PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 64

Example

Let us consider two students and call one of them system A and the other system C. Find out

whether both of them have the same temperature without bringing them to any thermal contact.

Solution

We use a thermometer as system B. We bring student A and student C in thermal contact with

the thermometer one by one, and then compare the measured values.

3.1.2 The first law of thermodynamics

There is a function of state calledinternal energyU.For its total differential dUwe write

dU= ̄dQ+ ̄dW , (3.1)

where the symbols ̄dQand ̄dW are not total differentials but represent infinitesimal values of
heatQand workW supplied to the system.
U Main unit:J

Note:Equation (3.1) does not apply to open systems. Extension of the first law to these

systems is covered in section6.4.1.

The integral form of equation (3.1) is

U=Q+W+ const (3.2)

Neither the integration constant nor the value ofU in a given thermodynamic state can be

obtained by any experimental method. Experimentally available are only changes in the internal

energy ∆Uon the system’s transition from one state to another [see3.2.8].

∆U=Q+W. (3.3)

Example

A closed system performed work of 400 J, with 1000 J of energy in the form of heat supplied to

it. How did the system’s internal energy change?