Thermodynamics and Chemistry

CHAPTER 3 THE FIRST LAW

3.8 ELECTRICALWORK 89

I

wel

bc

Figure 3.15 Work of electrical heating with a fixed magnitude ofQsysas a function

of the electric currentID∂Qsys=dt. The open circle indicates the limit of infinite

slowness.

cell

bb

L

R

C

(a)

cell

e

bb

 C

(b)

Figure 3.16 Galvanic cell and external electrical resistor.

(a) Open circuit with isolated cell in an equilibrium state.

(b) Closed circuit.

of irreversibledissipativework (page 83 ). Note the resemblance of Fig.3.15to Fig.3.11(b)
on page 83 for dissipative stirring work—they are the same graphs with different labels.
Suppose we redefine the system to be only the liquid. In this case, electric current passes
through the resistor but not through the system boundary. There is no electrical work, and
we must classify energy transfer between the resistor and the liquid asheat.

3.8.3 Electrical work with a galvanic cell

Agalvanic cellis an electrochemical system that, when isolated, exhibits an electric poten-
tial difference between the two terminals at the system boundary. The potential difference
has its source at the interfaces between phases within the cell.
Consider the combination of galvanic cell and electrical resistor in Fig.3.16, and let the
systembe the cell. When an electric current passes through the cell in either direction, a cell
reaction takes place in one direction or the other.
In a manner similar to the labeling of the conductors of a circuit, the cell terminals are
called therightterminal and theleftterminal. Thecell potentialEcellis the electric potential
difference between the terminals, and is defined by

EcelldefD RL (3.8.6)

When the cell is in an isolated zero-current equilibrium state, as in Fig.3.16(a), the cell
potential is theequilibrium cell potentialEcell, eq. When the cell is part of an electrical