Magnetism and magnetic fields Study Guide

Transformers are devices that change the potential difference (voltage) of an alternating current (AC) supply. They consist of two coils, a primary coil and a secondary coil, wound around a laminated soft iron core. When an alternating potential difference is applied across the primary coil, it generates an alternating magnetic field within the soft iron core. This alternating magnetic field continuously changes direction and strength, and it is channelled by the soft iron core through the secondary coil. According to Faraday's law of electromagnetic induction, a changing magnetic field through a coil induces an electromotive force (EMF) and thus a potential difference across that coil. Therefore, an alternating potential difference is induced across the secondary coil. In an ideal transformer, it is assumed that there is no energy loss, meaning the power in the primary coil equals the power in the secondary coil. The ratio of the potential differences across the primary and secondary coils is directly proportional to the ratio of the number of turns in each coil. Specifically, (Vp / Vs) = (Np / Ns), where Vp is the potential difference across the primary coil, Vs is the potential difference across the secondary coil, Np is the number of turns in the primary coil, and Ns is the number of turns in the secondary coil. This relationship arises because the same alternating magnetic flux passes through each turn of both coils. Therefore, the induced EMF per turn is the same in both coils. The total induced potential difference in each coil is proportional to the number of turns it has. If the secondary coil has more turns than the primary coil (Ns > Np), then Vs will be greater than Vp, creating a step-up transformer. Conversely, if the secondary coil has fewer turns than the primary coil (Ns < Np), then Vs will be less than Vp, creating a step-down transformer.

A solenoid is essentially a coil of wire. When an electric current flows through this wire, it generates a magnetic field, much like a bar magnet. The strength of this magnetic field can be significantly enhanced through several key arrangements. Firstly, increasing the number of turns (coils) in the solenoid strengthens the magnetic field because each turn contributes to the overall magnetic effect. More turns mean more 'loops' producing magnetism. Secondly, increasing the current flowing through the wire directly increases the magnetic field strength; a greater flow of charge creates a stronger magnetic influence. Thirdly, the magnetic effect is further intensified by inserting a soft iron core into the centre of the solenoid. Soft iron is a 'ferromagnetic' material, meaning it can be easily magnetised. When placed inside the solenoid, the magnetic field produced by the coils aligns the magnetic domains within the soft iron, causing the iron itself to become magnetised. This induced magnetism in the soft iron core adds substantially to the magnetic field generated by the current in the coils, leading to a much stronger electromagnet. These three factors—number of turns, current, and the presence of a soft iron core—are crucial for creating powerful electromagnets used in various applications, from relays and circuit breakers to lifting scrap metal.