Page 76 - Physics_Form_4

Electromagnetism

Therefore, when the current in the primary coil is Mode of action
changing, the magnetic field, through the secondary An induction coil produces
coil also changes. This produces a change in the high voltage in its secondary
magnetic flux in the secondary coil. In response, an coil by electromagnetic
e.m.f is induced in the secondary coil producing a induction. The direct current
current that in turn produces a magnetic field, B , that in the primary coil is switched
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2
tends to oppose the change in the magnetic flux, B . on and off by a make-and-
1
Primary coil Secondary coil break mechanism. This
produces changes in current
and magnetic field which are
B 1 B 1 necessary for electromagnetic
B induction to occur in the
2
secondary coil. When the
Figure 2.29: Mutual induction current in the primary coil
The e.m.f induced in the secondary coil is is switched on, the induced
proportional to the rate of change of current in the magnetism in the iron core
primary coil. The phenomenon of mutual induction attracts the soft-iron armature.
has many applications in real life. These include the The moving armature causes
induction coil, generator and transformer. the platinum contact to

separate and opens a gap
Induction coil
between the two contacts
An induction coil (Figure 2.30) is an electrical which breaks the primary
device consisting of two coils of insulated wires coil circuit. This switches off
wound around a common iron core. The primary the current. As the induced
coil, is made from relatively few (tens or hundreds) magnetism fades, the armature
turns of coarse wire. The secondary coil, typically springs back, thus closing the
consists of a larger number (thousands or millions) contacts that completes the
of turns of a fine wire. The secondary coil is normally circuit again. This allows the
wound on top of the primary coil. An induction coil
is used to produce high voltage alternating current current to flow in the primary
from low voltage direct current. coil again. This cycle of events
SG is repeated automatically and
SG - Spark gap an e.m.f is induced in the
S P - Primary coil
P P S - Secondary coil secondary coil. The induced
M - Soft iron core e.m.f is very large, usually
M A B - Battery
C - Capacitor in the order of hundreds of
K K - Platinum kilovolts (kV) causing a spark
B C contact
A - Soft iron to jump across the gap. Such
armature a high voltage is achieved
Figure 2.30: Induction coil because of two aspects:

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Physics Form 4.indd 69 14/07/2025 14:43