Magnetic fi elds and fl ux patterns and Electrical transformers

Lines of magnetic fl ux have no physical existence but were introduced by Michael Faraday as a way of
explaining the magnetic energy existing in space or in a material. The magnetic fi elds around a permanent
magnet, a current carrying conductor and a solenoid are shown in Figs 2.5, 2.6 and 2.7

Magnetism

 1. Draw in the magnetic fl ux patterns for the magnets shown below:

Electrical transformers
● A transformer is an electrical machine without moving parts, which is used to change the value of an
alternating voltage.
● A transformer will only work on an alternating supply.
● It will not normally work from a d.c. supply such as a battery.
● A transformer such as that shown below in Fig. 2.8 consists of two coils called the primary and second-
ary coils or windings, wound on to a common iron core.
● An alternating voltage applied to the primary winding establishes an alternating magnetic fl ux in the
core.
● The magnetic fl ux in the core causes a voltage to be induced in the secondary winding of the
transformer.
● The voltage in both the primary and secondary windings is proportional to the number of turns

● Because it has no moving parts, a transformer can have a very high effi ciency. 

● Large power transformers, used on electrical distribution systems, can have an effi ciency of better than 

90%.

● These power transformers need cooling to take the heat generated away from the core. This is often 

achieved by totally immersing the core and windings in insulating oil. A sketch of a power transformer 

can be seen in Fig. 5.18 (page 116) of Basic Electrical Installation Work .

● Very small transformers are used in electronic applications. 

● Small transformers are used as isolating transformers in shaver sockets. 

● Small transformers can also be used to supply SELV sources.