9/26/2016
P h ys i cs Acad em y
www.physicsacademy.org
Dr. Hazem Falah Sakeek
Al-Azhar University - Gaza
Unit 2: Heat and the first law of thermodynamics
Lecture 4: Heat and Internal Energy
Unit 2: Heat and the first law of thermodynamics

Heat and Internal Energy

Specific Heat and Calorimetry

Latent Heat

Work and Heat in Thermodynamic
Processes

The First Law of Thermodynamics

Some Applications of the First Law of
Thermodynamics

Energy Transfer Mechanisms in Thermal
Processes
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Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
Thermodynamics – Historical Background




Thermodynamics
and
mechanics
were
considered to be distinct branches of physics.
 Until about 1850
 Experiments by James Joule and others
showed a connection between them.
A connection was found between the transfer of
energy by heat in thermal processes and the
transfer of energy by work in mechanical
processes.
The concept of energy was generalized to
include internal energy.
The principle of conservation of energy emerged
as a universal law of nature.
DR. HAZEM FALAH SAKEEK || WWW.PHYSICSACADEMY.ORG|| WWW.HAZEMSAKEEK.NET
James Prescott Joule
1818 – 1889
British physicist
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Heat and Internal Energy
Internal Energy
Heat
Internal Energy is all the energy of a
system that is associated with its
microscopic components —atoms and
molecules— when viewed from a
reference frame at rest with respect to
the center of mass of the system.
Heat is defined as the transfer of energy
across the boundary of a system due to a
temperature difference between the
system and its surroundings. We also use
the term heat to represent the amount
of energy transferred (Q).
DR. HAZEM FALAH SAKEEK || WWW.PHYSICSACADEMY.ORG || WWW.HAZEMSAKEEK.NET
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Heat and Internal Energy
Heat is not in the following common quotes.
(1) Heat is not energy in a hot substance.
(2) Heat is not radiation.
(3) Heat is not warmth of an environment.
Note:
Heat, internal energy, and temperature are all different quantities.
 Be sure to use the correct definition of heat.

You cannot talk about the “heat of a system,” you can refer to
heat only when energy has been transferred as a result of a
temperature difference.
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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Units of Heat

Historically, the calorie was the unit used for heat.
 One calorie is the amount of energy transfer necessary to raise the temperature
g of water
from 14.5oC to
of 1
15.5oC.
 The “Calorie” used for food is actually 1 kilocalorie.

In the US Customary system, the unit is a BTU (British
Thermal Unit).

One BTU is the amount of energy transfer necessary to
raise the temperature of 1 lb of water from 63oF to 64oF.
(1 BTU = 1 055 joules).

The standard unit for heat, work, and internal energy
are measured in joules.
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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Mechanical Equivalent of Heat

Joule established the equivalence
between mechanical energy and internal
energy.

His experimental setup is shown at right.

The decrease in potential energy
associated of the system as the blocks fall
equals the work done by the paddle
wheel on the water.
DR. HAZEM FALAH SAKEEK || WWW.PHYSICSACADEMY.ORG || WWW.HAZEMSAKEEK.NET
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Mechanical Equivalent of Heat
o
o
Joule found that it took approximately 4.18 J of
mechanical energy to raise the water 1oC.
Later, more precise, measurements determined
the amount of mechanical energy needed to
raise the temperature of water from 14.5oC to
15.5oC.
1 cal = 4.186 J
o
This is known as the mechanical equivalent of
heat.
o A more proper name would be the
equivalence between mechanical energy and
internal energy, but the historical name is
well entrenched.
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Example
A student eats a dinner rated at 2000 (food) Calories. He whishes to do an equivalent
amount of work in the gymnasium by lifting 50Kg mass. How many times must he raise
the weight to expend this much energy? Assume that he raises the weight a distance of
2m each time and no work is done when the weight is dropped to the floor.
Solution
1 (food) Calories = 1000 cal
then the work required is 2000x1000 cal = 2x106 cal.
Converting this to joule, then the work required is
W = 2x106cal x 4.186J/cal = 8.37x106J
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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‫ والشغل الكلي لرفع األثقال عدة مرات‬mgh ‫ يساوي‬h ‫الشغل المبذول لرفع االثقال لمسافة‬
:‫ وعليه فإن عدد مرات رفع االثقال هو‬nmgh ‫يعطي بالعالقة‬
W = nmgh = 8.37x106J
Since m = 50 Kg, and h = 2m
n = 8.54x103 times
‫ مرة لحرق السعرات الحرارية المطلوبة ولو كان‬8500 ‫اي انه يلزم الطالب رفع الثقل ما يقارب‬
!!‫ ساعة إلنجاز المهمة‬12 ‫ ثواني فهذا يعني انه يلزمه‬5 ‫يؤدي كل رفعة في زمن مقداره‬
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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Problem to solve by yourself
1.
A 55.0-kg woman cheats on her diet and eats a 540 Calorie
(540 kcal) jelly doughnut for breakfast.
(a)
How many joules of energy are the equivalent of one jelly
doughnut?
(b)
How many steps must the woman climb on a very tall stairway to
change the gravitational potential energy of the woman–Earth
system by a value equivalent to the food energy in one jelly
doughnut? Assume the height of a single stair is 15.0 cm.
(c)
If the human body is only 25.0% efficient in converting chemical
potential energy to mechanical energy, how many steps must the
woman climb to work off her breakfast?
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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(٥) ‫إلى اللقاء مع المحاضرة‬
Specific Heat and Calorimetry
Dr. Hazem Falah Sakeek || www.physicsacademy.org || www.hazemsakeek.net
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