Document 6530469

11.2 specific heat
11.3 calorimetry
How much heat needed to raise T??
Exercise
calorimeter- insulating, ~ no heat exchanged w outside
2. A 50-g sample of copper is at 25°C. If 1,200 J of energy is added
to the copper by heat, what is its final temperature?
Define specific heat c:
SI unit: J/kg•ºC
Q
c = Q/m∆T (eq. 11.2)
mass of object
c depends on material
Exercise
read (p.336): c of H2O, effect on environment
Quick Quiz 11.1
Imagine you have 1 kg each of iron, glass, and
water, and that all three samples are at 10ºC.
(Rank the samples from lowest to highest
temperature after 100 J of energy is added to
each by heat. (b) Rank them from least to
greatest amount of energy transferred by heat
if enough energy is transferred so that each
increases in temperature by 20ºC.
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College Physics W04 Prof. Kinoshita
11.4 latent heat, phase changes
3 phases of matter: solid, liquid, gas
Phase change: solid ↔ liquid freezing/melting
liquid ↔ gas boiling/condensation
sublimation
solid ↔ gas
fluid, heat capacity cw, mass m -> Q from ∆T:
Q = m cw ∆T (from eq. 11.2)
object under study
Heat transferred from colder to warmer object
=>
Qcold = – Qhot (eq. 11.4)
15. An aluminum cup contains 225 g of water and a 40-g copper stirrer,
all at 27°C. A 400-g sample of silver at an initial temperature of 87°C is
placed in the water. The stirrer is used to stir the mixture gently until it
reaches its final equilibrium temperature of 32°C. Calculate the mass of
the aluminum cup.
18. A combination of 0.250 kg of water at 20.0°C, 0.400 kg of aluminum
at 26.0°C, and 0.100 kg of copper at 100°C is mixed in an insulated
container and allowed to come to thermal equilibrium. Neglect any
energy transfer to or from the container and determine the final
temperature of the mixture.
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College Physics W04 Prof. Kinoshita
11.4
all involve changes
to internal energy
Require heat to change phase of substance:
Exercise
23. What mass of steam that is initially at 120°C is needed to warm
350 g of water and its 300-g aluminum container from 20°C to
50°C?
cAl = 0.215 cal/g•ºC
Q = ± mL (eq. 11.5) (no change in T)
Latent heat - specific to phases, substance
Latent heat of
fusion
Lf
solid ↔ liquid
vaporization
Lv
liquid ↔ gas
So, if we add heat at const rate to e.g. solid H2O, T vs t (p. 338-9):
ice -> water
understandable at molecular level (p. 340)
water -> steam
College Physics W04 Prof. Kinoshita
Follow problem-solving strategy (p. 340):
1) consistent units
2) Q=mc∆T
only for no phase change
Q=mLf
for solid ↔ liquid
for liquid ↔ gas
Q=mLv
3) sign (±)
heat gain or loss www
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11.5 energy transfer by conduction
11.5
What about multiple layers with different materials? (read, p. 344)
3 ways to transfer heat: conduction, convection, radiation
conduction (read)
• molecular-level energy transfer over macroscopic distances
• no net movement of matter
Tc
heat conducts
warm -> cold
Th
Q
Define heat transfer rate:
"R value"
Read p. 344 on units for R value
area A
Exercise
what does P depend on, and how?
• ∆T
• distance L
• x-sectional area of conducting material
• material's conductivity, k
(work examples)
College Physics W04 Prof. Kinoshita
Ri
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39. A copper rod and an aluminum rod of equal
diameter are joined end to end in good thermal contact.
The temperature of the free end of the copper rod is
held constant at 100°C, and that of the far end of the
aluminum rod is held at 0°C. If the copper rod is 0.15
m long, what must be the length of the aluminum rod
so that the temperature at the junction is 50°C?
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