Name: _______________________
Date: ___________________
Physics I H
Mr. Tiesler
Solutions to Electromagnetic Induction Homework Problems 11-15
11.) A transformer has 320 turns in the primary coil and 120 in the secondary coil. What kind of
transformer is this, and by what factor does it change the voltage? By what factor does it change
the current?
Because NS  N P , this is a step-down transformer.
VS
VP

NS
NP

120 turns
320 turns
IS
 0.375
IP

NP
NS

320 turns
120 turns
 2.67
12.) A step-up transformer increases 25 V to 120 V. What is the current in the secondary coil as
compared to the primary coil?
VS
VP

NS
NP
;
IS
IP

NP
NS

VS
VP

IP
IS

IS
IP

VP
VS

25 V
120 V
 0.21
13.) Neon signs require 12 kV for their operation. To operate from a 240-V line, what must be
the ratio of secondary to primary turns of the transformer? What would the voltage output be if
the transformer were connected backward?
We find the ratio of the number of turns from
NS

NP
VS
VP

12000 V
240 V
 50
If the transformer is connected backward, the role of the turns will be reversed:
NS

NP
VS
VP
1

50

VS
240 V
 VS 
1
50
 240 V  
4.8 V
14.) A model-train transformer plugs into 120-V ac and draws 0.35 A while supplying 7.5 A to
the train. (a) What voltage is present across the tracks? (b) Is the transformer step-up or stepdown?
VS
VP

NS
NP
;
(b)
IS
IP

NP

NS
VS
VP

IP
IS
 VS  VP
IP
IS
 120 V 
0.35 A
7.5 A
 5.6 V
Because VS  VP , this is a step-down transformer.
15.) A transformer has 330 primary turns and 1340 secondary turns. The input voltage is 120 V
and the output current is 15.0 A. What are the output voltage and input current?
VS
VP

NS
NP
 VS  VP
IS
IP

NS
NP
NP
NS
 120 V 
1340 turns
330 turns
 I P  IS
NS
NP
 487 V
 15.0 A 
1340 turns
330 turns
 60.9 A