# Thermal Physics by liaoqinmei

VIEWS: 3 PAGES: 43

• pg 1
Thermal Physics
Thermometric property - a
characteristic of an object that varies
with temperature
Heat versus Temperature
 Temperature: average kinetic energy of
each particle in a substance
 degree of hotness or coldness
 indicates direction of heat flow
Heat versus Temperature
 Heat - total thermal energy (internal energy)
absorbed or transferred
 Internal energy - KE and PE
 PE due to bond energy (chemical energy stored
in bonds) and electromagnetic forces between
particles
 Example: bucket vs thimble of water at 100°C
Measurement of Temperature
 Alcohol vs. mercury thermometers
 Alcohol - used at lower temperatures
 Alcohol freezes at -114°C and boils at
78.5°C
 Mercury - used at higher temperatures
 Mercury freezes at -39°C and boils at
357°C(675F)
Thermometers

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Constructing Thermometers
 Bulb - thin glass for quick heat transfer
 Clinical thermometer - restriction
prevents backflow
 Vacuum above liquid
 Calibrated using two fixed points (0C and
100C), mark scale evenly
Thermometers-constructed
using thermometric properties
 Expansion of liquid in capillary tube
 Resistance in wire (thermistor)
 Different rates of expansion of metals
(bimetallic strips)
 Volume of gas at constant pressure (gas
expansion rate is linear)
 Color change of solid when
heated(pyrometers
Conversions
 K = °C + 273
 °F = 9/5 °C + 32
 Absolute zero - no motion of particles in
substance at 0 K
 Water freezing point 32°F, 0°C, 273 K
 Water boiling point 212°F, 100°C, 373 K
 Room temperature 20°C
Relating temperature to
velocity of particles
 Kinetic energy = 1/2 mv2
 Kinetic energy = 3/2 kT
 k = Boltzmann constant 1.38 x 10-23 J/K
 T = Temperature in Kelvin
 m = mass in kilograms
 v = root mean square velocity (rms)
rms velocity
 Number of particles at different speeds is
not a normal distribution - some particles
move VERY fast
 Peak velocity is most probable velocity
 Vrms = (vav2)1/2
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Heat Transfer - Conduction
 Occurs in solids, liquids, gases
 Temperature difference causes transfer of
thermal energy from hot to cold by
particle collision without net movement
of substance
Heat Transfer - Convection
 Occurs in fluids (liquids and gases)
 Temperature difference causes mass
movement of fluid particles - density
differences
 Convection cells (convection currents)
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Heat Transfer - Radiation
 No medium required
 Heat travels as electromagnetic waves
 Most reflected at atmosphere, some is
absorbed
Heat Capacity
 Objects with a high heat capacity take in heat at
a slower rate - heat slowly, and also cool slowly
 Heat capacity Q/T units are JK-1
 Specific heat capacity - heat capacity per unit
mass - heat required to raise the temperature of
1 kg by one Kelvin
 Variable for specific heat….c
Heat Equation
 Heat required to produce a temperature
change
 Q = mcT
 Can heat a substance using electrical
energy
 Electrical energy = VIt = mcT
Mixtures
 Heat lost by one substance equals heat
gained by the other substance - total heat
gained or lost by the system is 0
 Calorimeter - allows minimal energy loss
to surroundings
Calorimeter

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Example #1
Mix 100 grams of water at 20°C
and 200 grams of water at 40°C.
Find the final temperature of the
water.
Solution:
 Heat lost by one sample plus heat gained by
other sample equals 0
mcT  mcT  0
1    1
0.1kg(4180 kJkg K )(Tf  20) 
1 1
0.2kg(4180 kJkg K )(Tf  40)  0
418Tf  8360 836Tf  33440  0

