Electricity and Magnetism are linked to each other. This can be demonstrated by the following experiment:
- Place a straight thick copper wire between points X and Y in an electric circuit. The wire is kept perpendicular to the plane of the paper.
- Horizontally place a small compass near the copper wire.
- When the current is passed, the compass needle is deflected, indicating 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 a compass needle in the presence of an electric current. The unit of magnetic field strength is named oersted.
MAGNETIC FIELD AND FIELD LINES
- A compass needle is a small bar magnet. Its ends point towards the north and south directions. It gets deflected when brought near a bar magnet.
- The end pointing towards the north is called the north seeking (north pole). The other end pointing towards the south is called the south seeking (south pole). Like poles of magnets repel, and unlike poles attract each other.
Experiments to Obtain Magnetic Field & Field Lines
Experiment 1:
- Fix a white paper on a drawing board and place a bar magnet in its center.
- Sprinkle some iron filings uniformly around the bar magnet.
- Tap the board gently. The iron filings near the magnet align along the field lines.
Reason: The magnet exerts its influence in the surrounding region, causing the iron filings to experience a force and 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.
Experiment 2:
- Place a bar magnet on a white paper fixed on a drawing board. Mark the boundary of the magnet.
- Place a compass near the north pole of the magnet. The south pole of the needle points towards the north pole of the magnet. Mark the position of the two ends of the needle.
- Move the needle to a new position such that its south pole occupies the position previously occupied by its north pole. Proceed step by step until you reach the south pole of the magnet.
- Join the points marked on the paper by a smooth curve. This curve represents a field line.
- Repeat the procedure to draw multiple lines. These lines are called magnetic field lines.
- The deflection in the compass needle increases as the needle is moved towards the poles.
- A magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken as the direction in which the north pole of the compass needle moves. Thus, the field lines emerge from the north pole and merge at the south pole. Inside the magnet, the direction of field lines is from south to north, making magnetic field lines closed curves.
- The 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.