ch17 young freedman2
Post on 03-Jun-2018
215 Views
Preview:
TRANSCRIPT
-
8/12/2019 Ch17 Young Freedman2
1/14
Heat
Caution: in everyday usage, Temp & Heat are usuallyinterchangeable.
But, in physics, they are not the same!
Temperature: a macroscopic state variable ~ avg. KE ofmolecules in the system (later).
Heat: the transfer of energybetween bodies due to atemperature difference.
-
8/12/2019 Ch17 Young Freedman2
2/14
Mechanical Equivalent of Heat(Sir James Joules, 1818-1889)
Joule knew a mass above theground had potential energy. He
dropped a weight on a cord,
turning a paddle in water. Temp
changes were then monitored bya very accurate thermometer.
His conclusion: mechanical work
and heatare equivalent in raisingthe temperature of the water.
D colliding balls
-
8/12/2019 Ch17 Young Freedman2
3/14
Quantity of Heat
Energy transfer due to temp diff is called heat.
Historically, the unit of heat is defined in terms of temp changes
of water:
1 calorie (cal) = amount of energy transfer (heat) needed to raisethe temp of 1g of water from 14.5oC to 15.5oC.
From Joules experiment, we also know that this amount of heat isequivalent to 4.18 J amount of mechanical energy, i.e.,
1cal = 4.186 J
-
8/12/2019 Ch17 Young Freedman2
4/14
Specific Heat/Heat Capacity
Different type of materials will need different amount ofheat to raise its temp by 1oC (or 1K).
We can quantify this using specific heat:
orQ quantity of heat needed to raise Tfrom T1 to T2
(T = T2 T1)
m mass of the material
c specific heat is characteristic of the type of
material [ ]/J kg K
Q mc T dQ mc dT
-
8/12/2019 Ch17 Young Freedman2
5/14
Specific Heat Values
-
8/12/2019 Ch17 Young Freedman2
6/14
MolarSpecific Heat/Heat Capacity
One can also specify a certain amount of materialsby the number of molecules (or mole n) instead of itsmass (m in kg). With m=nM,
Q = mc T = (nM)c T = nCT
n number of mole
M molar mass (mass per mole)
C molar specific heat (note upper case)
(1 mole = )236.022 10 particles
:note cM C
-
8/12/2019 Ch17 Young Freedman2
7/14
Molar Specific Heat
Specific heat also depends on theprocessbywhich heat is being transferred into thesystem
Two often used molar specific heats:
Cp : molar specific heat at constant pressure
(heating a liquid in an open container)
Cv : molar specific heat at constant volume
(heating a gas in a closed container)
For most materials, Cp > Cv .
-
8/12/2019 Ch17 Young Freedman2
8/14
Specific Heat (examples)
Hot food on metal/wood plate Colliding metal balls
Physical Intuition for c:
For a given amount of heat flow Q, specific
heat c is a measure of the thermal sensitivityof the material !
D water balloon
note
-
8/12/2019 Ch17 Young Freedman2
9/14
Example (Iron vs. Wood Plate)Metal Plate Wood PlateHot food
m=0.25kg
ciron=470 J/kg K
Q=2 kJ
m=0.25kg
cwood=2500 J/kg K
Putting the same amount of heat (hot food) on the plate, what is T?
T Q mc
200017
0.25 470 /
JT K
kg J kg K
20003.2
0.25 2500 /
JT K
kg J kg K
much less temp increase !hot to the touch!
-
8/12/2019 Ch17 Young Freedman2
10/14
Example (Colliding Balls)
MM
m
+v -v
5
0.5
10 1 10
5 /
M kg
m mg kg
v m s
2500 /wood
c J kg K
KE delivered to the small piece of paper by the two balls:
2
2 212 0.5 5 / 12.52
KE Mv Mv kg m s J
KE Q, what is the temperature increase for the small piece of paper?
5
12.5
793.1512.5 / 1 10 2500 / 500520
wood o
Q KE J
KT Q mc J kg J kg K K C
paper will
burn!
(starting @
room T)
-
8/12/2019 Ch17 Young Freedman2
11/14
Extensive and Intensive Quantities
Extensive quantities: depend on the amount of
substance:
double the amount double the quantity
e.g., volume Vis an extensive quantityIntensive quantities: not depend on amount of substance
units are typically per kg or per mol
e.g. specific heat c and molar heat capacity C:
m o l k g
J k g K J m o l K
(Note: Typically, heat capacity is an extensive quality and specific heat is
intensive but your book does not make this distinction.)
-
8/12/2019 Ch17 Young Freedman2
12/14
Heat Exchanges during Phase Changes
Observation: Most of the time, when Q enters a system, T
increases. But, not always!
During Phase Changes, heat exchanged by substances does notproduce T.
During Phase Changes, energy exchanged is used for internalstructural changes (e.g., pulling molecules further apart) :
e.g. ice water or water steam
Q = m L
heat of fusion (water)5
3.34 10 /fL J kg
heat of vaporization (water)6
2.26 10 /vL J kg
-
8/12/2019 Ch17 Young Freedman2
13/14
Heats of Fusion & Heats of Vaporization
-
8/12/2019 Ch17 Young Freedman2
14/14
Calorimetry: Problem Solving with HeatExchanges
Main Concept: Conservation of Energy
Q = 0 (sum of all heat flows intoand out of system =0)
Sign Convention: heat enters an object is +heat leaves an object is
T = Tf Ti
Steps: 1. Identify all phase change pts2. Apply either (Q=mcTor Q=mL) for each processes
separately. (dont apply Q=mcTacross ph. changes!)
3. Use Q = 0 and sign convention to solve problem
top related