ABSTRACT
Electric circuits are an excellent example of employing the properties of electric charges, covered in Chapters 13 and 14, for practical purposes. Circuits generally consist of (1) a source of charge, for example, a battery or an electric generator, (2) a conducting path for charge and hence energy flow, (3) a “load,” that is, a charge/energy consuming device such as a radio, light bulb, and electric motor.
A simple chemical cell can be constructed by placing two dissimilar metals, say A and B, in a dilute acid. As an example, the metals (electrodes), generally in the shape of rods, could be copper and zinc. Most metals dissociate (dissolve) slightly in the acid. The dissociation results in a positive ion being removed from the metal and entering the acid solution. That metal ion leaves behind at least one electron and therefore negatively charges the rod from which it came. Over time, as the process continues, the rod becomes negative enough to attract some of the positive ions in solution back to it. Eventually, an equilibrium is reached so that the number of positive ions leaving the rod equals the number attracted back to it. The electrode now is negatively charged, so a potential difference exists between it and the solution, which is at a higher potential. A similar process occurs with rod B and it likewise reaches an ion dissociation-attraction equilibrium. But metal B is dissimilar to electrode A and suppose it is more electronegative, that is, retains more electrons per dissociation than A. Both A and B are negative, so they are at a lower potential than the solution, but because of its greater electron retention, B is at a lower potential than A. This potential difference is traditionally called the electromotive force (a misnomer) or emf of the cell. Thus, electrode A will be the positive and B will be the negative terminals of the cell. If now, a conducting wire is attached to rods A and B, electrons will flow from the lower potential B, through the wire to the higher potential at A (Figure 15.1). The connecting wire, often connected to an electrical circuit, is usually placed outside the solution. As electrons leave B, they destroy B’s dissociation-attraction equilibrium and allow more positive ions to leave B and go into solution. Additionally, as more electrons are deposited on A, they attract some of these ions which “plate out” on A. Thus, electrons migrate through the wire from B to A and an equal number of positive ions move through the solution from B to A. Therefore, the net charge on the electrodes remains unchanged. This process can continue until either B dissolves or its ions completely plate A. Then, the segments of the electrodes in solution are not “dissimilar.” Charge ceases to flow. Note that the operation of commercially produced cells is somewhat more complicated but they operate by the same basic principles as discussed above.
