16. Electromagnetism

Oersted's Discovery — The Magnetic Effect of Electric Current

For a very long time, electricity and magnetism were regarded as two entirely separate branches of physics with no connection between them. This changed dramatically in 1820, when the Danish physicist Hans Christian Oersted, during a classroom demonstration, noticed by accident that a compass needle placed near a current-carrying wire was deflected from its usual north-south orientation whenever current flowed through the wire, and it returned to its normal position when the current was switched off. This simple but momentous observation demonstrated for the first time that an electric current produces a magnetic field around itself, thereby establishing the link between electricity and magnetism and giving birth to the field of electromagnetism. This is an extremely frequently asked exam fact: Oersted's experiment (1820) proved that a current-carrying conductor has a magnetic field associated with it, exactly like a magnet.

Magnetic Field Due to a Straight Current-Carrying Conductor

When current flows through a long, straight wire, it produces a magnetic field around the wire in the form of concentric circles centred on the wire, lying in planes perpendicular to the wire. The strength of this field is directly proportional to the magnitude of the current and inversely proportional to the distance from the wire (it grows weaker as one moves farther from the wire). The direction of this circular magnetic field is given by the Right-Hand Thumb Rule (also called Maxwell's right-hand rule or the right-hand grip rule): if a current-carrying straight conductor is held (imagined to be gripped) in the right hand such that the thumb points in the direction of conventional current flow, then the direction in which the fingers curl around the conductor gives the direction of the magnetic field lines.

Magnetic Field Due to a Circular Loop

When a wire is bent into a circular loop and current is passed through it, the magnetic field lines are again circles around each small segment of the wire, but their combined effect is such that, at the centre of the loop, all the field contributions add up in the same direction, perpendicular to the plane of the loop, producing a comparatively strong and uniform field at the centre. The field's direction at the centre can also be found using a version of the right-hand rule: if the fingers of the right hand are curled in the direction of current flow around the loop, the extended thumb points in the direction of the magnetic field at the centre. The strength of the field at the centre increases if the current is increased, if the number of turns of wire in the loop is increased, or if the radius of the loop is decreased.

Magnetic Field Due to a Solenoid

A solenoid is a long coil of wire consisting of many circular turns wound closely and uniformly, one behind the other, typically around a cylindrical (often hollow) core. When current flows through a solenoid, the magnetic fields produced by each individual turn add together, and the resulting overall field pattern outside and inside the solenoid closely resembles that of a bar magnet: field lines emerge from one end (which behaves like a north pole) and re-enter at the other end (which behaves like a south pole), while inside the solenoid the field lines run essentially straight and parallel to the axis of the coil, making the magnetic field inside a solenoid remarkably strong and uniform — a key reason solenoids (and the electromagnets built from them) are so useful. The polarity of the ends (which end behaves as north, which as south) can again be determined by the right-hand thumb rule applied to the direction of current circulation around the coil, and the strength of the field can be substantially increased by inserting a soft iron core inside the solenoid, which is exactly the construction principle of an electromagnet, discussed in the applications section below.