations, we can say that for a magnetic field at a point, due to an electric current :

• Depends on the strength of the current

• Depends upon the distance from the conductor carrying current

• The lines of force are concentric.

• The direction of the field depends on the direction of the current.[1]

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