The relative strength of a magnetic field is represented by the spacing of the magnetic field lines. Closer spacing of the magnetic field lines indicates a stronger magnetic field. This is because a stronger magnetic field exerts a greater force on a moving magnetic dipole, resulting in the field lines being more concentrated. Conversely, wider spacing indicates a weaker magnetic field where the magnetic force is less pronounced.

Magnetic field lines are continuous and form closed loops due to the fundamental principle that magnetic monopoles (isolated North or South poles) have not been observed. Magnetic field lines always form closed loops. Therefore, the magnetic field lines emerging from the North pole must eventually enter the South pole, and vice versa. This implies that a bar magnet possesses both a North and a South pole, and these poles are intrinsically linked. The field lines cannot simply start and stop; they must complete a circuit. If a magnetic field line were to abruptly end, it would indicate the presence of a magnetic monopole, which is not known to exist.

An electromagnet is created by passing an electric current through a coil of wire. When current flows through the wire, it generates a magnetic field around the coil. The more turns in the coil (the more coils wound around the core), the stronger the magnetic field. The magnetic field is concentrated within the coil, acting like a magnet. A ferromagnetic core (like iron) significantly enhances the magnetic field by concentrating the magnetic flux lines.

Factors affecting the strength of an electromagnet include:

• Number of turns in the coil (N): Increasing the number of turns increases the magnetic field strength proportionally.
• Current (I): Increasing the current flowing through the wire increases the magnetic field strength proportionally.
• Core material (μ): Using a ferromagnetic core (e.g., iron) significantly increases the magnetic field strength by providing a path of lower reluctance for the magnetic flux.
• Cross-sectional area of the coil (A): Increasing the cross-sectional area of the coil also increases the magnetic field strength.

Practical applications of strong electromagnets include:

• Electric Motors: Electromagnets are used in electric motors to create the force that rotates the motor's shaft. The magnetic field generated by the electromagnet interacts with the magnetic field of permanent magnets or other electromagnets to produce torque.
• Lifting Magnets: Powerful electromagnets are used in cranes and scrap yards to lift and move heavy ferrous materials. The strength of the electromagnet can be controlled by adjusting the current.
• Magnetic Resonance Imaging (MRI): As mentioned previously, strong electromagnets are used to generate the powerful magnetic fields required for MRI machines.
• Magnetic Recording Heads: Electromagnets are used in hard disk drives to write data onto magnetic media.