CHM 3773 Lecture Notes - Thermal Energy, Isolated System, Potential Energy

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Published on 8 Feb 2013
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System: part of the universe chosen for study. It can be as large as all the oceans
on Earth or as small as the contents of a beaker.
Surroundings: part of the universe outside the system with which the system
interacts.
Open system: free exchanges energy and matter with its surroundings.
Closed system: can exchange energy with its surroundings, but not matter.
Isolated system: does not interact with its surroundings.
Energy: the capacity to do work.
Work: done when a force acts through a distance. Moving objects do work when
they slow down or are stopped.
Kinetic Energy: the energy of a moving object.
Potential energy: due to condition, position, or composition; it is an energy
associated with forces of attraction or repulsion between objects.
Thermal energy: kinetic energy associated with random molecular motion. When
the potential energy in the ball is converted to kinetic energy found in the ball, the
surface and the surrounding air. In general, thermal energy is proportional to the
temperature of a system. The more vigorous the motion of the molecules in the
system, the hotter the sample and the greater is its thermal energy. However, the
thermal energy of a system also depends on the number of particles present. A
small sample at a high temperature such as 75ºC may have less thermal energy
than a pool at 30ºC.
Heat: energy transferred between a system and its surroundings as a result of
temperature difference. At the molecular level, molecules of the warmer body,
through collisions, lose kinetic energy to those of the colder body. Thermal energy is
transferred until the average molecular kinetic energies of the two bodies are equal,
thus the temperatures are equal. Change in temperature can also change a state of
matter. Energy is required to overcome the attractive forces in a given state of
matter. During the transition phases from one state to the next, the temperature
remains constant, while the thermal energy is raised and used to overcome the
forces holding the matter together. This is said to be Isothermal. The energy of a
system is called internal energy. Heat is simply a form in which a quantity of energy
may be transferred across a boundary between a system and its surroundings.
Therefore, the quantity of heat, q, required to change the temperature of a
substance depends on:
How much the temperature is to be changed.
The quantity of the substance
The nature of the substance (type of atoms or molecules)
Calorie: the quantity of heat required to change the temperature of one gram of
water by one degree Celsius.
Heat capacity: quantity of heat required to change the temperature of a system
by one degree.
Specific Heat: quantity of heat required to change the temperature of one gram of
a system by one degree.
Q= m TcΔ
qsystem + qsurrounings = 0 qsystem =- qsurrounings
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