Magnetic Field
We know that when 2 magnets with like poles facing each other will tend to repel or unlike poles facing each other would attract each other. From this, we can see that magnets produce magnetic force fields around them. These magnetic force fields are not visible but we can prove they exist by simply sprinkling iron filings onto a bar magnet as can be seen on the left. The iron filings would reveal the pattern of the magnetic field around the bar magnet.
A magnetic field is a region in which a magnetic object placed within the influence of the field, experiences a magnetic force.
Plotting Magnetic Field Lines
We can plot the field lines of a bar magnet using a small plotting compass.
1. Place the magnet on a piece of paper so that its N pole faces North and its S pole faces South. This is to prevent the compass from being affected by the earth's magnetic field.
2. Starting near one pole, the positions of the ends, S and N, of the compass needle are marked by a pencil dots X and Y respectively. The compass is then moved so that the S end is at Y and the new position of the N end is marked with a third dot Z.
3. Repeat process of marking the dots. Join the dots and this will give the plot of the magnetic field lines.
Magnetic field lines between two magnets
Note that X is called a neutral point because the fields from both magnets cancel out each other.
Magnetic field lines tend to pass through magnetic materials (like iron). The figures on the left and above are two examples. The figure on the left shows the effect a thin sheet of iron has on a magnetic field. If a magnet is placed on one side of the sheet, the other side would be free from magnetic field. Hence this is a very efficient way of enclosing sensitive equipment and shielding it from surrounding magnetic fields like the figure above.