AC Transformers

1
AC Transformers
Source OSHA
2
Objective

• Relate input and output power, voltage, current,
  and number of windings in an AC transformer.

3
Flux Change Creates Potential
4
How a Transformer Works

• AC current in the primary creates a changing
  magnetic field.
• Changing field induces a potential in the
  secondary

5
Potential Proportional to Loops

• Same flux F and area A through both sets of
  windings
• Each loop adds potential
• Potentials V are proportional to the number of
  loops N

6
Energy is Conserved

• Ideally power in power out

V1I1 V2I2

• Realistic power in gt power out
• Efficiencies usually around 95

7
Transformer Summary

• Power in ? power out
• ? loops ? higher V, lower I
• ? loops ? lower V, higher I

8
CPS 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.

9
Group Work

• A transformer with 1000 primary windings converts
  AC at 110 V (primary) to 220 V (secondary).
• Which is greater the potential in the primary
  circuit or the potential in the secondary
  circuit?
• Where will the number of loops be greater in the
  primary or in the secondary?

10
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)?

11
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?

12
Formulas for Transformers

• Power V1I1 V2I2

13
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

14
Light

• Chapter 26

15
Objectives

• Describe the transverse medium of
  electromagnetic waves.

16
AC Creates an Oscillating Field
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E and B Fields Perpendicular

• Electric and magnetic fields are perpendicular
• to each other, and
• to the direction of propagation.

18
CPS Question

• Are electromagnetic waves transverse,
  longitudinal, or a combination of both?

• Transverse.
• Longitudinal.
• A combination of transverse and longitudinal.

19
Speed of Light

• c 2.9979 ? 108 m/s
• (in vacuum)

20
Electromagnetic Wave Energy

• Non-classical result
• E hf
• E energy
• f frequency
• h Planck constant 6.621 ? 10-34 J s

21
Electromagnetic Spectrum
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Temp Influences EM Radiation

• Higher T ? greater power
• P/A sT4
• s 5.67?108 W m2 K4
• Higher T ? higher peak frequency
• lmax b/T
• b 2.898 ?106 nm K

Source M. A. Seeds, Exploring the Universe
24
CPS Question

• How do warm colors (red, orange, yellow) and
  cool colors (green, blue) relate to temperature?

• At higher temperatures, more of the light emitted
  is warm-colored.
• At higher temperatures, more of the light emitted
  is cool-colored.

25
Group Work

• Why do cooler objects emit a greater fraction of
  their energy at long wavelengths (low
  frequencies)?