A transfer of energy (as heat or work)
from the system would change its internal
energy. To know ∆U the energy supplied to or
removed from the system need to be monitored.
i. The energy transferred to the system by
heating it or performing work on it is added
to the system.
ii. The energy transferred from the system
by cooling or by performing work on the
surroundings is removed from the system.
The following examples illustrate how to
determine ∆U.
i. 30 kJ of heat supplied to the system. It
would be added to internal energy of the
system and ∆U = +30 kJ.
ii. If 20 kJ of work is done on the system, it
is added to internal energy of the system.
Consequently, ∆U = + 20 kJ.
iii. Suppose a system releases 10 kJ of heat and
performs 15 kJ of work on the surroundings.
These quantities are removed from internal
energy of the system and ∆U = - 25 kJ
4.7 First law of thermodynamics : First law
of thermodynamics is simply the conservation
of energy. According to this law the total
energy of a system and surroundings remains
constant when the system changes from an
initial state to final state. The law is stated in
different ways as follows.
i. Energy of the universe remains constant
ii. The total internal energy of an isolated
system is constant
iii. Energy is neither created nor destroyed
and can only be converted from one form to
another.
All above statements are equivalent.
4.7.1 Formulation of first law of
thermodynamics : A system exchange energy
with its surroundings either by transfer of
heat or by doing work. An energy supplied
to the system increases its internal energy.
On the other hand, removal of heat or work
from the system decreases its internal energy.
Suppose (Q) is heat supplied to the
system and W work done on the system by
the surroundings. The internal energy of the
system would increase.
Increase in internal energy of the system
is equal to sum of the quantity of heat supplied
to the system and amount of work done on the
system or
∆U = Q + W (4.12)
where ∆U is an increase in internal energy
of the system. Eq. (4.12) is the first law of
thermodynamics. For infinitesimal changes.
dU = dQ +dW (4.13)
4.7.2 First law of thermodynamics for
various processes
i. Isothermal process : Temperature is constant
in such process, internal energy is constant.
Hence, ∆U = 0
For isothermal process
0 = Q +W or W = -Q (4.14)
The above equation implies that heat
absorbed by the system is entirely used for
doing work on the surroundings. When work
is done on the system by the surroundings it
results in release of heat.
ii. Adiabatic process : In adiabatic process,
there is no exchange of heat between system
and its surroundings that is, Q = 0. then- ∆U = -W (4.15)
Thus an increase in internal energy of
the system is the work done on it. If the work
is done by the system on the surroundings at
the expense of its internal energy, the internal
energy accompanying the adiabatic process
would decrease.
Try this...
25 kJ of work is done on the
system and it releases 10 kJ of
heat. What is ∆U?