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Magnetic field lines are fictitious lines that represent the distribution and direction of magnetic fields. The tangent to each line shows the local field direction, while line density indicates field strength. For a bar magnet, field lines emerge from the north pole and enter the south pole, decreasing in density as distance increases from the magnet's surface.

The right-hand rule states that if your thumb points along the current direction, your curling fingers indicate the magnetic field direction around a conductor. This rule applies to straight wires and solenoids. When a bar magnet is brought near a current-carrying conductor, it aligns with the generated local magnetic field according to this principle.

The Lorentz force is the force exerted by a magnetic field on a moving charge. It is proportional to the magnetic field strength, charge magnitude, and velocity. The force direction is perpendicular to both the charge's motion and the magnetic field, determined by the right-hand rule. This force also acts between current-carrying conductors.

Two parallel wires carrying current in the same direction attract each other due to the Lorentz force generated by their magnetic fields. When currents flow in opposite directions, the wires repel. The direction of attraction or repulsion is determined by applying the right-hand rule to the magnetic field produced by one wire acting on the current in the other wire.

Magnetic fields are generated by two primary sources: moving electrical charges, such as current in a conductor, and permanent magnetic objects like bar magnets. In bar magnets, microscopic dynamics of charges inside the material create the magnetic field. Both sources produce fields that can be visualized using field lines and interact with other charges and magnetic materials.

Compass needles align with local magnetic fields, revealing field direction and pattern. When placed around a current-carrying wire, needles form circular patterns reflecting the wire's magnetic field. Reversing current reverses needle orientation. Similarly, compass needles align with field lines from a bar magnet's poles, with orientation reversing when the magnet is flipped.

Magnetic fields enable navigation through compasses, which align with Earth's magnetic field to indicate geographic north. In medicine, magnetic resonance imaging uses strong magnetic fields and gradients to generate detailed body images. These applications demonstrate how fundamental magnetic field principles translate into essential technologies for navigation and clinical diagnostics.