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the field collapses back across the secondary coil. To use dc in
a transformer, the current must be continually interrupted to
produce a changing magnetic fi eld. Primary Secondary
When an alternating current or a continually interrupted coil coil
direct current is applied to the primary coil, the magnitude of 10 turns
the induced voltage in the secondary coil is proportional to the 1 turn 12 V
ratio of wire loops in the two coils. If they have the same num-
ber of loops, the primary coil produces just as many magnetic
fi eld lines as are intercepted by the secondary coil. In this case, 120 V
the induced voltage in the secondary coil will be the same as the A
voltage in the primary coil. Suppose, however, that the second-
ary coil has one-tenth as many loops as the primary coil. Th is Secondary
means that the secondary loops will move across one-tenth as coil

many field lines as the primary coil produces. As a result, the Primary
induced voltage in the secondary coil will be one-tenth the volt- coil 10 turns

age in the primary coil. This is called a step-down transformer
because the voltage was stepped down in the secondary coil. 1 turn 1,200 V
On the other hand, more wire loops in the secondary coil will

intercept more magnetic field lines. If the secondary coil has 120 V
10 times more loops than the primary coil, then the voltage
will be increased by a factor of 10. This is a step-up transformer. B

How much the voltage is stepped up or stepped down depends
on the ratio of wire loops in the primary and secondary coils
( Figure 6.34). Note that the volts per wire loop are the same in FIGURE 6.34 (A) This step-down transformer has 10 turns on
each coil. The relationship is the primary for each turn on the secondary and reduces the voltage

from 120 V to 12 V. (B) This step-up transformer increases the
volts
volts

__ ___ voltage from 120 V to 1,200 V, since there are 10 turns on the
primary
secondary

=

(number of loops) primary (number of loops) secondary secondary to each turn on the primary.
or
can see that when the voltage is stepped up, the current is cor-
V p
_ _ respondingly decreased, as
V s
=

N p N s
equation 6.8 power input = power output
watts input = watts output
(amps × volts) = (amps × volts)
out
in
EXAMPLE 6.9
or
A step-up transformer has 5 loops on its primary coil and 20 loops on its
V p I p = V s I s
secondary coil. If the primary coil is supplied with an alternating current
at 120 V, what is the voltage in the secondary coil?
equation 6.9
SOLUTION
EXAMPLE 6.10
V p N s
V p
V s
_ _ _

N p = 5 loops = ∴ V s = The step-up transformer in example 6.9 is supplied with an alternating
N p N s N p
N s = 20 loops current at 120 V and a current of 10.0 A in the primary coil. What cur-
(120 V)(20 loops)
__ rent flows in the secondary circuit?

V p = 120 V V s =
5 loops
V s = ?
120 × 20 V·loops
= _ _ SOLUTION

5 loops
= 480 V V p I p
_
V p = 120 V V p I p = V s I s ∴ I s =

V s
I p = 10.0 A __
120 V × 10.0 A
V s = 480 V I s =
A step-up or step-down transformer steps up or steps down 480 V
the voltage of an alternating current according to the ratio of I s = ? _ _
120 × 10.0 V·A

wire loops in the primary and secondary coils. Assuming no = 480 V
losses in the transformer, the power input on the primary coil = 2.5 A
equals the power output on the secondary coil. Since P = IV, you
6-25 CHAPTER 6 Electricity 163