medical physics

1. Construction and their working

2. What is transformer?
 A transformer is an electric device which is used for
changing the a.c. voltages and to render the current
earth-free
 A transformer which increase voltages is called as
step up transformer
 A transformer which decrease voltages is called as
step down transformer
 Principle: based on the principle of mutual induction

3. Circuit Symbol for Transformer

4. Construction of transformer
 Consist of two coils (primary
and secondary) of insulated wire
wound onto a laminated soft iron
frame
 Two coils may be wound on top
of one another on central bar or
on opposite side of the frame
 Frame is usually rectangular in
shape
 One coil usually contain more
turns than other

5. Working of Transformer
 An alternating current is passed through primary coil
which sets up magnetic field which cut the secondary coil
 Emf is induced in secondary coil due to change in
magnetic field of primary coil
 For an emf to be induced, current should be a.c.
 No electrical connection b/w two coils both are insulated
from each other, current is induced via EMI
 Soft iron core serves to concentrate the mag. lines of
forces
 Soft iron is laminated to prevent eddy currents

6.  Step up transformer: No. of turns in primary coil is less
than that in secondary coil
 Converts the low voltages into high voltages
 Primary coil is made of thick insulated copper wire with
less no. of turns
 Secondary coil is made of thin insulated copper wire with
larger no. of turns
 Step down transformer: No. of turns in primary coil is
larger than that in secondary coil
 Converts the high voltages into low voltages
 Secondary coil is made of thick insulated copper wire with
less no. of turns
 Primary coil is made of thin insulated copper wire with
larger no. of turns

8. Function of Transformer
 To render the current earth free: The main current
passes through primary coil induces emf in secondary coil,
as there is no elec. connection b/w primary and secondary
coil, which has no connection to earth so render the
current earth free
 If connection b/w earth and sec. coil made, no current
passes to earth as circuit is not complete, so no earth
shock occur
 To alter the voltage of an alternating current: via step
up and step down transformer
 Emf induced in secondary coil is directly proportional to
no. of turns in this coil

9. Types of transformer
 Static transformer (step up, step down and even ratio
transformer)
 Variable transformer
 Autotransformer

10. Variable transformer
 That can put out different amount of voltage from the same
input voltage
 Tapings may be provided so that either primary or secondary
coil can be varied in length
 To use the same apparatus on supply of different voltages by
varying no. of turns
 Higher the voltage of supply the more turns of wire are
included in primary coil
 Care should be taken to see that control is in right position for
supply used
 Tapings may be taken from different points on secondary coil
to supply different voltages to separate circuits in same
apparatus

11.  Used to speed up and slow down electric motor
 Make lights dimmer and brighter

12. Autotransformer
 Consist of a single coil of insulated wire wound over
laminated soft iron core
 Tapped at two points, C and D
 When used as a step up transformer , coil b/w C and D act
as primary and A and B act as secondary and vice versa
 It has certain disadvantages
 It can be used only for a small step up and down in
voltage
 Do not make current earth free in secondary coil

14. Choke coil
 It is included in circuit in order to produce a self induced EMF
 Low frequency choke coil: consist of many turns of insulated
wire wound on soft iron usually on central bar of rectangular
frame
 High frequency choke coil: consist of usually less no. of turns
of insulated wire wound on bobbin of non conducting material
as current varies rapidly
Uses:
 Even out variations in the intensity of current
 To prevent the flow of high frequency current and allow the
passage of lower one

15. Dynamo
 It is a device which is used for the production of an EMF
via electromagnetic induction
Construction
 A coil of wire lie b/w poles of magnet and by some mean
rotation of the coil is provided via motor, steam engine or
water power
 When coil rotates, emf is induced in coil first in one
direction than other
 So current produced is alternating type
 Collecting device is used to convey current from coil to
external circuit

16.  Supply both types of currents to external circuit either
a.c or by converting a.c into d.c
 Used on large scale for production of electricity
including main supply