Magnetic Fields from Electric Currents
In 1820, Hans Christian Oersted accidentally discovered that an electric current deflects a nearby compass needle. This was the first evidence that electricity and magnetism are connected. A current-carrying wire creates a circular magnetic field around it, with the field strength proportional to the current and inversely proportional to the distance from the wire.
Ampere's Law and the Right-Hand Rule
The magnetic field around a long straight wire is given by Ampere's law: B = μ₀I/(2πr). The direction follows the right-hand rule — point your thumb along the current, and your fingers curl in the field direction. This simple relationship is the foundation of all electromagnetic technology, from electric motors to MRI machines.
Parallel Wire Interactions
Two parallel wires carrying current interact through their magnetic fields. If the currents flow in the same direction, the wires attract; if in opposite directions, they repel. This force, F/L = μ₀I₁I₂/(2πd), was historically used to define the ampere: one ampere is the current that produces a force of 2 × 10⁻⁷ N per meter between two wires one meter apart.
Applications and Technology
The magnetic field around current-carrying conductors is the operating principle behind electromagnets, solenoids, transformers, and electric motors. By coiling wire into loops, the field is concentrated and amplified. Superconducting coils carrying thousands of amperes create the powerful fields used in MRI machines (1.5-3 T) and particle accelerators (up to 16 T at the LHC).