ch17 young freedman2

8/12/2019 Ch17 Young Freedman2

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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.


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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


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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


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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


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Specific Heat Values


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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


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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 .


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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


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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!


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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)


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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.)


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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


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Heats of Fusion & Heats of Vaporization


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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

ch17 young freedman2

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