Tf  33C
Example #2
A 100 gram block of Ag at
100°C is placed in 100 grams
of water at room temperature.
Find the final temperature.
Solution:
 Heat lost by silver plus heat gained by water
equals 0
mcT  mcT  0
1   1
0.1kg(235 kJkg K )(Tf 100 ) 
1 1
0.2kg(4180 kJkg K )(Tf  20)  0
23.5Tf  2350 418Tf  8360  0

Tf  24C
Kinetic Theory
 All matter is composed of extremely
small particles
 All particles are in constant motion
 If particles collide with other particles,
KE is conserved
 A mutually attractive force exists
between particles
Matter
 Matter - has mass and occupies space
 Four phases - solid, liquid, gas, plasma
 Plasma - made by heating gas atoms until
they ionize - separate into positively and
negatively charged particles - sun, other
stars composed of plasma
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Characteristic   Solid         Liquid       Gas
Shape            definite      variable     variable
Volume           definite      definite     variable
Compressibility almost          slightly     highly
incompressible compressible compressible

Diffusion        small         slow         fast
Density          highest       high         low
Bonds            strong        relatively   weak
strong
Particle         closely       larger       very large
spacing          packed        spaces       spaces
PE               high          higher       highest
Thermal          least         middle       most
energy
Latent Heat
 Latent heat of transformation: heat required to
change 1kg of a substance from one phase to
another
 Equation: Q = mL
 No temperature change during a phase change -
heat is used to change PE - heat needed to break
bonds, heat released when bonds are formed
Latent Heat
 Latent heat of fusion: solid to liquid (heat
is absorbed) or liquid to solid (heat is
released)
 Latent heat of vaporization: liquid to gas
(heat is absorbed) or gas to liquid (heat is
released)
 Heating curves:
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Example #1
Find the heat required to melt
10 kg of gold. (latent heat of
fusion = 6.3 x 104 JKg-1)
Solution
Q  mH f
1
10kg(6.3x10 Jkg )
4

 6.3x10 J
5
Example #2
Find the heat required to
vaporize 100 grams of lead
(latent heat of vaporization =
2.04 x 104 JKg-1)
Solution

Q  mHv
1
 0.1kg(2.04x10 Jkg )
4

 2.04x10 J
3



Example #3
 Find the heat required to change 2 kg of
ice at -10°C to steam at 120°C
 Specific heat of ice 2060 Jkg-1K-1
 Latent heat of fusion 3.34 x 105 Jkg-1
 Specific heat of water 4180 Jkg-1K-1
 Latent heat of vaporization 2.26 x 106
Jkg-1
 Specific heat of steam 2020 Jkg-1K-1
Solution:
Heat ice from -10°C to 0°C
Melt ice
Heat water from 0°C to 100°C
Evaporation of water
Heat vapor from 100°C to 120°C
Q  mcT  mH f  mcT  mHv  mcT
1   1
2 kg(2060Jkg K )(10C) 
1
2kg(3.34x10 Jkg ) 
5

1   1
2 kg(4180Jkg K )(100C) 
1
2kg(2.26x10 Jkg ) 
6

1       1
2 kg(2020Jkg K )(20C) 
                                    6

5.5x10 J
Example #4
 Heat is added to a mass of 5 kg at room
temperature (20°C) at a rate of 500 watts for 1
minute until the substance begins to melt at
300°C. The substance takes 3 minutes to melt.
 A. Sketch a graph of temperature vs. time
 B. Find the specific heat of the solid
 C. Find the latent heat of fusion
Solution-Part A.

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300°C

20°C
1     4
Times and temperatures on graph?
B. Q  mcT
500watts x 60s  5kg(c)(280C)
1   1
c  21.4Jkg K
C. Q  mH f
            500watts(180sec) 5kg(H f )
1
Hf  1.8x10 Jkg
4


Evaporation
Evaporation takes place at all
temperatures and results in the cooling
of a liquid
Evaporation
 Change from liquid state to gaseous state,
occurs at a temperature below boiling
point
 Particles near surface have enough KE to
overcome attractive forces of nearby
particles, lowers KE of substance

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