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Heat and Heat Transfer

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Heat and Heat Transfer Where would you rather go for break, Montreal or Southern France? * Thermal Radiation The most important source of heat on Earth is the Sun. – PowerPoint PPT presentation

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Title: Heat and Heat Transfer


1
Heat and Heat Transfer
  • Where would you rather go for break, Montreal or
    Southern France?

2
Heat Capacity
  • Why do you burn your
  • mouth (but not your
  • hand) when you eat hot
  • pie or pizza?

3
Why do we go to the beach to cool off?
  • All because water has a high heat capacity!

4
Which heats up more quickly?
  • Put a pot of water on the stove 15 minutes to
    boil
  • Put same mass of iron 2 minutes to reach 100o
    C
  • Substances have different ability to absorb or
    lose heat.

5
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6
Heat Capacity and Specific Heat
For many substances, under normal circumstances
?T?Q. Example heat pot of water Or Q C?T
where C is the heat capacity. Takes more Q to
boil 2 cups than 1 cup The specific heat
capacity, or just specific heat, of a substance
is the heat capacity per unit mass.
or
7
  • Which has a higher specific heat iron or water?
    A) Iron B) water
  • Which has a higher specific heat water or sand?
    A) water B) sand

8
Water has a very high specific heat
9
Example If 125.6 kJ of heat are supplied to
5.00?102 g of water at 22 ?C, what is the final
temperature of the water?
10
Example A 0.400 kg aluminum teakettle contains
2.00 kg of water at 15.0 ?C. How much heat is
required to raise the temperature of the water
(and kettle) to 100 ?C?
11
Example A 0.400 kg aluminum teakettle contains
2.00 kg of water at 15.0 ?C. How much heat is
required to raise the temperature of the water
(and kettle) to 100 ?C?
The heat needed to raise the temperature of the
water to Tf is
The heat needed to raise the temperature of the
aluminum to Tf is
Then Qtotal Qw QAl 732 kJ.
12
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17
Heat of transformation Heat needed to change a
substance from solid to liquid or liquid to
gas. Depends on substance Q ML M is mass of
substance Lf or Lv Heat of fusion f, heat of
vaporization v Lf for water 3 x 105 J/kg Lv
(water) 20 x 105 J/kg Evaporation is similar
to vaporization
18
Thermal expansion
Most objects including liquids and solids expand
when their Temperature increases
19
An objects length after its temperature has
changed is
? is the coefficient of linear expansion. It is
usually very small
?T T?T0 and L0 is the length of the object at
a temperature T0.
The fractional change in volume due to a
temperature change is
For solids ? 3?
20
Heat Transfer
Conduction Convection (evaporation) Radiation
21
Conduction
22
Thermal Conduction
Through direct contact, heat can be conducted
from regions of high temperature to regions of
low temperature. Energy is transferred by
collisions between neighboring atoms or
molecules.
23
The rate of energy transfer by conduction is
where ? is the thermal conductivity, A is the
cross-sectional area, and ?T/L is the temperature
gradient, the temperature change per unit
distance. ? depends on the material. Some
materials conduct heat better than others
24
Thermal Conduction
Through direct contact, heat can be conducted
from regions of high temperature to regions of
low temperature. Energy is transferred by
collisions between neighboring atoms or
molecules.
25
Example A metal rod with a diameter of 2.30 cm
and a length of 1.10 m has one end immersed in
ice at 0 ?C and the other end in boiling water at
100 ?C. If the ice melts at a rate of 1.32 grams
every 175 s, what is the thermal conductivity of
the metal? Assume no heat loss to the
surrounding air.
26
We are given geometrical information, A and
L ?T Rate of ice melting which is related to
rate of heat transfer Find ?
27
Example A metal rod with a diameter of 2.30 cm
and a length of 1.10 m has one end immersed in
ice at 0 ?C and the other end in boiling water at
100 ?C. If the ice melts at a rate of 1.32 grams
every 175 s, what is the thermal conductivity of
the metal? Assume no heat loss to the
surrounding air.
Heat is conducted to the ice at a rate of Qc is
the heat necessary to melt the ice.
The heat conducted to the ice in a time period ?t
is
The heat needed to melt a given mass of ice is Lf
333 x 103 J/kg
28
Example continued
Since all the heat conducted by the rod is
absorbed by the ice,
29
Question
  • You are trying to transfer heat from a hot
    reservoir to a cold reservoir. You have at your
    disposal an aluminum rod and a copper rod of the
    same size. Which should you choose to have the
    highest rate of energy transfer?
  • A) The copper rod alone
  • B) The aluminum rod only
  • C) The rods in parallel.
  • D) The rods in series.

30
Question
  • You are trying to transfer heat from a hot
    reservoir to a cold reservoir. You have at your
    disposal an aluminum rod and a copper rod of the
    same size. Which should you choose to have the
    highest rate of energy transfer?
  • A) The copper rod alone
  • B) The aluminum rod only
  • C) The rods in parallel.
  • D) The rods in series.

31
Thermal Convection
Hot air rises. We can see the ripples in the air
above a hot road. The rising air transfers heat.
Convection is the movement of heat by fluid
currents. Material is transported from one place
to another.
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33
Convection can set up convection cells. Hot
fluids rise and cool fluids sink. Important for
cooking, weather, Sun, Ocean currents etc
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35
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36
Thermal Radiation
The most important source of heat on Earth is
the Sun. How can the heat get here? There is
nothing (vacuum between us and Sun) Not by
conduction or convection. Heat gets here by
radiation
37
Radiation is a an electromagnetic (EM)
wave. Light is one example of EM radiation Even
if I turn out the lights this room is full of EM
radiation
38
Examples of radiation
All bodies emit electromagnetic (EM) radiation.
The amount and type of radiation emitted depends
on the temperature of the object.
39
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40
Question
  • A wood-burning fireplace has a chimney which
    allows the heated air to rise and escape the
  • house. How then does a fireplace then heat the
    room?
  • A) convection
  • B) conduction
  • C) radiation
  • D) all of the above

41
The rate of energy emission by a body is
(Stefans Law)
where A is the surface area of the emitting body,
T is its temperature, and e is the emissivity
e ranges from 0 to 1 (perfect emitter). ?
5.670?10?8 W/m2 K4 is the Stefan-Boltzmann
constant.
42
The net energy gained or lost by a body at a
temperature T is
where Ts is the temperature of the surroundings.
43
Question
  • Which of the following do not emit radiant
    energy?
  • A) Sun
  • B) Earth
  • C) Cup of hot chocolate
  • D) Ice cube
  • E) all of the above emit radiation
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