MAGNETIC EFFECTS OF ELECTRIC CURRENT
Electricity and Magnetism are linked to each other. It can be shown by the following experiment.
Place a straight thick copper wire b/w the points X & Y in an electric circuit. The wire is kept perpendicular to the plane of paper.
Horizontally place a small compass near to copper wire.
On passing the current, the compass needle is deflected. It means that the electric current through the copper wire produces a magnetic effect.
An electric current-carrying wire behaves like a magnet.
Hans Christian Oersted (1820) accidentally discovered the deflection of compass needle in presence of electric current. Unit of magnetic field strength is oersted.
A compass needle is a small bar magnet. Its ends point towards north and south directions. It gets deflected when brought near a bar magnet.
The end pointing towards north is called north seeking (north pole). The other end pointing towards south is called south seeking (south pole). Like poles of magnets repel and unlike poles attract each other.
Experiments to obtain Magnetic field & Field lines
o Sprinkle some iron filings uniformly around bar magnet.
o Tap the board gently. Iron filings near the magnet align along the field lines.
Reason: The magnet exerts its influence in the surrounding region. So, the iron filings experience a force. It makes iron filings to arrange in a pattern.
The area around a magnet that has magnetic force is called a magnetic field. The lines along which the iron filings align themselves represent magnetic field lines.
o Place a bar magnet on white paper fixed on a drawing board. Mark the boundary of the magnet.
o Place a compass near the north pole of the magnet. The south pole of the needle points towards the north pole of magnet. Mark the position of two ends of the needle.
o Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole. In this way, proceed step by step till reach the south pole of the magnet.
o Join the points marked on the paper by a smooth curve. This curve represents a field line.
o Repeat the above procedure and draw many lines. These lines are called magnetic field lines.
o The deflection in the compass needle increases as the needle is moved towards the poles.
Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken as the direction in which north pole of the compass needle moves. Thus, the field lines emerge from north pole and merge at south pole. Inside the magnet, the direction of field lines is from south to north. Thus, the magnetic field lines are closed curves.
Relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger (i.e., greater force acting on the pole of another magnet) where the field lines are crowded.
Two field-lines do not cross each other because the compass needle cannot point towards two directions at the point of intersection.