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Basic Terms And Concept - II

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Basic Terms And Concept - II - Lesson Summary

A system is a part of the universe, in which observations are made. In order to make these observations, it is important that we know the state of the system.

The state of a system means the condition of the system in terms of observable properties such as temperature, pressure and volume.

The first state of the system, that is, the state before the change, is called the initial state, and the last state, that is, the state after the change, is called the final state.

As a change in the magnitude of these properties changes the state of a system, these properties are called state variables or state functions.

Macroscopic properties, on which the state of a system depends, are further divided into extensive properties and intensive properties.

Extensive properties:

These properties depend upon the quantity of the matter contained in a system.

Ex: mass, volume and heat capacity.

Intensive properties:

These properties independent of the amount of the substance present in the system.

Ex: Temperature, pressure, freezing point and boiling point.

When the state of a system changes, it implies that some thermodynamic process has occurred on the system, and energy has been either added to it or removed from it.

The energy stored within a substance or a system is known as the internal energy of the system. It is represented by U.

The absolute value of internal energy cannot be found. However, change in it, represented by ∆U.

Internal energy U changes when:

· Work is done on or by the system

· Heat passes into or out of the system

· Matter enters or leaves the system.

The change in internal energy brought out by work done on the system.

Take a certain amount of water in a thermos flask or an insulated vessel that does not allow heat exchange with the surroundings. Such a system is called an adiabatic.

The state of such a system changes without an exchange of heat, and thus, the process is called an adiabatic process.

∆ q = 0

We can bring a change in internal energy in a system by two ways.

In the first method, internal energy can be increased by doing some mechanical work

Let U A be the initial internal energy of the system at temperature T A. Final internal energy changes to U B as temperature increases to T B.

The change in internal energy ∆U =UB -UA

In the second method, internal energy can be increased by the same amount by doing electrical work.

The change in temperature = TB-TA

The change in internal energy ∆U =UB-UA

Thus, internal energy changes to same extent either by doing mechanical work or electric work

∴ Internal energy is a state function.

If work is done on the system, then U increases and w is positive.

If work is done by the system, then U decreases and w is negative.

A change in internal energy can also be brought out by the transfer of heat. The exchange of energy due to temperature difference is called heat q.

∆ U = q

q is positive if heat transfers from the surroundings to the system and negative if heat transfers from the system to the surroundings.


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