Board Work

1. If the magnetic field B intensifies, in what direction will a current be induced around the loop?

B

Which direction is the flux change?

What current would oppose it?

AC Transformers

Source: OSHA

Objective

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

Flux Change Creates Potential

Rapidly changing fieldhigh induced potential

unchanging field zero induced potential

How a Transformer Works

• Alternating current in the primary coil creates a changing magnetic field.

• The changing field induces an electric potential in the secondary coil.

Potential Proportional to Loops

• Same flux and area A through both sets of windings

• Each loop adds potential• Potentials V are proportional to the

number of loops N

V1 N1

V2 N2=

Energy is Conserved

• Ideally: power in = power out

V1I1 = V2I2

• Realistic: power in > power out

• Efficiencies usually around 95%

Transformer Summary

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

Poll 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?

A. 200 W.

B. 660 W.

C. 1100 W.

D. 2400 W.

Board Work

2. A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).

a. Which is greater: the potential in the primary circuit or the potential in the secondary circuit?

b. Where will the number of loops be greater: in the primary or in the secondary?

Board Work

2. A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).

c. What is the voltage ratio V2/V1?

d. What is the loops ratio N2/N1?

e. How many loops are in the secondary circuit (N2)?

Board Work

2. A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary).

f. What is the current ratio I2/I1?

Formulas for Transformers

• Power: V1I1 = V2I2

• Current: I2 = I1V1

V2

= I1N1

N2

• Potential: V2 = V1

I1I2

= V1

N2

N1

• Loops:V1

V2

N1

N2

=

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

Light

Chapter 26

Objectives

• Describe the transverse “medium” of electromagnetic waves.

• Thought experiment: charging parallel plates

Last Piece of EM Theory

• A changing electric field acts as a current

• Charges accumulate on the plates

• A changing E field creates a B field!

Maxwell’s pivotal insight

I I

+ –EB

• The E field between the plates increases• The changing E field is sort of a virtual current

Electromagnetic Fields

• Faraday’s law: a changing magnetic field creates an electric field

B E

• Virtual Current: a changing electric field creates a magnetic field

What They Mean

• Faraday’s Law = Lorentz Force– A transversely-moving B field makes an E

field

– The electric field is proportional to the magnetic field strength

B

What They Mean

• Virtual current works the same way– A transversely-moving E field makes a B field

– The magnetic field is proportional to the electric field strength (note the directions)

Electromagnetic Field Interplay

• A moving B field creates an E field– E direction = B v direction

Bv

EB

v

E

vB

E

• The fields are self-propagating– v direction = E B direction

• A moving E field creates a B field– B direction = v E direction

AC Creates Oscillating B Field

Observe here

The End Result

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

Poll Question

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

A. Transverse.

B. Longitudinal.

C. A combination of transverse and longitudinal.

Speed of Light

c = 2.9979 108 m/s

(in vacuum)

Electromagnetic Wave Energy

Non-classical result:

E = hf

E = energy

f = frequency

h = Planck constant = 6.621 10-34 J s

Electromagnetic Spectrum

Temp Influences Spectrum

• Higher T greater power– P/A = T4

– = 5.6710–8 W m–2 K–4

• Higher T higher peak frequency– max = b/T

– b = 2.898 106 nm K

Source: M. A. Seeds, Exploring the Universe

Poll Question

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

A. At higher temperatures, more of the light emitted is “warm”-colored.

B. At higher temperatures, more of the light emitted is “cool”-colored.

Something to Ponder

Why do hotter objects emit a greater fraction of their energy at short wavelengths (high frequencies)?

Reading for Next Time

• Color– How we see color– Why things are colored