Thermal Energy

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Air currents at the beach

Water currents in a saucepan while heating

When the movement is forced by a fan or a pump, it is called forced convection (fluid is pushed around by mechanical means – fan or pump)

Forced-air heating systems

Hot-water baseboard heating

Blood circulation in the body

(although air currents move under natural convection)

Slide 20

Thermal Radiation

Thermal radiation transfers energy through emission of electromagnetic waves – does not require physical contact

All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules

At ordinary temperatures (~20°C) nearly all the radiation is in the infrared (wavelengths longer than visible light)

At 800°C a body emits enough visible radiation to be self-luminous and appears “red-hot”

At 3000°C (incandescent lamp filament) the radiation contains enough visible light so the body appears “white-hot”

An ideal emitter and absorber of radiation is called a blackbody (would appear black)

Slide 21

Thermal Radiation

The rate at which energy is radiated is given by Stefan’s Law:

P is the rate of energy transfer (power), in Watts

σ = Stefan-Boltzmann constant = 5.6696 x 10–8 W/m2K4

A is the surface area of the object

e is a constant called the emissivity, and ranges from 0 to 1 depending on the properties of the object’s surface

T is the temperature in Kelvin

Objects absorb radiation as well

Net rate of energy gained or lost given by:

T0 = temperature of environment

Slide 22

Applications of Thermal Radiation

Choice of clothing

Black fabric acts as a good absorber, so about half of the emitted energy radiates toward the body

White fabric reflects thermal radiation well

Thermography as medical diagnostic tool

Measurement of emitted thermal energy using infrared detectors, producing a visual display (see Fig. 11.13)

Areas of high temperature are indicated, showing regions of abnormal cellular activity

Measuring body temperature

Radiation thermometer measures the intensity of the infrared radiation from the eardrum (see Fig. 11.14)

Eardrum is good location to measure temperature since it is near hypothalamus (body’s temperature control center)

Slide 23

Resisting Energy Transfer

Dewar flask/thermos bottle

Designed to minimize energy transfer to surroundings

Space between walls is evacuated to minimize conduction and convection

Silvered surface minimizes energy transfer by radiation

Neck size is reduced