Magnetic Field is the region around a magnet where other magnetic material will experience a force.
A magnetic field can be graphically represented by magnetic field lines which indicates its strength and direction.
Note: Magnetic field is a vector quantity! (It has both magnitude AND direction!)
- When the field lines are close together at a point, the point is said to have a strong magnetic field.
- Arrows in the field lines outside the magnets show the direction in which a free north pole would move (from north pole to south pole).
- Field lines NEVER cross over.
- Compass is used to find the direction and pattern of magnetic field. It has a permanent magnet needle which is free to rotate in a horizontal plane. The north pole of compass magnet (arrow head) will align and point along the magnetic field line direction.
IMPORTANT: Please note that for the last two diagrams, the field lines are NOT pushing against one another. Do NOT be tempted to say that the like poles repel because the field lines push against one another. It is NOT correct!
Magnetic field strength can be measured using a teslameter.
Plotting of magnetic field lines with a compass
Apparatus Needed: Bar magnet, plotting paper and plotting compass.
- Place the bar magnet at the centre of the piece of paper so that its north pole is aligned as shown.
- Place the compass near one pole of the magnet, and mark the positions of the ends N and S, of the compass needle by pencil dots. Then, move the compass until the end of the compass is over the second dot, and mark the new position of the other with a third dot.
- Repeat the above until reaching the other pole. Join the series of dots and this will give a field line of the magnetic field. Use this method to plot other field lines on both sides of this magnet.