Download Internal Energy Energy is defined as the capacity to do work. There

Internal Energy
Energy is defined as the capacity to do work. There are various forms of energy- potential
energy by virtue of position, kinetic energy due to motion of the body, thermal energy, nuclear energy,
mechanical energy etc.
The energy possessed by a system due to translational, vibrational and rotational motions of the
molecules along with the energy of the electrons and nuclei is known as its internal energy or intrinsic
energy or energy content of the system. It is denoted by U or E and depends on the internal structure
and constitution of the system. It also depends upon its T, P and V. The energy of a system can be
denoted by any two of the state variables T, P and V.
U = f1(P,T) = f2(V,T) = f3(P,V)
Energy is an extensive property. If a system changes from state A to another state B, the energy change
∆U = UB - UA
The change in energy ∆U or ∆E is independent of the path by which the transformations are carried out.
It depends only on the initial and final states of the system. Hence energy is a state function. The actual
value of internal energy cannot be determined. In thermodynamics, the absolute value of internal
energy is not of any significance. The change in internal energy during a chemical or a physical process is
of interest and it is a measurable quantity.
Heat (Q)
Heat is a form of energy and can be produced from work or can be partly converted into work.
Heat flows from a region of higher temperature to a region of lower temperature until thermal
equilibrium is attained. Heat cannot be completely converted into work without producing permanent
changes either in the system or in the surroundings.
Heat is denoted by Q. When the system absorbs heat from the surroundings, Q is positive. When the
system releases heat, Q is negative. Q is a path function.
First law of thermodynamics
The first law of thermodynamics is the law of conservation of energy.
Statement – Energy can neither be created nor be destroyed. It can only be transformed from one form
into another.
According to the first law, the total energy of a system and its surroundings is conserved. It also
implies that it is impossible to construct a perpetual motion- a machine which can produce energy
without expenditure of energy.
Mathematical formulation of the first law
Consider a system in state A with internal energy UA. Let the system changes to state B where its
energy is UB. Suppose the system while changing from state A to state B absorbs heat ‘q’ from the
surroundings and also performs some work ‘w’. The absorption of heat by the system tends to increase
the energy of the system. But the performance of work by the system tends to lower the energy of the
system because performance of work requires expenditure of energy.
The change in internal energy during the above process is ∆U = UB - UA
Generally if the quantity of heat transferred from the surroundings to the system during a
given process is q and the work done in the process is w, then the change in internal energy is
∆U = q + w This is the mathematical statement of the first law of thermodynamics.
During expansion of a gas, work is done by the system on the surroundings and w is taken as
negative. Then ∆U = q- w
During compression of a gas, work is done by the surroundings on the system and w is taken as
positive. Then ∆U= q + w