# L27_EM - Barransclass.com

Urban and Civil

Nov 16, 2013 (4 years and 6 months ago)

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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 field

high 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

F

and
area

A

through both sets
of windings

Each loop adds potential

Potentials

V

are proportional to the
number of loops

N

V
1

N
1

V
2

N
2

=

Energy is Conserved

Ideally:
power in

=
power out

V
1
I
1
=
V
2
I
2

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
V
2
/
V
1
?

d.
What is the
loops

ratio
N
2
/
N
1
?

e.
How many

loops

are in the secondary circuit
(
N
2
)?

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
I
2
/
I
1
?

Formulas for Transformers

Power:
V
1
I
1

=

V
2
I
2

Current:
I
2

=

I
1

V
1

V
2

=

I
1

N
1

N
2

Potential:
V
2

=

V
1

I
1

I
2

=

V
1

N
2

N
1

Loops:

V
1

V
2

N
1

N
2

=

Example

A transformer with 5000 primary coils and
100 secondary coils has an input voltage of
50 kV. What is the
output

voltage
?

V
out

=
V
in

N
out
/
N
in

= (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

+

E

B

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

D
B

D
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

D
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

B

v

E

B

v

E

v

B

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

10
8
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

=
s
T
4

s

=
5.67

10

8

W m

2
K

4

Higher
T

higher
peak frequency

l
max

=
b
/
T

b

=
2.898

10
6

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