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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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The Daniell cell, illustrated in Figure 10-5, uses the electrochemical method of avoiding polarization. The Daniell cell is based on the theory that whenever a current passes from a metal to a solution of its own salt, metallic atoms are dissolved into the solution.
The zinc plate is placed in a dilute solution of zinc sulfate (ZnSO4) with a little sulfuric acid (H2SO4). The copper plate is placed in a porous, unglazed earthenware cup. This will pass ions, but not the solution. The solution in the cup is copper sulfate (blue vitriol, CuSO4), with some crystals of CuSO4 placed in the cup containing the copper plate.
The zinc dissolves, leaving electrons on the zinc plate, and gives off Zn++ ions. These ions combine with the SO4= ions to form zinc sulfate.
Primary and Secondary Cells 121
Figure 10-5 Daniell cell.
Zn s Zn+ + + 2e and Zn++ + SO4 s ZnSO4
The Cu++ ions of the depolarizer take electrons from the copper plate, leaving it positive, and deposit metallic copper on the positive plate:
CuSO4 - Cu+ + + SO4=
Cu+ + 2e s Cu T
The hydrogen ions from the sulfuric acid (H2SO4) combine with the SO4= ions of the depolarizer and make sulfuric acid:
2H+ + SO4= s H2SO4
During the use of the cell, the zinc wastes away while the copper gains weight.
The gravity cell has the same action as the Daniell cell, but zinc sulfate is lighter than copper sulfate, so the two keep separated by specific gravity. See Figure 10-6. The gravity cells were used extensively in telegraphy.
The Leclanche cell uses zinc and carbon plates. The solution contains sal ammoniac, which is ammonia chloride (NH4Cl). See Figure 10-7. The depolarizer used is manganese oxide (MnO2). This cell supplies about 1.5 V emf. The accompanying formulas are
Zn++ + 2Cl s ZnCl2 (near negative plate)
2NH4+ + 2e + MnO2 + H2O s 2NH4OH + MnO
122 Chapter 10
Dry cells are in reality a form of the Leclanche cell. The outer case is composed of zinc, which is the negative electrode. The center post is carbon and is the positive post (electrode). The chemicals are manganese dioxide (depolarizer), ground coke, sal ammoniac, and zinc chloride. The zinc chloride is added to lengthen the life of the cell by retarding local action.
There are other primary cells, but since the basic theories have been discussed, no further subject matter will be covered.
Figure 10-7 Leclanche cell.
Secondary Cells
If a charging current is sent through a Daniell cell in reverse direction to the normal flow, this will dissolve some of the copper electrode into the sulfuric acid, making copper sulfate. At the same time some of the zinc atoms will be taken out of the zinc sulfate solution and will be deposited on the zinc electrode. The Daniell
Primary and Secondary Cells 123
cell may be replenished by recharging, instead of by replenishing the zinc plate and electrolyte.
A storage or secondary cell is rechargeable, instead of needing to have the chemicals or the like replenished. We associate the secondary cell or secondary or storage battery with the automobile battery.
The automobile battery is a lead and sulfuric acid battery. It is made up of two lead electrodes immersed in a solution of sulfuric acid (H2SO4) and water (H2O). The plates are connected to a battery charger as shown in Figure 10-8.
A charged storage cell consists of a positive plate of lead oxide (PbO2), a negative plate of spongy lead (Pb), and sulfuric acid (H2SO4). See Figure 10-9. The positive plate is dark brown and the negative plate is light gray.
As the cell discharges, both plates have the active materials turned into lead sulfate (PbSO4) (see Figure 10-10), and it is this condition in which they appear when it becomes necessary to recharge the cells.
The chemical symbols for lead and some of its compounds are as follows:
Lead Pb
Red lead Pb3O4
Peroxide of lead or lead dioxide PbO2
Monoxide of lead PbO
Sulfate of lead PbSO4
Hard sulfate of lead Pb2SO4
124 Chapter 10
Figure 10-10 Discharged cell at the time of putting on charge.
From previous information, we found that H2SO4, when charging or discharging a cell, dissociates into hydrogen H+ ions and sulfate SO4= ions. The lead dioxide (PbO2) in Figure 10-9 reduces to lead monoxide (PbO). This occurs in the positive plate.
The equations associated with the discharge as are follows:
Negative Plate:
Pb s Pb++ + 2e Pb= + SO4= s PbSO4
Positive Plate:
PbO2 + 2H+ + 2e s PbO + H2O
Primary and Secondary Cells 125
PbO2 is inactive with H,SO4 but PbO, which forms at the positive plate when reduced from ’ PbO2 reacts readily with H2SO4. Thus, ’
PbO + H2SO4 s PbSO4 + H2O
So, on discharge, both plates are coated with PbSO4 and the H2SO4 is partially converted to water (H2O).
The voltage of each cell starts at 2.1 V and drops to 2.0 V, where it remains throughout the discharging cycle until toward the end, and then it drops off rapidly.
During the charging from a charging source, the PbSO4 on the negative plate is restored to lead (Pb) and the positive plate is restored to lead oxide (PbO2) (see Figure 10-9); and the H2SO4 which was diluted with H2O, reverts to its original concentration. Thus, 2
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