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Heat 14 (01 of 32)

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Title: Heat 14 (01 of 32)


1
Heat
Physics Lecture Notes
Heat 14 (01 of 32)
2
Heat
1) Heat As Energy Transfer
2) Internal Energy
3) Specific Heat
4) Calorimetry
5) Latent Heat
6) Heat Transfer Conduction
Heat 14 (02 of 32)
3
Heat As Energy Transfer
Heat is random thermal Energy
Unit of heat calorie (cal)
1 cal is the amount of heat necessary to raise
the temperature of 1 g of water by 1 Celsius
degree.
1 kcal is the amount of heat necessary to raise
the temperature of 1 kg of water by 1 Celsius
degree.
Heat 14 (13 of 32)
4
Heat As Energy Transfer
Heat is a form of energy and can be equated to
mechanical energy.
Heat 14 (13 of 32)
5
Heat As Energy Transfer
Definition of heat Heat is thermal energy
transferred from one object to another because of
a difference in temperature.
The sum total of all the energy of all the
molecules in a substance is its internal (or
thermal) energy.
Temperature measures molecules average kinetic
energy
Internal energy total energy of all molecules
Heat transfer of energy due to difference in
temperature
Heat 14 (13 of 32)
6
Heat As Energy Transfer
m
Heat 14 (13 of 32)
7
Chapter 14 Page 387
Heat 14 (13 of 32)
8
Heat As Energy Transfer
Problem
A 50 g piece of cadmium is at 20 oC. If 400 J of
heat is added to the cadmium, what is its final
temperature
Heat 14 (13 of 32)
9
Heat As Energy Transfer
Problem
A 100 g lead bullet traveling at 300 m/s is
stopped by a large tree. Half the kinetic energy
of the bullet is transformed into heat energy and
remains with the the bullet while the other half
is transmitted to the tree. What is the increase
in temperature of the bullet?
Heat 14 (13 of 32)
10
Heat As Energy Transfer
Problem
A 3.0 kg block of iron is dropped from rest from
the top of a cliff. When the block hits the
ground it is observed that its temperature
increases by 0.50 oC. Assume that all the
potential energy is used to heat the block. How
high is the cliff?
Heat 14 (13 of 32)
11
Heat As Energy Transfer
Problem
A 1.5 kg copper block is given an initial speed
of 30 m/s on a rough horizontal surface Because
of friction, the block finally comes to rest. If
the block absorbs 85 of its initial kinetic
energy in the form of heat, Calculate its
increase in temperature?
Heat 14 (13 of 32)
12
Calorimetry
Problem
A 0.40 kg iron horseshoe that is initially at 500
oC is dropped into a bucket containing 20 kg of
water at 22 oC. What is the final equilibrium
temperature? Neglect any heat transfer to for
from the surroundings.
Heat 14 (13 of 32)
13
Calorimetry
Problem
A 200 g block of copper at a temperature of 90 oC
is dropped into 400 g of water at 27 oC. The
water is contained in a 300 g glass container.
What is the final temperature of the mixture
Heat 14 (13 of 32)
14
Example 1 A 500-g copper coffee mug is filled
with 200-g of coffee. How much heat was required
to heat cup and coffee from 20 to 960C?
1. Draw sketch of problem.
2. List given information.
3. List what is to be found
15
Example 1(Cont.) How much heat needed to heat
cup and coffee from 20 to 960C? mm
0.2 kg mw 0.5 kg.
4. Recall applicable formula or law
5. Decide that TOTAL heat is that required to
raise temperature of mug and water (coffee).
Write equation.
QT mmcm Dt mwcw Dt
6. Look up specific heats in tables
16
Example 1(Cont.) How much heat needed to heat
cup and coffee from 20 to 960C? mc
0.2 kg mw 0.5 kg.
