PHYSICS 103: Lecture 17

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PHYSICS 103 Lecture 17
Agenda for Today

• Temperature and Heat
• Heat Transfer
• Radiation

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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
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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
casesroom temperature 300 K surface of Sun
5800 K
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Heat
Heat is the energy that flows from a substance of
higher temperature to a substance of lower
temperature
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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
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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

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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?
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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

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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
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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
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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). Doesnt need to be in basement of house.
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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
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Blackbody Spectrum
The distribution of wavelengths emitted and the
intensity (how bright) depend on the temperature
of the object
P e ? s ? T4 ? A
1) s is the Stefan-Boltzmann constant. 2) s 5.7
x 10-8 W/m2.K4
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Wiens 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 l 480 nm (v) object at 60,000K l
48 nm (uv)
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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

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Main Points from Todays 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.