Thermal Energy Transmission - Radiation

• Wavelength and frequency

• Wien’s Law

• Power/Intensity

Unit 05 Thermal Radiation - Slide 1

Electro-magnetic waves

*fv Velocity =How long it took you

How far you go=

Period

Wavelength

T

Unit 05 Thermal Radiation - Slide 2

Example 5.1 Wave Length

The frequency of blue visible light is around 620 X 1012

Hz. What is the wavelength of its radio wave?

The frequency of the country music station “US 99”

is 99.5 MHz (99.5 X 106 cycles/sec). What is the

wavelength of its radio wave?

Remember! All electromagnetic waves have the same speed!

v=c=3X108 m/secUnit 07 Electricity - Slide 3

Longer wavesHigher frequency

Enough

energy to

damage

cells

Thermal Radiation – Unit 05, Slide 4

60Hz =5X106m

How much energy via radiation?

4*** TAreaPower Heat per time

(Watts)

Emissivity

Stefan’s constant

= 5.67 X 10-8 W/(m2-K4)

-> amount absorbed (1 for black)

Stefan-Boltzman’s Law

Unit 07 Electricity - Slide 5

Everything Emits Radiation!

Thermal Radiation – Unit 05, Slide 6

As long as T = 0!

Example 5.2: Power from the sun

4*** TAreaP

What is the rate of energy released from

the sun via radiation?

• Tsun = 5887K

• sun~1

• rsun=7X108m

24 rSurface area of Sphere =

(on formula sheet)

Unit 05 Thermal Radiation - Slide 7

Example 5.3: Temperature of the Earth

Unit 05 Thermal Radiation - Slide 8

4*** TAreaP

We just calculated that the sun puts out

4.14X1026 Watt of power via radiation.

How much of that power hits the earth?

The radius of the earth is rearth=6.4X106m.

The earth is rorbit=1.5X1011m from the sun.

A) 1.9X1017Watts

B) 0.9X1017Watts

C) 2.4X1012Watts

D) 3.4X1011Watts

Example 5.3: Temperature of the Earth

Unit 05 Thermal Radiation - Slide 9

4*** TAreaP

We just calculated that 1.9X1017Watts of power

hits the earth via radiation. 30% is reflected by

the atmosphere, so the earth absorbs

1.3X1017 Watts

If the same amount of power must leave the

earth, what should the temperature of the earth

be?

A) 410K

B) 350K

C) 260K

D) 150K

earth~1, rearth=6.4X106m

24 rSurface area of Sphere =

(on formula sheet)

Wien’s Law

T

mmKpeak

9.2

Why is earth not freezing all the time?

Wien’s Law – peak depends on T

• out is longer than in

• earth is smaller for longer

• “Green house effect”

Unit 05 Thermal Radiation - Slide 10

Example 5.4: Peak

The temperature of the sun is 5887K.

What is the peak wave length, peak, of the

radiation beginning transmitted?

What is its frequency of the wave, f ?

What is the period of the wave, T ?

What is the velocity of the wave?

Unit 05 Thermal Radiation - Slide 11

Easy! All electromagnetic waves have the same speed v=c=3X108 m/sec

Longer wavesHigher frequency

Enough

energy to

damage

cells

Thermal Radiation – Unit 05, Slide 12

Wien’s Law

Thermal Radiation – Unit 05, Slide 13

Example 5.5: Peak

The temperature of the earth is around 287K.

What is the peak wave length of the radiation

beginning transmitted?

Unit 05 Thermal Radiation - Slide 14

Longer waves

Higher frequency

Enough

energy to

damage

cells

Thermal Radiation – Unit 05, Slide 15

Question

Thermal Radiation – Unit 05, Slide 16

We just determined that the peak wave

length of the electromagnetic waves coming

from the earth peaks at around 4.9mm.

What is the velocity of this wave?

A) 4.9 m/sec

B) 4.9 mm/sec

C) 3X108m/sec

D) 4.9 X 108m/sec

E) 1/4.9 mm/sec

Example 5.6: Do you glow?

Do you emit light?

The temperature of your skin is

around 98.6oF. What is the

peak wave length of the

radiation you transmit?

Unit 05 Thermal Radiation - Slide 17

Question

Unit 05 Thermal Radiation - Slide 18

The temperature of your skin is

around 98.6oF. What is the

temperature of your skin in Kelvins?

A) 0K

B) 98.6K

C) 37K

D) 310K

E) 522K

Question

Unit 05 Thermal Radiation - Slide 19

The temperature of your skin is

around 98.6oF. What is the peak

wavelength of the light you emit?

A) 2.9X10-3m

B) 2.9X10-6m

C) 3.1X10-6m

D) 6.2X10-6m

E) 9.4X10-6m

Longer wavesHigher frequency

Enough

energy to

damage

cells

Thermal Radiation – Unit 05, Slide 20

Look for heat losses?

Thermal Radiation – Unit 05, Slide 21

Thermal Radiation – Unit 05, Slide 22

Heat Loss at my house

Thermal Radiation – Unit 05, Slide 23

Thermal Radiation – Unit 05, Slide 24

Older Window

(double pane)

Colder at center

Modern Window

(triple pane)

Colder at frame

Embarrassing item #1

8” of concrete (e.g. uninsulated basement wall) has R ~ 1.35

Thermal Radiation – Unit 05, Slide 25

Brand new zillion

dollar window

R~5.1

Exposed concrete

basement wall

R~1.35

Embarrassing item #2

2” of wood (e.g. uninsulated band board + siding) has R ~ 2.2

Thermal Radiation – Unit 05, Slide 26

Really BIG brand new

zillion dollar window

R~5.1

Exposed concrete

basement wall

R~1.35

Insulated Wall

R~13

2”X10” Bandboard +

siding

R~2.2

(Note: Exterior Shot)

Embarrassing item #3

A wood-on-wood connection makes a very poor air seal

Thermal Radiation – Unit 05, Slide 27

Cold air pouring into basement at joint

between band board and foundation

wall

Cold air pouring into kitchen at joint

between wall frame and floor

(Note: Photos taken during 5Pa vacuum air

tightness test)

Embarrassing item #4

Insulation doesn’t always stay where you put it and in the condition you put it there

Thermal Radiation – Unit 05, Slide 28

New insulation fell during sheetrock

installation?Insulation damaged by

past water leak