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resistance. When resistors are connected in series the resistance of
the circuit is greater than the highest individual value of resistance
in the circuit and the resistance of the circuit is less than the lowest
individual value of resistance the circuit.

5. Electromagnetism

A magnetic field in a ferromagnetic material is produced
by magnetic domains. Lines of magnetic flux are said to leave a N-
pole and enter a S-pole. Like magnetic poles repel one another and
unlike magnetic poles attract one another.
The magneto motive force (m.m.f.) produced by an
electromagnet causes a magnetic flux to be established in the
magnetic circuit. The effective “resistance” of a magnetic circuit to
magnetic flu is known as its reluctance (S).The relationship
between the flux (F) the reluctance and the m.m.f. (F) is (Ohm’s
law for the magnetic circuit) Р=F·8. Equipment can be screened
from a strong magnetic field by surrounding it with a material of

low reluctance. An e.m.f. may be induced in a circuit either by
self-induction or induction by motion in a magnetic field or by
mutual induction. The magnitude and “direction” of the induced
e.m.f. can be predicted using Faraday’s laws and Lens’s law.

6. Electrical Generators and Power Distribution

Motors action is caused by the force acting on a current-
carrying conductor in a magnetic field. The direction of the force
can be predicted by Fleming’s left hand rule.
A d.c. motor consists of a rotating part (the armature) and a
fixed part (the frame). Electrical connection to the armature is
made via carbon brushes and the commutator. When the armature
rotates a back e.m.f. is induced in the armature conductors (this is
by generator action) which opposes the applied voltage.
The four main types of dc motor are the separately excited,
the shunt wound, the series wound and compound wound -
machines.

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