For Ma'am Torralba's classes

1.  A transformer is
made of an iron core
with two coils wound
on it. One is
connected to a
source of electrical
power and is called
the 'primary coil'; the  There is no electrical connection
other supplies the between the two coils. However they are
connected by the magnetic field in the
power to a load and iron core. The current in the primary coil
is called the produces a magnetic field that is carried
'secondary coil'. by the iron core into the secondary coil.

2. Voltage in secondary
 When the voltage and current in the primary
coil is sinusoidally varying (like a cosine
function, for example), it sets up an alternating
field. Effectively, the flux through the
secondary coil changes, inducing an emf
(voltage) and current in it
∆B
V2 = − N 2 A
∆t

3. Voltage in primary
 The flux through the
primary coil also
changes. The rate of
change in flux ∆B
V1 = − N1 A
through both coils are
the same as the coils
essentially share the ∆t
same magnetic field.

4.  Eliminating the time rate of change in field, one
arrives at
V1 V2 V1 N1
= =
N1 N 2 V2 N 2

5. To summarize,

6. If the secondary coil
If the secondary coil has less number of turns, we
 
have a STEP-DOWN transformer
has greater number of
turns, we have a
STEP-UP transformer