# Notes 21-07 Transformers

```Physics Lesson Plan
Teacher
Howard
Unit Title
Length
Goal(s)/PLO(s):
Course
Block/Period
 solve problems that deal with an ideal
transformer and that involve
– primary voltage
– secondary voltage
– number of primary windings
– number of secondary windings
– primary current
– secondary current
– identify a transformer as step-up or stepdown
Phys 12
12
Date
Class Size
Lesson #, of
21-07
 give examples of the use of transformers in
the home, workplace, and community
Materials:
Transformers
Timeline
Class Activities
Introduction
Body
Notes 21-07
Closure
Questions 17-19, Problems 31-37 odd
21-07 Transformers and Transmission of Power
A transformer is a device for increasing or decreasing
an AC voltage.
• found in many electrical devices, on utility poles
and at the generator sites
• consists of two coils of wire
o primary (input voltage)
o secondary (output voltage)
Transformers are designed so that “all” the
(really &gt; than 99%) magnetic flux produced
by the alternating current in the primary coil passes through the
secondary coil.
The frequency of the induced emf in the second coil is the same as
in the primary coil but if the number of coils is different then, by
Vs = N s
Where N is the number of coils and
∆Φ
∆t
∆Φ
∆t
is the rate of change of
flux.
The input voltage in the primary coil is
Vp = N p
∆Φ
∆t
If we assume that little or no flux is lost then we can divide the two
equations and ∆Φ disappears.
∆t
Transformer Equation
The secondary (output) voltage of a transformer is related to the
primary (input) voltage as the ratio of the numbers of turns in the
secondary and primary coils.
Vs N s
=
Vp N p
Transformers only work with AC because the alternating current
produced a change in the magnetic flux. DC current does not
produce a change in flux while it is flowing.
If the secondary coil has more turns
than the primary coil (N s &gt;N p ) then
the transformer is a step up
transformer.
• the secondary voltage is higher
than the primary voltage
If the secondary coil has fewer turns than
the primary coil (N s &lt;N p ) then the
transformer is a step down transformer.
• the secondary voltage is lower than
the primary voltage
The power output (in a perfect world) is the same as the input so
P p =P s
IpVp= IsVs
2nd Transformer Equation
The secondary (output) current of a transformer is related to the
primary (input) current as the ratio of the numbers of turns in the
primary and secondary coils.
I s Vp N p
=
=
I p Vs N s
Example 21-11
There is a time when a DC current can work with a transformer.
The moment the circuit is closed there is a change in flux
• Ignition systems use this to
create high voltage spikes
and produce a spark across a
gap in a internal combustion
engine
• 12V battery creates a
30000V spike across the
spark plug
The main reason our electrical grid uses AC instead of DC is that
transformers work with AC. If power can be transported long
distances at low currents (high voltage) then the power loss from
P=I2R is minimized. But high voltage is
dangerous in town or in your home because
the electricity is more likely to arc so the
voltage is stepped down in town and then
further stepped down before it enters your
house.
• safety trumps efficiency
Example 21-12
Questions 17-19, Problems 31-37 odd
```