- Количество слайдов: 17
PHYSICS 103: Lecture 17 Agenda for Today: • • • Temperature and Heat Transfer Radiation
Temperature 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 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 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 contact Convection - heat flow along with fluids Radiation - 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? Hot air is blown out to ducts 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 Gas or oil is burned Room air blown past “heat exchanger” and into metal ducts that heat your house Blower sucks in cold room air and sends it up to heat exchanger
Gravity warm air heating 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
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 Click here
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 = e s T 4 A 1) s is the Stefan-Boltzmann constant. 2) s » 5. 7 x 10 -8 W/m 2. K 4
Wien’s Law Wavelength of peak emission µ 1/temperature or l max = 0. 29 / T Note: . . . l is in cm …T is in Kelvins Example: object at 6000 K object at 60, 000 K l ~ 480 nm (v) l ~ 48 nm (uv)
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 greater the power radiated (ie, more 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 heat You 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 Radiation You 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.