Maths, Physics Revision Physicss Maths Physics Physics A Level Physics Home Maths Maths Revision hut Magnetic fields Opposite poles (north and south)  attract each other. The same poles repel each other. The field lines are imaginary  lines which represent the  magnetic field. The field is the strongest where the lines are  close together (at the poles of the magnet) Play the animation Relay Switch A relay switch is an example of the use of an  electromagnet. Play the animation and see how it  can be used to allow a small electric circuit to switch  on a larger circuit.