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Phy 202: General Physics IIPowerPoint Presentation

Phy 202: General Physics II

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### Phy 202: General Physics II

Ch 13: Heat Transfer

Jean Baptiste Fourier (1768-1830)

- French mathematician
- Originally wanted to become a priest but drawn to mathematics
- Contemporary of Laplace, Lagrange, Biot & Poisson
- Served as Precept under Napoleon
- Worked on:
- mathematics
- heat conduction

- Yesterday was my 21st birthday, at that age Newton and Pascal had already acquired many claims to immortality.

Types Heat Transfer

- Convection
- Heat energy that is carried from place to place by the bulk movement of a fluid

- Conduction
- Heat energy that is transferred directly through a material

- Radiation
- Heat energy that is transferred by means of electromagnetic waves (radiant energy or light)

Conduction

- Heat flow due to collisions between neighboring atoms (a sort of “domino effect”)
- The rate of conductive heat flow (Q/Dt) is:
- proportional to temperature difference between 2 regions in a conducting pathway material (DT)
- proportional to cross sectional area of material (A)
- proportional to ability of material to conduct heat (k) {called thermal conductivity}
- inversely proportional to length of conducting pathway (L)

- Combining all of these elements forms Fourier’s Heat Equation (1609):
Q/Dt = k (A.DT) / L (Rate of Heat Flow in W)

Or

Q = [k (A .DT) / L] .Dt (Heat Flow in J)

Conduction depends on temperature difference between 2 regions & how far apart those regions are separated

Increasing the cross-sectional area increases amount of heat that will flow in a given time

Conductive Heat Flow- Of course, the relative ability to conduct heat is an intrinsic property of different materials !!

Convection regions & how far apart those regions are separated

- When a fluid is warmed:
- Volume expands (thermal volume expansion)
- Density decreases

- Buoyant forces exerted by cooler (denser) fluids causes warmer fluids to rise
- Remember Archimedes’ Principle ???

- As warmer fluids rise they cool and descend warmer fluids beneath push them out of the way
- The net result is a natural mixing that occurs, called convection
- Convection is a very efficient form of thermal transfer
- Heat energy gets rapidly dispersed throughout the bulk of a fluid

- When mixing is induced artificially (i.e. with a fan) convection occurs more rapidly & efficiently, this is called forced convection

Convection Currents in heating & cooling appliances regions & how far apart those regions are separated

Convection currents in a saucepan

Examples of ConvectionRadiation regions & how far apart those regions are separated

- All objects emit(or radiate) energy in the form of electromagnetic waves
- The rate of radiant energy emitted is related:
- Surface area (A)
- The 4th power of the surface temperature (T4) {in kelvin!!}
- The ability of an object absorb/emit radiant energy, called emissivity (e)
e is 0 for a perfect reflector

e is 1 for a absorber/emitter

e is between 0 & 1 for normal substances and varies with wavelength

- Combined together we have Stefan-Boltzmann’s Law of Radiation:
Q/Dt = e (s A.T4) (Rate of Radiant Heat Flow in W)

where s = 5.67 x 10-8 J/s.m2.K4(the Stefan-Boltzmann Constant)

Effect of Emissivity on Absorption of Radiant Energy regions & how far apart those regions are separated

- All bodies emit as well as absorb radiant energy
- When a body is in thermal equilibrium:
Qabsorbed = Qemitted

- Dark objects will reach higher equilibrium T than lighter objects (e.g. consider pavement and concrete on a hot day!)
- Good emitters are also good absorbers of radiant energy (e ~ 1)
- Poor emitters are also good poor of radiant energy (e ~ 0)

The Greenhouse Effect regions & how far apart those regions are separated

- The presence of our atmosphere helps the Earth maintain a moderate & livable temperature range, due to a process called the Greenhouse Effect:
- Radiant energy (short wavelength) from the sun is absorbed by the surface of earth
- The earth’s surface radiates energy (long wavelength)
- Some of the long wavelength radiation is reflected by the earth’s atmosphere by so called “Greenhouse Gases”
- The reflected long wavelength radiation is reabsorbed by the earth’s surface

- This positive feedback process heats the surface of the earth and keeps the surface warmer at night
- Do not confuse Greenhouse Effect (Good!) with Global Warming (Bad!)
- Global warming is believed to be due to an increased production of greenhouse gases which may increased the amount of long wavelength radiation reflected back to the Earth

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