PHYSICS 103: Lecture 17

• Temperature and Heat

• Heat Transfer

• Radiation

• Temperature and Heat

• Heat Transfer

• Radiation

Agenda for Today:

Matter: Composed of molecules, atoms & sub-atomic particles. Continuously in motion.

Temperature: is a measure of this motion (thermal energy)

The hotter the object --- the higher its temperature --- faster the motion of the constituent particles

Temperature

Temperature Scale

Fahrenheit (°F) & Centigrade (°C)( calibrated by the properties of water)

…water freezes at 0 °C (32 °F)…water boils at 100 °C (212 °F)

Absolute Scale (K for Kelvin)(calibrated by ‘intrinsic’ property of all matter i.e. cessation of all atomic motion)

Absolute zero occurs at ~ -273 °C

Conversions:TK = T C + 273

Special cases:…room temperature ~ 300 K…surface of Sun ~ 5800 K

Heat

Heat is the energy that flows from a substance of higher temperature to a substance of lower temperature

Heat Transfer Mechanisms

Heat transfers from warmer to cooler things and the objects involved tend to reach a common temperature. This can occur through:

Conduction - heat flow through materials in contactConvection - heat flow along with fluidsRadiation - heat flow through light waves

Conduction

• Heat moves through material but atoms do not• In metal, mobile electrons move and collide with molecules, transferring energy• Heat flows quickly from hot to cold

Convection

• Heat is transferred by actual motion of fluid•If fluid is heated from below, the molecules at the bottom increase in speed• Heated fluid becomes less dense and is pushed up by the denser cooler fluid that takes its place at the bottom• As hot air rises and expands, it cools, becomes denser, sinks•cycle continues

Why does rising air cool?

Radiation

• Heat is transferred by electromagnetic waves (“light”)• Type of wave (radio, IR, X-ray, Gamma-rays…) depends on temperature•Higher the temperature => more radiated heat

How does a modern furnace work?

Blower sucks in cold room air and sends it up to heat exchanger

Room air blown past “heat exchanger” and into metal ducts that heat your house

Hot burned air passes (is blown) through curved metal tubing which gets hot through conduction and is sent out of the house through a “chimney”. Hot burned air gets cold by the time it leaves

Hot air is blown out to ducts

Gas or oil is burned

Furnace is in basement, buoyont forces move hot air up through

ducts; cold air accumulates near the floor and falls into cold air

registers and returns to furnace

Gravity warm air heating

Forced-air heating

Forced-air heating uses a fan to force air into furnace (vacuum it in), passes through heat exchanger, and is blown throughout the house in wide ducts (this is what is in most modern houses). Doesn’t need to be in basement of house.

Thermal Radiation

•All matter contains electric charges•These particles are in constant motion and are accelerating•Accelerating charge produces electromagnetic radiation• All materials emit electromagnetic radiation

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Electromagnetic Waves

Shorter wavelength light has higher energy

Blackbody Spectrum

The distribution of wavelengths emitted and the intensity (how bright) depend on the temperature of the object

P = T4 A

1) is the Stefan-Boltzmann constant.

2) 5.7 x 10-8 W/m2.K4

Wien’s LawWavelength of peak emission

1/temperature

or

max = 0.29 / T

Note:... is in cm…T is in Kelvins

Example:object at 6000 K ~ 480 nm (v)object at 60,000K ~ 48 nm (uv)

•The hotter the object --- the higher its temperature --- faster the motion of the constituent particles --- the greater the power radiated (ie, more light)

Summary of Radiation

•The sun, stars, you, everything in this room radiates electromagnetic waves (light)

•The hotter the object --- the higher its temperature --- faster the motion of the constituent particles --- the shorter the wavelength at which most of the light is radiated

Main Points from Today’s Lecture

• Temperature and heatYou should understand that temperature is a measure of the internal kinetic energy of particles in matter. Heat is the transfer of energy from hot to cold things.

• Heat Transfer

You should understand that heat can be transferred through three different means: conduction, convection, and radiation. You should understand how these work in simple everyday cases like how your house is heated.

• Thermal RadiationYou should understand that all things radiate electromagnetic waves (or light). Light comes in different “colors” (wavelengths). The hotter the object, the greater the power of radiation emitted and the shorter the wavelength at which most of the light is radiated.

• Temperature and heatYou should understand that temperature is a measure of the internal kinetic energy of particles in matter. Heat is the transfer of energy from hot to cold things.

• Heat Transfer

You should understand that heat can be transferred through three different means: conduction, convection, and radiation. You should understand how these work in simple everyday cases like how your house is heated.

• Thermal RadiationYou should understand that all things radiate electromagnetic waves (or light). Light comes in different “colors” (wavelengths). The hotter the object, the greater the power of radiation emitted and the shorter the wavelength at which most of the light is radiated.