Magnetism and magnetic fields Study Guide

Transformers are electrical devices that change the voltage of an alternating current (AC). They operate on the principle of electromagnetic induction. A transformer consists of two coils of wire, called the primary coil and the secondary coil, wound around a laminated soft iron core. When an alternating current flows through the primary coil, it produces a continuously changing magnetic field in the core. The soft iron core is easily magnetised and demagnetised, effectively channelling this changing magnetic field through the secondary coil. The constantly changing magnetic field passing through the secondary coil induces an alternating electromotive force (e.m.f.) across its ends, and if the secondary circuit is closed, an alternating current will flow. This phenomenon is called mutual induction. The magnitude of the induced e.m.f. in the secondary coil depends on the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. If the secondary coil has more turns than the primary, it's a step-up transformer, increasing the voltage. If it has fewer turns, it's a step-down transformer, decreasing the voltage. The iron core must be laminated (made of thin sheets separated by insulation) to reduce energy losses due to eddy currents.

The Earth behaves like a giant magnet, generating its own magnetic field. This field is responsible for the way a magnetic dipping compass behaves. A dipping compass, unlike a standard compass that only shows horizontal direction, also indicates the angle at which the Earth's magnetic field lines enter or leave the Earth's surface. At the magnetic equator, the needle of a dipping compass would be horizontal, as the magnetic field lines are parallel to the Earth's surface. As you move towards the magnetic poles, the magnetic field lines become progressively steeper. Consequently, the dipping compass needle tilts downwards (dips) more and more, pointing towards the Earth at the magnetic north pole and away from the Earth at the magnetic south pole. This dipping behaviour provides strong evidence for the existence of a magnetic field originating from the Earth's core. If the Earth were not magnetic, or if its magnetism arose from surface rocks, the regular and predictable dipping pattern observed across the globe would not occur. The molten iron and nickel in the Earth's outer core, moving in convection currents, generate these electric currents that create the Earth's magnetic field through a process called the geodynamo effect. This internal, dynamic process accounts for the global magnetic field and the observed dipping phenomenon of a compass.