- Intensive Properties: Properties of the system which depend only on the nature of matter but not on the quantity of matter are called Intensive properties, e.g., pressure, temperature, specific heat, etc.
- Extensive Properties: Properties of the system which are dependent on the quantity of matter are called extensive properties, e.g., internal energy, volume, enthalpy, etc.
Internal Energy (E or U)
It is the total energy within the substance. It is the sum of many types of energies like vibrational energy, translational energy, etc. It is a extensive property and state function.
Its absolute value cannot be determined but experimentally change in internal energy (∆) can be determined by
For exothermic process, ∆U = -ve
For endothermic process ∆U = +ve
U depends on temperature, pressure, volume and quantity of matter.
The Internal Energy as a State Function
In thermodynamics, we call it the internal energy, U of the system, which may change, when
- Heat passes into or out of the system,
- Work is done on or by the system,
- Matter enters or leaves the system.
Work is the energy spent to overcome an external force. When the system does work against an external pressure (expansion) it tends to reduce the internal energy and on the other hand when the system contracts due to the external pressure it tends to increase the internal energy.
Adiabatic Process: an Adiabatic process is a process in which there is no transfer of heat between the system and surroundings.
Change in Internal Energy by Doing Work
Let us bring the change in the internal energy by doing work.
Let the initial state of the system is state A and Temp. TA , Internal energy = UA
On doing some mechanical work the new state is called state B and the temp. TB. It is found to be
TB > TA
UB is the internal energy after change.
∴ ∆U = UB – UA
Heat is the energy transferred due to temperature difference between the system and the surrounding. On heating, the kinetic energy of the molecules increases and therefore the internal energy increases.
Change in Internal Energy by Transfer of Heat
Internal energy of a system can be changed by the transfer of heat from the surrounding to the system without doing work.
∆U = q
Where q is the heat absorbed by the system. It can be measured in terms of temperature difference. Q is +ve when heat is transferred from the surrounding to the system. q is -ve when heat is transferred from system to surroundings.
When change of state is done both by doing work and transfer of heat.
∆U = q + W