1. AC Transformers
Source: OSHA

2. Flux Change Creates Emf
Rapidly changing field
high induced emf
unchanging field
zero induced emf

3. How a Transformer Works
AC current in the primary coils creates a changing magnetic field.
The magnetic flux inside the secondary coils changes
Changing flux induces emf in the secondary coils

4. Potential Proportional to Loops
Same flux F through both sets of windings
Each loop adds emf
Emf’s V are proportional to the number of loops N V1 N1 V2 N2 =

5. Energy is Conserved Ideally: power in = power out V1I1 = V2I2
Realistic: power in > power out
Efficiencies usually around 95%

6. Transformer Summary Power in  power out  loops  higher V, lower I
 loops  lower V, higher I

7. Question
A “step-down” transformer converts input at 120 V to output at 20 V. If the input circuit has 1100 W, how much power is available at the output?
200 W.
660 W.
1100 W.
2400 W.

8. General Physics L18_Faraday
Group Work
A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).
Which is greater: the voltage in the primary circuit or the voltage in the secondary circuit?
Where will the number of loops be greater: in the primary or in the secondary?

9. Group Work
A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).
What is the voltage ratio V2/V1?
What is the loops ratio N2/N1?
How many loops are in the secondary circuit (N2)?

10. Group Work
A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).
What is the current ratio I2/I1?

11. Formulas for Transformers
Loops: V1 V2 N1 N2 =
Power: V1I1 = V2I2
Potential: V2 = V1 I1 I2
= V1 N2 N1
Current: I2 = I1 V1 V2
= I1 N1 N2

12. Example
A transformer with 5000 primary coils and 100 secondary coils has an input voltage of 50 kV. What is the output voltage?
Vout = Vin Nout/Nin
= (50 kV)(100/5000)
= (50 kV)/50
= 1 kv = 1,000 V