Final answer:
The incorrect statement is (c) because parallel and equidistant magnetic field lines signify a uniform magnetic field, not zero field strength. Magnetic field lines are closed curves that indicate the direction and relative strength of the magnetic field.
Step-by-step explanation:
The incorrect statement regarding magnetic lines of field is (c) if magnetic field lines are parallel and equidistant, they represent zero field strength. This statement is incorrect because parallel and equidistant magnetic field lines actually represent a uniform magnetic field, not one of zero strength. Magnetic field lines help us visualize magnetic forces and the magnetic field, which is often referred to as the B-field.
Magnetic field lines are very useful for visualizing the strength and direction of the magnetic field. They are defined to have the direction in which a small compass needle points when placed in the field. At any given point, the direction of the magnetic field is taken to be the direction in which the north pole of a magnetic compass needle points (a).
Magnetic field lines are indeed closed curves (b), which means they loop from the north pole to the south pole outside the magnet and continue through the magnet from the south pole back to the north pole. The relative strength of the magnetic field is indicated by the degree of closeness of the field lines (d); closer lines indicate a stronger magnetic field. The more dense the lines, the stronger the magnetic field felt at that point.