7. Substitute info / solve problem
Water (0.20 kg)(4186 J/kgC0)(76 C0)
Dt 960C - 200C 76 C0
Cup (0.50 kg)(390 J/kgC0)(76 C0)
QT 78.4 kJ
QT 63,600 J 14,800 J
17
Calorimetry
m1
m2
TH
TL
Conservation of thermal energy
Final Temperature
Heat 14 (13 of 32)
18
Latent Heat - Stored / Hidden
Energy is required for a material to change
phase, Even though its temperature is not
changing.
Heat 14 (13 of 32)
19
The water problem
20
Table of latent heats The following table shows
the latent heats and change of phase temperatures
of some common fluids and gases.
Substance Latent HeatFusionkJ/kg MeltingPointC Latent HeatVaporizationkJ/kg BoilingPointC
Alcohol, ethyl 108 -114 855 78.3
Ammonia 339 -75 1369 -33.34
Carbon dioxide 184 -78 574 -57
Helium 21 -268.93
Hydrogen(2) 58 -259 455 -253
Lead8 24.5 327.5 871 1750
Nitrogen 25.7 -210 200 -196
Oxygen 13.9 -219 213 -183
R134a -101 215.9 -26.6
Toluene -93 351 110.6
Turpentine 293
Water 334 0 2260 100
21
Latent Heat
Heat of fusion, Lf heat required to change 1.0
kg of material from solid to liquid
Heat of vaporization, Lv heat required to change
1.0 kg of material from liquid to vapor
Chapter 14 - Page 392
Heat 14 (13 of 32)
22
Latent Heat
Heat 14 (13 of 32)
23
Latent Heat
Heat required to convert 1 kg of ice at -50 oC
to steam at 150 oC
Q1 1.05 x 105 J Q2 3.33 x 105 J Q3 4.19 x
105 J Q4 22.6 x 105 J Q5 1.01 x 105 J
3.22 x 106 J
Heat 14 (13 of 32)
24
Latent Heat
Problem
A large block of ice at 0 oC has a hole chipped
in it, and 400 g of aluminum pellets at a
temperature of 30 oC are poured into the hole.
How much of the ice melts?
Heat 14 (13 of 32)
25
Conduction
Heat conduction can be visualized as occurring
through molecular collisions.
The heat flow per unit time is given by
Heat 14 (13 of 32)
26
Conduction
The constant k is called the thermal conductivity.
Materials with large k are called conductors
those with small k are called insulators.
Heat 14 (13 of 32)
27
Conduction
Problem
A window has a glass surface of 1.6 x 103 cm2 and
a thickness of 3.0 mm. Find the rate of heat
transfer by conduction through this pane when the
temperature of the inside surface of the glass is
20 oC and the outside temperature is 40 oC.
Heat 14 (13 of 32)
28
Conduction
Problem
A glass window pane has an area of 3.0 m2 and a
thickness of 0.60 cm. If the temperature
difference between its faces is 25 oC, how much
heat flows through the window per hour?
Heat 14 (13 of 32)
29
Conduction
Building materials are measured using R-values
rather than thermal conductivity
Where, L is the thickness of the material.
Heat 14 (13 of 32)
30
Summary
Internal energy U refers to the total energy of
all molecules in an object. For an ideal
monatomic gas,
Heat is the transfer of energy from one object to
another due to a temperature difference. Heat can
be measured in joules or in calories.
Specific heat of a substance is the energy
required to change the temperature of a fixed
amount of matter by 1 C.
Heat 14 (13 of 32)
31
Summary
In an isolated system, heat gained by one part of
the system must be lost by another.
Calorimetry measures heat exchange quantitatively.
Energy in involved in phase changes even though
the temperature does not change.
Heat of fusion amount of energy required to melt
1 kg of material.
Heat of vaporization amount of energy required
to change 1 kg of material from liquid to vapor.
Heat 14 (13 of 32)
32
Summary
Heat transfer takes place by conduction,
convection, and radiation.
In conduction, energy is transferred through the
collisions of molecules in the substance.
Heat 14 (13 of 32)
33
Internal Energy
Heat 14 (13 of 32)
34
END
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