6 Joule’s Experiment
• The main aim of Joules Experiment was to determine the relationship
between the work done and the quantity of heat produced in mechanical
system.
• Prior to Joule, heat was considered to be a invisible fluid known as
caloric and flows from a body of higher caloric to one with a lower
caloric. Caloric was also thought of as a weightless gas that could pass
in and out of pores in solids and liquids(Caloric theory of heat).
In 1844-1854 the English scientist J. P. Joule conducted experiments
which were destined to play an important role in science. The objective
of Joule’s experiment was to establish a relation between the amount of work
spent to bring about the liberation of heat and the amount of the heat liber-
ated. The layout of Joule’s experiment was as in figure 6. Paddle wheel ’2’
was submerged in the heat-insulated vessel ’1’ to the walls of which vanes ’3’
were fastened, the vanes interfering with the motion of water due to rotation
of the paddle. Rotation was imparted to the paddle (stirrer) by the falling
load ’4’ of weight G = mg, connected to the paddle by means of a rope and
pulley ’5’. As the weight falls through a distance Δh, the work done by it
(and, consequently, by the stirrer) is equal to the decrease in the potential
energy of the weight GΔh. The heat liberated in the water-filled vessel is
calculated from the rise in water temperature, measured with a thermometer.
Now the thermal insulation is removed, and the heat is rejected from
the system until it restores the initial state i.e., the temperature of water
decreases to initial value. Thus, the system has undergone a cycle. Similar
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, experiments by Joule led to an important conclusion that the net amount
of heat removed from the system is proportional to the net amount of work
transferred into the system.
thus, � �
δQ = J δW (9)
where J = Joule’s equivalent (constant) or mechanical equivalent of
heat � �
In SI system both δQ and δW are expressed in joule. Therefore, the
Joule’s constant is unity
� i.e., J = 1 �
In MKS System δW is expressed in kgm and δQ is in kcal, then the
value of J is 427, i.e., J = 427kgm/kcal.
Importance of Joule’s Experiments:
• Joule’s experiments conclusively established that heat is a form of en-
ergy.
• Always the same amount of heat was produced by spending a given
amount of mechanical work. It is immaterial what type of arrangement
is used for doing mechanical work.
• It defines the relationship between Joule and Calories.
• Joule’s experiment laid the foundation of the first law of thermody-
namics.
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• The main aim of Joules Experiment was to determine the relationship
between the work done and the quantity of heat produced in mechanical
system.
• Prior to Joule, heat was considered to be a invisible fluid known as
caloric and flows from a body of higher caloric to one with a lower
caloric. Caloric was also thought of as a weightless gas that could pass
in and out of pores in solids and liquids(Caloric theory of heat).
In 1844-1854 the English scientist J. P. Joule conducted experiments
which were destined to play an important role in science. The objective
of Joule’s experiment was to establish a relation between the amount of work
spent to bring about the liberation of heat and the amount of the heat liber-
ated. The layout of Joule’s experiment was as in figure 6. Paddle wheel ’2’
was submerged in the heat-insulated vessel ’1’ to the walls of which vanes ’3’
were fastened, the vanes interfering with the motion of water due to rotation
of the paddle. Rotation was imparted to the paddle (stirrer) by the falling
load ’4’ of weight G = mg, connected to the paddle by means of a rope and
pulley ’5’. As the weight falls through a distance Δh, the work done by it
(and, consequently, by the stirrer) is equal to the decrease in the potential
energy of the weight GΔh. The heat liberated in the water-filled vessel is
calculated from the rise in water temperature, measured with a thermometer.
Now the thermal insulation is removed, and the heat is rejected from
the system until it restores the initial state i.e., the temperature of water
decreases to initial value. Thus, the system has undergone a cycle. Similar
11
, experiments by Joule led to an important conclusion that the net amount
of heat removed from the system is proportional to the net amount of work
transferred into the system.
thus, � �
δQ = J δW (9)
where J = Joule’s equivalent (constant) or mechanical equivalent of
heat � �
In SI system both δQ and δW are expressed in joule. Therefore, the
Joule’s constant is unity
� i.e., J = 1 �
In MKS System δW is expressed in kgm and δQ is in kcal, then the
value of J is 427, i.e., J = 427kgm/kcal.
Importance of Joule’s Experiments:
• Joule’s experiments conclusively established that heat is a form of en-
ergy.
• Always the same amount of heat was produced by spending a given
amount of mechanical work. It is immaterial what type of arrangement
is used for doing mechanical work.
• It defines the relationship between Joule and Calories.
• Joule’s experiment laid the foundation of the first law of thermody-
namics.
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