Transformers - Wellington High School

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Practice Problems
 A horizontal wire is moving vertically upwards in a
horizontal magnetic field of strength 0.150 tesla which is
perpendicular to the length of the wire. Calculate the
voltage induced in the wire if its length is 50.0 cm and it is
moving with a speed of 30.0 m s-1.
 A coil of 1000 turns of mean area 20.0 cm2 is placed with its
plane perpendicular to a uniform magnetic field of strength
1.50 T. Calculate the emf induced in the coil if the field falls
uniformly to zero in 1.50 x 10-2 s.
2.25 V; 200 V
Transformers
Introduction to Inductance
Transformer Theory and Notation
Efficiency of Transformers
Mutual Inductance
 If two coils are placed side by side, and the current in one
coil is changed, the magnetic flux in the second coil will
change, inducing a current in the second coil.
 Demonstration
Transformers
 A transformer consists of two coils wound onto an iron core.
Circuit Symbol
Transformer Theory
 Because the input current is
changing direction (AC), the
magnetic flux in the iron core is
changing.
 This results in an induced
current in the secondary coil.
Transformer Equation
 The voltage induced in the
secondary coil is related to the
voltage supplied to the primary
coil by the following equation:
 N represents the number of
windings in the coils.
Vs N s

Vp N p
Problem: A battery charger contains a transformer to convert
240V to 12 V. If the primary coil has 1200 turns, how many are
there on the secondary?
60 turns
Three Types of Transformers
 A step-up transformer
 Higher voltage in the secondary coil
 An isolating transformer
 Same voltage in both
 A step-down transformer
 Lower voltage in the secondary coil
Transformer Efficiency
 In an ideal transformer the
power input is the same as the
output.
Vp I p  Vs I s
If a transformer steps up the
voltage there is a
corresponding decrease in the
current supplied
 In reality, some energy is lost (as
heat) in the wire used and in the
core*
Vs I s
input power

1
V p I p output power
Problems
 Complete Questions 1-4 on page 247 of the study guide.
 Questions 1 and 2* in worksheet 4

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